Neuromuscular transmission of excitation. We have already shown above that the conduction of excitation in nerve and muscle fibers is carried out with the help of electrical impulses propagating along the surface membrane. The transmission of excitation from the nerve to the muscle is based on a different mechanism. It is carried out as a result of the release of highly active chemical compounds by the nerve endings - mediators of the nerve impulse. In skeletal muscle synapses, such a mediator is acetylcholine (ACh).

In the neuromuscular synapse, there are three main structural elements - presynaptic membrane on the nerve postsynaptic membrane on the muscle, between them - synaptic cleft . The shape of the synapse can be varied. At rest, ACh is contained in the so-called synaptic vesicles inside the end plate of the nerve fiber. The cytoplasm of the fiber with synaptic vesicles floating in it is separated from the synaptic cleft by the presynaptic membrane. When the presynaptic membrane is depolarized, its charge and permeability change, the bubbles come close to the membrane and pour into the synaptic cleft, the width of which reaches 200-1000 angstroms. The mediator begins to diffuse through the gap to the postsynaptic membrane.

The postsynaptic membrane is not electrogenic, but has a high sensitivity to the mediator due to the presence in it of the so-called cholinergic receptors - biochemical groups that can selectively react with ACh. The latter reaches the postsynaptic membrane in 0.2-0.5 msec. (so-called "synaptic delay") and, interacting with cholinergic receptors, causes a change in the membrane permeability for Na, which leads to depolarization of the postsynaptic membrane and the generation of a depolarization wave on it, which is called excitatory postsynaptic potential, (EPSP), the value of which exceeds the Ek of neighboring, electrogenic sections of the muscle fiber membrane. As a result, an AP (action potential) arises in them, which spreads over the entire surface of the muscle fiber, then causing its contraction, initiating the process of the so-called. electromechanical interface (Kapling). The mediator in the synaptic cleft and on the postsynaptic membrane works for a very short time, as it is destroyed by the enzyme cholinesterase, which prepares the synapse to receive a new portion of the mediator. It has also been shown that part of the unreacted ACh can return to the nerve fiber.

With very frequent stimulation rhythms, postsynaptic potentials can be summed up, since cholinesterase does not have time to completely break down the ACh released in the nerve endings. As a result of this summation, the postsynaptic membrane becomes more and more depolarized. At the same time, neighboring electrogenic sections of the muscle fiber come into a state of depression, similar to that which develops during prolonged action of the direct current cathode. (Verigo's cathodic depression).

Functions and properties of striated muscles.

The striated muscles are the active part of the musculoskeletal system. As a result of the contractile activity of these muscles, the body moves in space, the parts of the body move relative to each other, and the posture is maintained. In addition, during muscular work, heat is generated.

Each muscle fiber has the following properties: excitability , those. the ability to respond to the action of the stimulus by generating AP, conductivity - the ability to conduct excitation along the entire fiber in both directions from the point of irritation, and contractility , i.e. the ability to contract or change its tension when excited. Excitability and conductivity are functions of the surface cell membrane - the sarcolemma, and contractility is a function of the myofibrils located in the sarcoplasm.

Research methods. Under natural conditions, the excitation and contraction of muscles is caused by nerve impulses. In order to excite a muscle in an experiment or in a clinical study, it is subjected to artificial stimulation with an electric current. Direct irritation of the muscle itself is called direct, and irritation of the nerve is called indirect irritation. Due to the fact that the excitability of muscle tissue is less than that of nervous tissue, the application of electrodes directly to the muscle does not yet provide direct irritation - the current, spreading through the muscle tissue, acts primarily on the endings of the motor nerves located in it. Pure direct irritation is obtained only with intracellular irritation or after poisoning of the nerve endings with curare. Registration of muscle contraction is carried out using mechanical devices - myographs, or special sensors. When studying muscles, electron microscopy, recording of biopotentials during intracellular recording, and other subtle techniques are used to study the properties of muscles both in the experiment and in the clinic.

Mechanisms of muscle contraction.

The structure of myofibrils and its changes during contraction. Myofibrils are the contractile apparatus of the muscle fiber. In striated muscle fibers, myofibrils are divided into regularly alternating sections (discs) with different optical properties. Some of these sections are anisotropic, i.e. have double refraction. In ordinary light they look dark, but in polarized light they are transparent in the longitudinal direction and opaque in the transverse direction. Other areas are isotropic, and appear transparent in ordinary light. Anisotropic regions are denoted by the letter A, isotropic - I. In the middle of disk A there is a light strip H, and in the middle of disk I there is a dark stripe Z, which is a thin transverse membrane through the pores of which myofibrils pass. Due to the presence of such a support structure, parallel single-valued disks of individual myofibrils within one fiber do not move relative to each other during contraction.

It has been established that each of the myofibrils has a diameter of about 1 micron and consists of an average of 2500 protofibrils, which are elongated molecules polymerized by the protein myosin and actin. Myosin filaments (protofibrils) are twice as thick as actin filaments. Their diameter is approximately 100 angstroms. In the resting state of the muscle fiber, the filaments are located in the myofibril in such a way that the thin long actin filaments enter with their ends into the gaps between the thick and shorter myosin filaments. In such a section, each thick thread is surrounded by 6 thin ones. Due to this, disks I consist only of actin filaments, and disks A also consist of myosin filaments. The light stripe H is a zone free from actin filaments during the dormant period. Membrane Z, passing through the middle of disc I, holds the actin filaments together.

Numerous cross-bridges on myosin are also an important component of the ultramicroscopic structure of myofibrils. In turn, there are so-called active centers on actin filaments, at rest covered, like a sheath, with special proteins - troponin and tropomyosin. Contraction is based on the sliding of actin filaments relative to myosin filaments. Such sliding is caused by the work of the so-called. "chemical gear", ie. periodically occurring cycles of changes in the state of cross bridges and their interaction with active centers on actin. ATP and Ca+ ions play an important role in these processes.

When the muscle fiber contracts, the actin and myosin filaments do not shorten, but begin to slide over each other: the actin filaments move between the myosin filaments, as a result of which the length of the I disks is shortened, and the A disks retain their size, approaching each other. The H strip almost disappears, because the ends of the actin are in contact and even go behind each other.

The role of AP in the occurrence of muscle contraction (the process of electromechanical coupling). In skeletal muscle, under natural conditions, the initiator of muscle contraction is the action potential, which propagates upon excitation along the surface membrane of the muscle fiber.

If the tip of the microelectrode is applied to the surface of the muscle fiber in the area of ​​the Z membrane, then when a very weak electrical stimulus is applied that causes depolarization, the I disks on both sides of the stimulation site will begin to shorten. in this case, the excitation propagates deep into the fiber, along the Z membrane. Irritation of other sections of the membrane does not cause such an effect. From this it follows that the depolarization of the surface membrane in the region of disc I during AP propagation is the trigger of the contractile process.

Further studies have shown that an important intermediate link between membrane depolarization and the onset of muscle contraction is the penetration of free CA++ ions into the interfibrillar space. At rest, most of the Ca++ in the muscle fiber is stored in the sarcoplasmic reticulum.

In the mechanism of muscle contraction, a special role is played by that part of the reticulum, which is localized in the region of the Z membrane. triad (T-system), each of which consists of a thin transverse tube centrally located in the Z membrane region, running across the fiber, and two lateral cisterns of the sarcoplasmic reticulum, in which bound Ca ++ is enclosed. AP propagating along the surface membrane is conducted deep into the fiber along the transverse tubules of the triads. Then the excitation is transferred to the cisternae, depolarizes their membrane and it becomes permeable to CA++.

It has been experimentally established that there is a certain critical concentration of free Ca++ ions, at which the contraction of myofibrils begins. It is equal to 0.2-1.5*10 6 ions per fiber. Increasing the concentration of Ca++ to 5*10 6 already causes the maximum reduction.

The onset of muscle contraction is timed to the first third of the ascending AP knee, when its value reaches approximately 50 mV. It is believed that it is at this depolarization level that the concentration of Ca++ becomes the threshold for the beginning of the interaction between actin and myosin.

The Ca++ release process stops after the end of the AP peak. Nevertheless, the contraction continues to grow until the mechanism that ensures the return of Ca ++ to the reticulum cisterns comes into action. This mechanism is called the "calcium pump". To carry out its work, the energy obtained from the breakdown of ATP is used.

In the interfibrillar space, Ca++ interacts with proteins that close the active centers of actin filaments - troponin and tropomyosin, providing an opportunity for the reaction of myosin cross-bridges and actin filaments.

Thus, the sequence of events leading to contraction and then to relaxation of the muscle fiber is currently drawn as follows:

Irritation - the occurrence of AP - its conduction along the cell membrane and deep into the fiber through the tubules of T-systems - depolarization of the membrane sarcoplasmic reticulum - Ca++ release from triads and its diffusion to myofibrils - Ca++ interaction with troponin and ATP energy release - interaction (sliding) of actin and myosin filaments - muscle contraction - decrease in Ca++ concentration in the interfibrillar space due to the work of the Ca-pump - muscle relaxation .

The role of ATP in the mechanism of muscle contraction. In the process of interaction between actin and myosin filaments in the presence of Ca++ ions, an important role is played by the energy-rich compound, ATP. Myosin has the properties of the enzyme ATPase. When ATP is broken down, about 10,000 calories are released. per 1 mol. Under the influence of ATP, the mechanical properties of myosin filaments also change - their extensibility sharply increases. It is believed that the breakdown of ATP is the source of energy necessary for the sliding of threads. Ca++ ions increase the ATP-ase activity of myosin. In addition, the energy of ATP is used to operate the calcium pump in the reticulum. Accordingly, ATP-cleaving enzymes are localized in these membranes, and not only in myosin.

The resynthesis of ATP, which is continuously split during muscle work, is carried out in two main ways. The first is the enzymatic transfer of the phosphate group from creatine phosphate (CP) to ADP. CF is contained in the muscle in much larger quantities than ATP, and ensures its resynthesis within thousandths of a second. However, during prolonged muscle work, CF reserves are depleted, so the second way is important - slow ATP resynthesis associated with glycolysis and oxidative processes. Oxidation of lactic and pyruvic acids formed in the muscle during its contraction is accompanied by phosphorylation of ADP and creatine, i.e. resynthesis of CP and ATP.

Violation of ATP resynthesis by poisons that suppress glycolysis and oxidative processes leads to the complete disappearance of ATP and CP, as a result of which the calcium pump stops working. The concentration of Ca ++ in the area of ​​myofibrils increases greatly and the muscle enters a state of long-term irreversible shortening - the so-called. contractures.

Heat generation during the contraction process. According to its origin and time of development, heat generation is divided into two phases. The first is many times shorter than the second and is called the initial heat generation. It starts from the moment of excitation of the muscle and continues throughout the entire contraction, including the relaxation phase. The second phase of heat generation occurs within a few minutes after relaxation, and is called delayed, or restorative heat generation. In turn, the initial heat generation can be divided into several parts - activation heat, shortening heat, and relaxation heat. The heat generated in the muscles maintains the temperature of the tissues at a level that ensures the active flow of physical and chemical processes in the body.

Types of abbreviations. Depending on the conditions in which the reduction occurs,

nie, there are two types of it - isotonic and isometric . Isotonic is the contraction of the muscle, in which its fibers are shortened, but the tension remains the same. An example is shortening without load. An isometric contraction is such a contraction in which the muscle cannot shorten (when its ends are fixedly fixed). In this case, the length of the muscle fibers remains unchanged, but their tension increases (lifting an unbearable load).

Natural muscle contractions in the body are never purely isotonic or isometric.

Single cut. Irritation of a muscle or motor nerve innervating it with a single stimulus causes a single muscle contraction. It distinguishes two main phases: the contraction phase and the relaxation phase. The contraction of the muscle fiber begins already during the ascending branch of the AP. The duration of contraction at each point of the muscle fiber is tens of times greater than the duration of AP. Therefore, there comes a moment when the AP has passed along the entire fiber and ended, while the contraction wave has covered the entire fiber and it continues to be shortened. This corresponds to the moment of maximum shortening or tension of the muscle fiber.

The contraction of each individual muscle fiber during single contractions obeys the law " all or nothing". This means that the contraction that occurs both with threshold and with supra-threshold stimulation has a maximum amplitude. The magnitude of a single contraction of the entire muscle depends on the strength of the irritation. With threshold stimulation, its contraction is barely noticeable, but with an increase in the strength of irritation it increases, until it reaches a certain height, after which it remains unchanged (maximum contraction). This is due to the fact that the excitability of individual muscle fibers is not the same, and therefore only part of them is excited with weak irritation. At maximum contraction, they are all excited. The speed of the wave of muscle contraction is the same with the speed of propagation of AP.In the biceps muscle of the shoulder, it is 3.5-5.0 m/sec.

Contraction summation and tetanus. If, in an experiment, an individual muscle fiber or the entire muscle is affected by two rapidly following each other strong single stimuli, then the resulting contraction will have a greater amplitude than the maximum single contraction. The contractile effects caused by the first and second irritation seem to add up. This phenomenon is called the summation of contractions. For summation to occur, it is necessary that the interval between stimuli has a certain duration - it must be longer than the refractory period, but shorter than the entire duration of a single contraction, so that the second stimulus acts on the muscle before it has time to relax. In this case, two cases are possible. If the second stimulation arrives when the muscle has already begun to relax, on the myographic curve the top of the second contraction will be separated from the first by a depression. If the second irritation acts when the first contraction has not yet reached its peak, then the second contraction, as it were, merges with the first, forming together with it a single summed peak. Both with full and incomplete summation, PDs are not summed up. Such a summed contraction in response to rhythmic stimuli is called tetanus. Depending on the frequency of irritation, it is serrated and smooth.

The reason for the summation of contractions in tetanus lies in the accumulation of Ca ++ ions in the interfibrillar space up to a concentration of 5 * 10 6 mM / l. After reaching this value, further accumulation of Ca++ does not lead to an increase in the tetanus amplitude.

After the termination of tetanic irritation, the fibers do not relax completely at first, and their original length is restored only after some time has passed. This phenomenon is called post-tetanic, or residual contracture. She is connected to it. that it takes more time to remove all Ca ++ from the interfibrillar space, which got there with rhythmic stimuli and did not have time to completely withdraw into the cisterns of the sarcoplasmic reticulum by the work of Ca-pumps.

If, after reaching a smooth tetanus, the frequency of stimulation is increased even more, then the muscle at a certain frequency suddenly begins to relax. This phenomenon is called pessimism. It occurs when each next impulse falls into refractoriness from the previous one.

Motor units. We have considered the general scheme of the phenomena underlying tetanic contraction. In order to get to know in more detail how this process takes place under the conditions of the natural activity of the body, it is necessary to dwell on some features of the innervation of the skeletal muscle by the motor nerve.

Each motor nerve fiber, which is an outgrowth of the motor cell of the anterior horns of the spinal cord (alpha motor neuron), branches in the muscle and innervates a whole group of muscle fibers. Such a group is called the motor unit of the muscle. The number of muscle fibers that make up the motor unit varies widely, but their properties are the same (excitability, conductivity, etc.). Due to the fact that the speed of propagation of excitation in the nerve fibers innervating the skeletal muscles is very high, the muscle fibers that make up the motor unit come into a state of excitation almost simultaneously. The electrical activity of the motor unit has the form of a palisade, in which each peak corresponds to the total action potential of many simultaneously excited muscle fibers.

It should be said that the excitability of various skeletal muscle fibers and the motor units consisting of them varies significantly. She is more in the so-called. fast and less in slow fibers. At the same time, the excitability of both is lower than the excitability of the nerve fibers that innervate them. It depends on the fact that in the muscles the difference between E0-E k is greater, and, therefore, the rheobase is higher. PD reaches 110-130 mV, its duration is 3-6 ms. The maximum frequency of fast fibers is about 500 per second, most skeletal fibers - 200-250 per second. The duration of AP in slow fibers is about 2 times longer, the duration of the contraction wave is 5 times longer, and the speed of its conduction is 2 times slower. In addition, fast fibers are divided depending on the speed of contraction and lability into phasic and tonic.

Skeletal muscles in most cases are mixed: they consist of both fast and slow fibers. But within one motor unit, all fibers are always the same. Therefore, motor units are divided into fast and slow, phasic and tonic. The mixed type of muscle allows the nerve centers to use the same muscle both to carry out fast, phasic movements and to maintain tonic tension.

There are, however, muscles that are predominantly composed of fast or slow motor units. Such muscles are often also called fast (white) and slow (red). The duration of the contraction wave of the fastest muscle - the internal rectus muscle of the eye - is only 7.5 ms, for the slow soleus - 75 ms. The functional significance of these differences becomes apparent when considering their responses to rhythmic stimuli. To obtain a smooth tetanus of a slow muscle, it is enough to irritate it with a frequency of 13 stimuli per second. in fast muscles, smooth tetanus occurs at a frequency of 50 stimuli per second. In tonic motor units, the duration of contraction for a single stimulus can be up to 1 second.

Summation of motor unit contractions in a whole muscle. Unlike muscle fibers in a motor unit, which fire simultaneously in response to an incoming impulse, muscle fibers of different motor units in a whole muscle fire asynchronously. This is explained by the fact that different motor units are innervated by different motor neurons, which send impulses at different frequencies and at different times. Despite this total contraction of the muscle as a whole, under conditions of normal activity, it has a fused character. This is because the neighboring motor unit (or units) always has time to contract before those that are already excited have time to relax. The strength of muscle contraction depends on the number of motor units involved in the reaction at the same time, and on the frequency of excitation of each of them.

Skeletal muscle tone. At rest, outside of work, the muscles in the body are not

completely relaxed, but retain some tension, called tone. The external expression of tone is a certain elasticity of the muscles.

Electrophysiological studies show that the tone is associated with the supply of rare nerve impulses to the muscle, which alternately excite various muscle fibers. These impulses arise in the motor neurons of the spinal cord, the activity of which, in turn, is supported by impulses coming from both higher centers and proprioreceptors (muscle spindles, etc.) located in the muscles themselves. The reflex nature of skeletal muscle tone is evidenced by the fact that transection of the posterior roots, through which sensitive impulses from muscle spindles enter the spinal cord, leads to complete relaxation of the muscle.

Muscle work and strength. The amount of contraction (degree of shortening) of the muscle at a given strength of stimulation depends both on its morphological properties and on the physiological state. Long muscles contract more than short ones. Moderate stretching of the muscle increases its contractile effect, with strong stretching, the contracted muscles relax. If, as a result of prolonged work, muscle fatigue develops, then the magnitude of its contraction falls.

To measure muscle strength, either the maximum load that it is able to lift, or the maximum tension that it can develop under conditions of isometric contraction, is determined. This power can be very great. Thus, it has been established that a dog with its jaw muscles can lift a load exceeding its body weight by 8.3 times.

A single muscle fiber can develop tension up to 100-200 mg. Considering that the total number of muscle fibers in the human body is approximately 15-30 million, they could develop a tension of 20-30 tons if they all pulled in one direction at the same time.

Muscle strength, other things being equal, depends on its cross section. The greater the sum of the cross sections of all its fibers, the greater the load that it is able to lift. This means the so-called. physiological cross section, when the line of section is perpendicular to the muscle fibers, and not to the muscle as a whole. The strength of muscles with oblique fibers is greater than with straight fibers, since its physiological cross section is greater with the same geometric. To compare the strength of different muscles, the maximum load (absolute muscle strength) that the muscle is able to lift is divided by the physiological cross-sectional area (kg / cm2). Thus, the specific absolute strength of the muscle is calculated. For the human gastrocnemius muscle, it is 5.9 kg / cm2, the shoulder flexor - 8.1 kg / cm2, the triceps muscle of the shoulder - 16.8 kg / cm2.

Muscle work is measured by the product of the lifted load by the amount of shortening of the muscle. Between the load that the muscle lifts and the work it performs, there is the following pattern. The external work of a muscle is zero if the muscle contracts without load. As the load increases, the work first increases and then gradually decreases. The muscle performs the greatest work at some average loads. Therefore, the dependence of work and power on the load is called rules (of law) medium loads .

The work of the muscles, in which the movement of the load and the movement of the bones in the joints, is called dynamic. The work of the muscle, in which the muscle fibers develop tension, but almost do not shorten - static. An example is hanging on a pole. Static work is more tedious than dynamic work.

Muscle fatigue. Fatigue is a temporary decrease in working capacity

function of a cell, organ or whole organism, which occurs as a result of work and disappears after rest.

If for a long time an isolated muscle, to which a small load is suspended, is irritated with rhythmic electrical stimuli, then the amplitude of its contractions gradually decreases until it drops to zero. The fatigue curve is recorded. Along with a change in the amplitude of contractions during fatigue, the latent period of contraction increases, the period of muscle relaxation lengthens, and the stimulation threshold increases, i.e. excitability decreases. All these changes do not occur immediately after the start of work, there is a certain period during which there is an increase in the amplitude of contractions and a slight increase in muscle excitability. At the same time, it becomes easily stretchable. In such cases, they say that the muscle is "worked in", i.e. adapts to work in a given rhythm and strength of irritation. After a period of workability, a period of stable performance begins. With further prolonged irritation, fatigue of the muscle fibers occurs.

The decrease in the efficiency of a muscle isolated from the body during its prolonged irritation is due to two main reasons. The first of these is that during contractions, metabolic products accumulate in the muscle (phosphoric acid, which binds Ca ++, lactic acid, etc.), which have a depressing effect on muscle performance. Some of these products, as well as Ca ions, diffuse outward from the fibers into the pericellular space and have a depressing effect on the ability of the excitable membrane to generate AP. So, if an isolated muscle placed in a small volume of Ringer's fluid is brought to complete fatigue, then it is enough just to change the solution washing it to restore muscle contractions.

Another reason for the development of fatigue in an isolated muscle is the gradual depletion of energy reserves in it. With prolonged work, the content of glycogen in the muscle decreases sharply, as a result of which the processes of ATP and CP resynthesis, which are necessary for the contraction, are disrupted.

It should be noted that under the natural conditions of the organism's existence, fatigue of the motor apparatus during prolonged work develops in a completely different way than in an experiment with an isolated muscle. This is due not only to the fact that in the body the muscle is continuously supplied with blood, and, therefore, receives the necessary nutrients with it and is released from metabolic products. The main difference is that in the body, excitatory impulses come to the muscle from the nerve. The neuromuscular synapse gets tired much earlier than the muscle fiber, due to the rapid depletion of the accumulated mediator. This causes a blockade of the transmission of excitations from the nerve to the muscle, which prevents the muscle from exhaustion caused by prolonged work. In a whole organism, the nerve centers (nerve-nerve contacts) get tired even earlier during work.

The role of the nervous system in the fatigue of the whole organism is proved by studies of fatigue in hypnosis (kettlebell-basket), the establishment of the influence of "active rest" on fatigue, the role of the sympathetic nervous system (the Orbeli-Ginetsinsky phenomenon), etc.

Ergography is used to study muscle fatigue in humans. The shape of the fatigue curve and the amount of work done vary enormously in different individuals and even in the same subject under different conditions.

Working muscle hypertrophy and inactivity atrophy. Systematic intensive work of the muscle leads to an increase in the mass of muscle tissue. This phenomenon is called working muscle hypertrophy. It is based on an increase in the mass of the protoplasm of muscle fibers and the number of myofibrils contained in them, which leads to an increase in the diameter of each fiber. At the same time, the synthesis of nucleic acids and proteins is activated in the muscle and the content of ATP and CPA, as well as glycogen, increases. As a result, the strength and speed of contraction of the hypertrophied muscle increase.

An increase in the number of myofibrils during hypertrophy is facilitated mainly by static work, which requires a lot of stress (power load). Even short-term exercises carried out daily in an isometric mode are sufficient for an increase in the number of myofibrils. Dynamic muscle work, performed without much effort, does not lead to muscle hypertrophy, but can affect the entire body as a whole, increasing its resistance to adverse factors.

The opposite of working hypertrophy is muscle atrophy from inactivity. It develops in all cases when the muscles somehow lose the ability to do their normal work. This happens, for example, with prolonged immobilization of a limb in a plaster cast, a long stay of the patient in bed, transection of the tendon, etc. With muscle atrophy, the diameter of muscle fibers and the content of contractile proteins, glycogen, ATP and other substances important for contractile activity in them decreases sharply. With the resumption of normal muscle work, atrophy gradually disappears. A special type of muscle atrophy is observed during denervation of the muscle, i.e. after transection of her motor nerve.

Smooth muscles Functions of smooth muscles in different organs.

Smooth muscles in the body are located in the internal organs, blood vessels, and skin. Smooth muscles are capable of relatively slow movements and prolonged tonic contractions.

Relatively slow, often rhythmic contractions of the smooth muscles of the walls of hollow organs (stomach, intestines, ducts of the digestive glands, ureters, bladder, gallbladder, etc.) ensure the movement of contents. Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of hollow organs; shrinking them prevents the contents from escaping.

The smooth muscles of the walls of blood vessels, especially arteries and arterioles, are also in a state of constant tonic contraction. The tone of the muscle layer of the walls of the arteries regulates the size of their lumen and thus the level of blood pressure and blood supply to the organs. The tone and motor function of smooth muscles is regulated by impulses coming through the autonomic nerves, humoral influences.

Physiological features of smooth muscles. An important property of smooth muscle is its large plastic , those. the ability to maintain the length given by stretching without changing the stress. Skeletal muscle, on the other hand, shortens immediately after the load is removed. A smooth muscle remains stretched until, under the influence of some kind of irritation, its active contraction occurs. The property of plasticity is of great importance for the normal activity of hollow organs - thanks to it, the pressure inside a hollow organ changes relatively little with different degrees of its filling.

There are different types of smooth muscles. In the walls of most hollow organs there are muscle fibers 50-200 microns long and 4-8 microns in diameter, which are very closely adjacent to each other, and therefore, when viewed under a microscope, it seems that they are morphologically one. Electron microscopic examination shows, however, that they are separated from each other by intercellular gaps, the width of which can be equal to 600-1500 angstroms. Despite this, smooth muscle functions as a single entity. This is expressed in the fact that AP and slow waves of depolarization propagate freely from one fiber to another.

In some smooth muscles, for example, in the ciliary muscle of the eye, or the muscles of the iris, the fibers are located separately, and each has its own innervation. In most smooth muscles, motor nerve fibers are located on only a small number of fibers.

The resting potential of smooth muscle fibers with automaticity exhibits constant small fluctuations. Its value at intracellular assignment is 30-70 mV. The resting potential of smooth muscle fibers that do not have automaticity is stable and equal to 60-70 mV. In both cases, its value is less than the resting potential of the skeletal muscle. This is due to the fact that the membrane of smooth muscle fibers at rest is characterized by a relatively high permeability to Na ions. Action potentials in smooth muscles are also somewhat lower than in skeletal ones. The excess over the resting potential is no more than 10-20 mV.

The ionic mechanism of AP occurrence in smooth muscles is somewhat different from that in skeletal muscles. It has been established that the regenerative depolarization of the membrane, which underlies the action potential in a number of smooth muscles, is associated with an increase in the permeability of the membrane for Ca++ ions, rather than Na+.

Many smooth muscles are characterized by spontaneous, automatic activity. It is characterized by a slow decrease in the resting membrane potential, which, when a certain level is reached, is accompanied by the onset of AP.

Conduction of excitation along smooth muscle. In nerve and skeletal muscle fibers, excitation propagates through local electric currents that arise between the depolarized and neighboring resting sections of the cell membrane. The same mechanism is characteristic of smooth muscles. However, unlike in skeletal muscle, in smooth muscle an action potential originating in one fiber can propagate to adjacent fibers. This is due to the fact that in the membrane of smooth muscle cells in the area of ​​contacts with neighboring ones there are areas of relatively low resistance through which the current loops that have arisen in one fiber easily pass to the neighboring ones, causing depolarization of their membranes. In this respect, smooth muscle is similar to cardiac muscle. The only difference is that in the heart, the entire muscle is excited from one cell, while in smooth muscles, AP that has arisen in one area propagates only a certain distance from it, which depends on the strength of the applied stimulus.

Another essential feature of smooth muscles is that propagating AP occurs downward only if the applied stimulus simultaneously excites a certain minimum number of muscle cells. This "critical zone" has a diameter of about 100 microns, which corresponds to 20-30 parallel cells. The rate of excitation conduction in various smooth muscles ranges from 2 to 15 cm/sec. those. much less than in skeletal muscle.

As well as in skeletal muscles, in smooth action potentials they have a starting value for the start of the contractile process. The connection between excitation and contraction is also carried out here with the help of Ca ++. However, in smooth muscle fibers, the sarcoplasmic reticulum is poorly expressed; therefore, the leading role in the mechanism of contraction is assigned to those Ca ++ ions that penetrate into the muscle fiber during AP generation.

With a large force of a single irritation, smooth muscle contraction may occur. The latent period of its contraction is much longer than the skeletal period, reaching 0.25-1 sec. The duration of the contraction itself is also large - up to 1 minute. Relaxation is especially slow after contraction. The contraction wave propagates through the smooth muscles at the same speed as the excitation wave (2-15 cm/sec). But this slowness of contractile activity is combined with a great force of smooth muscle contraction. So, the muscles of the stomach of birds are capable of lifting 2 kg per 1 sq. mm. its cross section.

Due to the slowness of contraction, smooth muscle, even with rare rhythmic stimulation (10-12 per minute), easily passes into a long-term state of persistent contraction, resembling tetanus of skeletal muscles. However, the energy costs of such a reduction are very low.

The ability to automate smooth muscles is inherent in their muscle fibers and is regulated by nerve elements that are located in the walls of smooth muscle organs. The myogenic nature of automaticity has been proven by experiments on strips of muscles of the intestinal wall, freed from nerve elements. Smooth muscle responds to all external influences by changing the frequency of spontaneous rhythm, resulting in contraction or relaxation of the muscle. The effect of irritation of the smooth muscles of the intestine depends on the ratio between the frequency of stimulation and the natural frequency of spontaneous rhythm: with a low tone - rare spontaneous AP - the applied irritation increases the tone, with a high tone, relaxation occurs in response to irritation, since an excessive increase in impulses leads to the fact that each next impulse falls into the phase of refractoriness from the previous one.

Smooth muscle irritants. One of the important physiologically adequate stimuli of smooth muscles is their rapid and strong stretching. It causes depolarization of the muscle fiber membrane and the occurrence of propagating AP. As a result, the muscle contracts. A characteristic feature of smooth muscles is their high sensitivity to certain chemical stimuli, in particular, to acetylcholine, norepinephrine, adrenaline, histamine, serotonin, prostaglandins. The effects caused by the same chemical agent in different muscles and in their different states may be different. So, ACh excites the smooth muscles of most organs, but inhibits the muscles of blood vessels. Adrenaline relaxes the non-pregnant uterus but contracts the pregnant one. These differences are due to the fact that these agents react on the membrane with different chemical receptors (cholinergic receptors, alpha and beta adrenoreceptors), and as a result, they change the ion permeability and membrane potential of smooth muscle cells in different ways. In cases where the irritating agent causes membrane depolarization, excitation occurs, and, conversely, membrane hyperpolarization under the influence of a chemical agent leads to inhibition of activity and relaxation of smooth muscle.

Gradient Law

Stimuli are characterized not only by the strength and duration of action, but also by the rate of growth in time of the force of impact on the object, i.e. gradient.

The relationship between the steepness of the increase in the strength of irritation and the magnitude of excitation is determined Vgradient law:the reaction of a living system depends on the gradient of stimulation: the higher the steepness of the rise of the stimulus in time, the greater, to certain limits, the magnitude of the functional response. In general terms, the physiological foundations of the gradient law can be represented as follows.

A decrease in the steepness of the increase in the strength of the stimulus leads to an increase in the threshold of excitation, as a result of which, the response of the biosystem disappears altogether at a certain minimum steepness. This phenomenon is called accommodation.

17. The threshold of irritation is the minimum strength of the stimulus at which excitation occurs.

b) Reobase is the minimum strength of the stimulus that causes excitation during its action for an indefinitely long time. In practice, threshold and rheobase have the same meaning. The lower the threshold of irritation or less reobase, the higher the excitability of the tissue.

c) Useful time - the minimum time of action of the stimulus with a force of one rheobase during which excitation occurs.

d) Chronaxia - this is the minimum time of action of the stimulus with a force of two rheobases, necessary for the onset of excitation.

18. PHYSIOLOGY OF MUSCLE TISSUE

Moving the body in space, maintaining a certain posture, the work of the heart and blood vessels and the digestive tract in humans and vertebrates is carried out by two main types of muscles: striated (skeletal, cardiac) and smooth, which differ from each other in cellular and tissue organization, innervation and in a certain degree mechanisms of functioning. At the same time, the molecular mechanisms of muscle contraction between these types of muscles have much in common.

Functions and properties of skeletal muscles

Skeletal muscles are an integral part of the human musculoskeletal system. In this case, the muscles perform the following functions:

1) provide a certain posture of the human body;

2) move the body in space;

3) move separate parts of the body relative to each other;

4) are a source of heat, performing a thermoregulatory function.

Skeletal muscle has the following essential properties:

1) excitability - the ability to respond to the action of the stimulus by changing the ionic conductivity and membrane potential. Under natural conditions, this stimulus is the mediator acetylcholine, which is released in the presynaptic endings of the axons of motor neurons. In laboratory conditions, electrical muscle stimulation is often used. During electrical stimulation of the muscle, nerve fibers are initially excited, which secrete acetylcholine, i.e., in this case, indirect muscle irritation is observed. This is due to the fact that the excitability of nerve fibers is higher than muscle fibers. For direct stimulation of the muscle, it is necessary to use muscle relaxants - substances that block the transmission of a nerve impulse through the neuromuscular synapse;

2) conductivity - the ability to conduct an action potential along and deep into the muscle fiber along the T-system;

3) contractility - the ability to shorten or develop tension when excited;

4) elasticity - the ability to develop stress when stretched.

4. Structural organization of the muscle fiber

A muscle fiber is a multinuclear structure surrounded by a membrane and containing a specialized contractile apparatus - myofibrils. In addition, the most important components of the muscle fiber are mitochondria, a system of longitudinal tubules - the sarcoplasmic reticulum (reticulum) and a system of transverse tubules - the T-system. The functional unit of the contractile apparatus of the muscle cell is the sarcomere, the myofibril consists of sarcomeres.

Mechanisms of muscle fiber contraction

In resting muscle fibers, in the absence of motor neuron firing, transverse myosin bridges are not attached to actin myofilaments. Tropomyosin is located in such a way that it blocks actin sites that can interact with myosin cross-bridges. Troponin inhibits myosin - ATP-ase activity and therefore ATP is not split. Muscle fibers are in a relaxed state.

When the muscle contracts, the length of the A-disks does not change, the J-disks shorten, and the H-zone of the A-disks can disappear. These data were the basis for creating a theory explaining muscle contraction by the sliding mechanism (slip theory) thin actin myofilaments along thick myosin. As a result, myosin myofilaments are drawn between the surrounding actin myofilaments. This leads to a shortening of each sarcomere, and hence the entire muscle fiber.

Molecular mechanism of contraction muscle fiber consists in the fact that the action potential arising in the region of the end plate propagates through the system of transverse tubules deep into the fiber, causes depolarization of the membranes of the cisterns of the sarcoplasmic reticulum and the release of calcium ions from them. Free calcium ions in the interfibrillar space trigger the contraction process. The set of processes that cause the propagation of the action potential deep into the muscle fiber, the release of calcium ions from the sarcoplasmic reticulum, the interaction of contractile proteins and the shortening of the muscle fiber are called "electromechanical interface". The time sequence between the appearance of the action potential of the muscle fiber, the flow of calcium ions to the myofibrils and the development of fiber contraction is shown in Figure 4.4.

When the concentration of Ca 2+ ions in the intermyofibrillar space is below 10″, tropomyosin is located in such a way that it blocks the attachment of transverse myosin bridges to actin filaments. The cross bridges of myosin do not interact with actin filaments. There is no movement of actin and myosin filaments relative to each other. Therefore, the muscle fiber is in a relaxed state. When the fiber is excited, Ca 2+ leaves the cisterns of the sarcoplasmic reticulum and, consequently, its concentration near the myofibrils increases. Under the influence of activating Ca 2+ ions, the troponin molecule changes its shape in such a way that it pushes tropomyosin into the groove between two actin filaments, thereby freeing up sites for attaching myosin cross-bridges to actin. As a result, cross bridges attach to actin filaments. Since the myosin heads make "stroke" movements towards the center of the sarcomere, actin myofilaments are "pulled" into the gaps between the thick myosin filaments and the muscle is shortened.

The muscular system has a number of physical and physiological properties. The main physical properties include:

Birefringence(anisotropy). Formed by disks A, it consists in the fact that in ordinary light the anisotropic fates look dark, and in polarized light they look bright if the light is transmitted in the longitudinal direction, and dark if it passes in the transverse direction. The alternation of anisotropic and isotropic discs creates a transverse striation to the muscles.

Extensibility. Associated with the presence in the muscles of the elastic component of the membrane, strips, sarcoplasmic reticulum, ...).

Elasticity. This property is associated with extensibility and lies in the fact that after stretching the muscle returns to its original position.

Elasticity. This property of the muscle is associated with its compression. After contraction, the muscle is able to return to its original state.

Plastic. It lies in the fact that the muscle is able to retain the artificial shape given to it for some time. The plastic properties of skeletal muscles are very weakly expressed, they are more inherent in smooth muscles. In some diseases (catatonic form of schizophrenia), the plastic properties of skeletal muscles become pronounced.

The physiological properties of muscles include - excitability, conductivity and contractility.

Muscle work. Since the ends of the muscle are attached to the bones, the points of its origin and attachment approach each other during contraction, while the muscles themselves perform certain work. Thus, the human body or its parts, when the corresponding muscles contract, change their position, move, overcome the resistance of gravity or, conversely, yield to this force. In other cases, when the muscles contract, the body is held in a certain position without movement. Based on this, there are overcoming, yielding and holding the work of the muscles.

Overcoming muscle work performed in the event that the force of muscle contraction changes the position of a body part, limb or its link, with or without a load, overcoming the resistance force.

Yielding called the work in which the force of the muscle is inferior to the action of the gravity of the body part (limb) and the load it holds. The muscle works, but it does not shorten at the same time, but, on the contrary, lengthens; for example, when it is impossible to lift or hold on weight an object that has a large mass. With a great effort of the muscles, it is necessary to lower this body to the floor or to another surface.

Retention work is performed if the force of muscle contractions holds the body or load in a certain position without moving in space. For example, a person stands or sits without moving, or holds a load in the same position. The strength of muscle contractions balances the mass of the body or load. In this case, the muscles contract without changing their length (isometric contraction).

Overcoming and yielding work, when the force of muscle contractions moves the body or its parts in space, can be considered as dynamic work. Holding work, in which the movement of the whole body or part of the body does not occur, is a static work.

Bones connected by joints act as levers when muscles contract. In biomechanics, a lever of the first kind is distinguished, when the points of resistance and application of muscle force are on opposite sides of the fulcrum, and a lever of the second kind, in which both forces are applied on the same side of the fulcrum, at different distances from it.

Lever of the first kind two shoulders is called "balance lever". The fulcrum is located between the point of application of force (the force of muscle contraction) and the point of resistance (gravity, organ mass). An example of such a lever is the connection of the spine with the skull. Equilibrium is achieved under the condition that the torque of the applied force (the product of the force acting on the occipital bone and the length of the arm, which is equal to the distance from the fulcrum to the point of application of the force) is equal to the torque of gravity (the product of the force of gravity and the length of the arm, equal to the distance from fulcrum to the point of application of gravity).

Lever of the second kind one-shouldered. In biomechanics (as opposed to mechanics) it comes in two forms. The type of such a lever depends on the location of the point of application of force and the point of action of gravity, which in both cases are on the same side of the fulcrum. The first type of lever of the second kind (power lever) takes place if the shoulder of application of muscle force is longer than the shoulder of resistance (gravity). Considering the foot as an example, it can be seen that the heads of the metatarsal bones serve as the fulcrum (axis of rotation), and the calcaneus is the point of application of muscle force (the triceps muscle of the lower leg). The point of resistance (weight of the body) falls on the place of articulation of the bones of the lower leg with the foot (ankle joint). In this lever, there is a gain in strength (the arm of the force is longer) and a loss in the speed of movement of the point of resistance (its arm is shorter). The second type of single-arm lever (lever speed) the muscle force application arm is shorter than the resistance arm, where the opposing force, gravity, is applied. To overcome the force of gravity, the point of application of which is at a considerable distance from the point of rotation in the elbow joint (fulcrum), a significantly greater force of the flexor muscles attached near the elbow joint (at the point of application of force) is required. In this case, there is a gain in the speed and range of motion of a longer lever (point of resistance) and a loss in the force acting at the point of application of this force.

muscle strength

Force is the product of a mass times the acceleration imparted to it. When performing some labor and sports movements, the greatest muscle strength is achieved either due to the greatest increase in the mass of the load being lifted or moved, or due to an increase in acceleration, i.e., changing the speed to a maximum value. In the first case, the tension of the muscle increases, and in the second, the speed of its contraction. Human movements usually occur with a combination of muscle contraction and muscle tension. Therefore, as the contraction speed increases, the voltage also increases proportionally. The greater the mass of the load, the less the acceleration reported to it by a person.

The maximum strength of a muscle is measured by determining the mass of the maximum load that it can displace. Under such isometric conditions, the muscle almost does not contract, and its tension is the limit. Therefore, the degree of muscle tension is an expression of its strength.

Power movements are characterized by maximum stress with an increase in the mass of the load and a constant speed of its movement.

The strength of a muscle does not depend on its length, but depends mainly on its thickness, on the physiological diameter, that is, on the number of muscle fibers per largest cross-sectional area. The physiological diameter is the cross-sectional area of ​​​​all muscle fibers. In feathery and semi-feathery muscles, this diameter is larger than the anatomical one. In fusiform and parallel muscles, the physiological diameter coincides with the anatomical one. Therefore, the strongest feathery muscles, then semi-feathery, fusiform, and finally the weakest muscles with a parallel course of fibers. The strength of a muscle also depends on its functional state, on the conditions of its work, on the limiting frequency and magnitude, on the spatial and temporal summation of nerve impulses flowing to it, causing it to contract, on the number of functioning neuromotor units, and on impulses that regulate metabolism. Muscle strength increases with exercise and decreases with fasting and fatigue. Initially, it increases with age, and then decreases with age.

The strength of the muscle at its maximum tension, developed at its greatest excitation and the most favorable length before the start of its tension, is called absolute.

Absolute muscle strength is defined in kilograms or newtons (N). The maximum muscle tension in a person is caused by volitional effort.

Relative muscle strength is calculated as follows. Having determined the absolute force in kilograms or newtons, divide it by the number of square centimeters of the cross section of the muscle. This allows you to compare the strength of different muscles of the same organism, the strength of the same muscles of different organisms, as well as changes in the strength of the same muscle of a given organism depending on changes in its functional state. The relative strength of the skeletal muscle of the frog is 2-3 kg, the extensor of the human neck is 9 kg, the masticatory muscle is 10 kg, the biceps of the shoulder is 11 kg, the triceps of the shoulder is 17 kg.

Dynamometry is a method of measuring the force of contraction of various muscle groups.
For dynamometers, there are various types of dynamometers. The most common spring dynamometer (Fig. 1).
The subject squeezes it with an outstretched hand. The compression force is indicated by an arrow on a special scale. Another modification is the Sternberg dynamometer (Fig. 2), which has two wide parallel handles, which the subject also squeezes with a brush.
There are mercury dynamometers (Fig. 3), in which the pressure force on the sensor is determined using a mercury manometer.
A variation of dynamometry - dynamography - is a method that allows you to register the force of muscle contraction graphically in the form of a series of curves. This method reflects the dynamics of long-term muscular effort of a certain muscle group.
Dynamometry is used in anthropology, anthropometry, neuropathology, balneology, etc.

20. Muscle fatigue

Fatigue is a temporary decrease or loss of efficiency, i.e. the result of previous work. Muscle fatigue in the body under conditions of blood circulation depends not only on the amount of long-term work done by it, but on the number of excitation waves that come to it, causing its contraction. With the same frequency of stimulation and other equal conditions, fatigue appears earlier with a greater load on the muscle. With the same load and other conditions being equal, fatigue sets in earlier with more frequent stimuli. At the beginning of work, the height of contractions increases, and then the signs of developing fatigue are a gradual decrease in the height of contractions, an increase in their duration, and an increase in contracture. The development of fatigue depends on changes in metabolism, blood circulation, temperature and other conditions. The higher the metabolism and better blood circulation, the later fatigue occurs. It occurs much earlier when the muscle is contracting, being stretched by a load during isometric contraction, and later when it is contracting without a load, and therefore without tension.

If the muscle is brought to complete fatigue by irritation with an electric current, then after a change in the direction of the current, its performance is immediately restored. This recovery is explained by a change in the state of muscle proteins and shifts of ions at the current poles. An isolated muscle reduces its work or even stops contracting when the glycogen store is half the original amount. These facts do not support the exhaustion theory (Schiff, 1868), which explains muscle fatigue by the use of substances that release energy for its work. However, glycogen reserves in the human body are limited and amount to 300-400 g. With very intensive work, they are consumed in 1.5-2 hours, which leads to such a decrease in blood sugar, in which work becomes impossible. The introduction of sugar into the body restores its performance.

The theory of muscle poisoning during fatigue with a special poison accumulating in it - kenotoxin (Weihardt, 1904) turned out to be unfounded. But there is evidence that fatigue is sometimes associated with poisoning of excitatory structures by metabolic products, mainly phosphoric and lactic acids at the time of their formation. Residual products of metabolism, as it were, clog the body and cause fatigue - the theory of clogging (Pfluger, 1872).

The accumulation of phosphoric and lactic acids reduces muscle performance. An isolated muscle fiber, unlike a whole muscle, gets tired much later with the same number of irritating impulses. This is due to the fact that the end products of metabolism are removed from it faster. In a trained muscle, due to the large acceleration of the analysis and synthesis of substances that ensure its work, fatigue occurs later. After washing the blood vessels of an isolated muscle, brought to complete fatigue, therefore, after removing some of the residual metabolic products from it, it again begins to contract despite the fact that the supply of carbohydrates and oxygen has not been restored. These facts prove that the residual decay products of substances formed in the working muscle are one of the causes of its fatigue.

There is also a theory of suffocation (M. Vervorn, 1903), which attributes the main role in fatigue to a lack of oxygen. It is known that work can continue for tens of minutes and even hours without fatigue, when the level of oxygen consumption is below the limit of its intake, possible for the worker (true steady state). When oxygen consumption reaches a maximum, it can be at a constant level, but does not provide the body's need for oxygen (apparent, or southern, stable state) and work in this case can last no more than 10-40 minutes.

Fatigue is a normal physiological process that leads to the cessation of work. During breaks in work, the working capacity of the muscles is restored. Therefore, the validity of the participation of petty and phosphoric acids in the onset of fatigue does not allow us to draw an absurd conclusion that labor is harmful, since it allegedly leads to poisoning. It is impossible to put an equal sign between the fatigue of an isolated muscle and the fatigue of the whole organism, in which the onset of fatigue depends on a change in the functions of the nervous system and endocrine glands and on a change in the regulation of the central nervous system of metabolism, blood circulation and respiration. The development of fatigue depends on a decrease in the efficiency of the circulatory system, especially the heart, and the respiratory system.

Under normal conditions, during prolonged physical work, muscle excitation and contraction are two interrelated processes that occur when oxygen is consumed, since they are carried out due to very complex chemical processes, culminating in the oxidation of residual metabolic products. Muscle performance after fatigue is restored as a result of the oxidation of these products. Therefore, oxygen consumption during muscular work increases significantly. If there is not enough oxygen, then with intensive muscular work there is a lack of oxygen - oxygen debt. In conditions of oxygen deficiency during work, the functions of the nervous system decrease, which is the main cause of fatigue. Oxygen debt is repaid due to increased blood circulation and breathing, not only during work, but also after its completion. This repayment of the oxygen debt ends only after the complete oxidation of the residual metabolic products formed during work, and the complete completion of the recovery processes.

In the neuromuscular preparation, fatigue develops in the region of the myoneural junction. The basic theory of fatigue, attributing the main role to its development in the central nervous system of the whole organism, was formulated by I, M, Sechenov (1902).

There is numerous evidence of the leading role of the central nervous system in the development of fatigue. Tired occurs under the action of conditioned stimuli. With fatigue, the inhibition of conditioned and unconditioned reflexes increases. The development of fatigue is affected by the influx of afferent impulses; in the brain, emotions. Conscious, voluntary muscle activity is more tiring than involuntary, automatic. Essential for the onset of fatigue is the functional state of the brain, which changes: with hypoxemia, hypoglycemia, hyperthermia, accumulation of metabolites in the blood, shifts in the functions of internal organs, especially the cardiovascular and respiratory systems.

The law of average loads and average contraction rates is of great importance for work and sports.

Physical activity causes a complex of somatovegetative changes in the body: heart rate, stroke volume of the heart, blood pressure, O 2 consumption by the body, respiratory rate, etc. increase. With moderate physical exertion, metabolism follows the aerobic pathway.

Hard work is accompanied by the activation of anaerobic oxidation, as a result of which lactic acid accumulates in the muscles, muscle fatigue develops.

Fatigue is a physiological state of a person that occurs as a result of hard or prolonged work, which is expressed in a temporary decrease in performance.

Muscular (physical) and central (nervous-psychic) ​​fatigue are usually combined.

Fatigue is characterized by a decrease in muscle strength and endurance, impaired coordination of movements, weakening of working memory, attention, and a decrease in the speed of information processing. It is assumed that the causes of fatigue may be the depletion of the glycogen depot and the weakening of the ATP resynthesis process, the accumulation of acidic metabolic products, the depletion of the calcium depot, and the fatigue of the nerve centers that regulate contractions of individual muscle groups. Subjectively, fatigue is felt in the form of fatigue and the need for sleep.

Rest is a state of rest or a specially organized type of activity that reduces fatigue and gradually returns body functions to normal.

THEM. Sechenov found that the work of some muscle groups of the limbs eliminates the fatigue of other groups associated with their work. This provision formed the basis for the definition of 2 types of recreation: passive and active. The first of them provides for relative peace, the second - the performance of a type of work that is significantly different from the work usually performed.

The main morpho-functional element of the neuromuscular apparatus of skeletal muscles is the motor unit (MU). It includes the motor neuron of the spinal cord with muscle fibers innervated by its axon. Inside the muscle, this axon forms several terminal branches. Each such branch forms a contact - a neuromuscular synapse on a separate muscle fiber. Nerve impulses coming from a motor neuron cause contractions of a certain group of muscle fibers. The motor units of small muscles that perform fine movements (muscles of the eye, hand) contain a small amount of muscle fibers. In large ones, there are hundreds of times more. All DUs, depending on their functional features, are divided into 3 groups:

I. Slow tireless. They are formed by "red" muscle fibers, in which there are fewer myofibrils. The rate of contraction and strength of these fibers are relatively small, but they are not very fatiguable. Therefore, they are referred to as tonic. The regulation of contractions of such fibers is carried out by a small number of motor neurons, the axons of which have few terminal branches. An example is the soleus muscle.

IIB. Fast, easily fatigued. Muscle fibers contain many myofibrils and are called "white". Contract quickly and develop great strength, but tire quickly. Therefore, they are called phase. The motor neurons of these DUs are the largest, have a thick axon with numerous terminal branches. They generate nerve impulses of high frequency. Muscles of the eye.

IIA. Fast, fatigue resistant. They occupy an intermediate position.

22Mechanism of muscle contraction

Skeletal muscle is a complex system that converts chemical energy into mechanical work and heat. At present, the molecular mechanisms of this transformation are well studied.

Structural organization of the muscle fiber. A muscle fiber is a multinuclear structure surrounded by a membrane and containing a specialized contractile apparatus - myofibrils. In addition, the most important components of the muscle fiber are mitochondria, a system of longitudinal tubules - the sarcoplasmic reticulum (reticulum) and a system of transverse tubules - the T-system. The functional unit of the contractile apparatus of the muscle cell is the sarcomere (Fig. 2.20, A); The myofibril is made up of sarcomeres. Sarcomeres are separated from each other by Z-plates. Sarcomeres in the myofibril are arranged in series, so the contraction of sarcomeres causes contraction of the myofibril and an overall shortening of the muscle fiber.

The study of the structure of muscle fibers in a light microscope made it possible to reveal their transverse striation. Electron microscopic studies have shown that the transverse striation is due to the special organization of the contractile proteins of myofibrils - actin (molecular weight 42,000) and myosin (molecular weight about 500,000). Actin filaments are represented by a double thread twisted into a double helix with a pitch of about 36.5 nm. These filaments, 1 μm long and 6–8 nm in diameter, numbering about 2000, are attached to the Z-plate at one end. In the longitudinal grooves of the actin helix are filamentous molecules of the protein tropomyosin. With a step of 40 nm, a molecule of another protein, troponin, is attached to the tropomyosin molecule. Troponin and tropomyosin play an important role in the mechanisms of interaction between actin and myosin. In the middle of the sarcomere, between the actin filaments, there are thick myosin filaments about 1.6 µm long. In a polarizing microscope, this region is visible as a dark stripe (due to birefringence) - an anisotropic A-disk. A lighter H-stripe is visible in its center. There are no actin filaments in it at rest. On both sides of the A-disk, light isotropic stripes are visible - I-discs formed by actin filaments. At rest, the actin and myosin filaments slightly overlap each other so that the total length of the sarcomere is about 2.5 μm. Electron microscopy in the center of the H-band revealed an M-line, a structure that holds myosin filaments. On a cross section of a muscle fiber, you can see the hexagonal organization of the myofilament: each myosin filament is surrounded by six actin filaments (Fig. 2.20, B).

Electron microscopy shows that protrusions called transverse bridges are found on the sides of the myosin filament. They are oriented with respect to the axis of the myosin filament at an angle of 120°. According to modern concepts, the transverse bridge consists of a head and a neck. The head acquires a pronounced ATPase activity upon binding to actin. The neck has elastic properties and is a swivel, so the head of the cross bridge can rotate around its axis.

The use of microelectrode technique in combination with interference microscopy made it possible to establish that the application of electrical stimulation to the area of ​​the Z-plate leads to a contraction of the sarcomere, while the size of the disc A zone does not change, and the size of the H and I bands decreases. These observations indicated that the length of myosin filaments does not change. Similar results were obtained during muscle stretching - the proper length of actin and myosin filaments did not change. As a result of these experiments, it turned out that the region of mutual overlap of actin and myosin filaments changed. These facts allowed N. Huxley and A. Huxley to independently propose the theory of filament sliding to explain the mechanism of muscle contraction. According to this theory, during contraction, a decrease in the size of the sarcomere occurs due to the active movement of thin actin filaments relative to thick myosin filaments. At present, many details of this mechanism have been elucidated, and the theory has received experimental confirmation.

mechanism of muscle contraction. In the process of muscle fiber contraction, the following transformations occur in it:

A. Electrochemical conversion:

1. Generation of PD.

2. Propagation of PD along the T-system.

3. Electrical stimulation of the contact zone of the T-system and the sarcoplasmic reticulum, activation of enzymes, the formation of inositol triphosphate, an increase in the intracellular concentration of Ca2+ ions.

B. Chemomechanical transformation:

4. Interaction of Ca2+ ions with troponin, release of active centers on actin filaments.

5. Interaction of the myosin head with actin, head rotation and development of elastic traction.

6. Sliding of actin and myosin filaments relative to each other, a decrease in the size of the sarcomere, the development of tension or shortening of the muscle fiber.

The transfer of excitation from the motor neuron to the muscle fiber occurs with the help of the mediator acetylcholine (ACh). The interaction of ACh with the cholinergic receptor of the end plate leads to the activation of ACh-sensitive channels and the appearance of an end plate potential, which can reach 60 mV. In this case, the area of ​​the end plate becomes a source of irritating current for the muscle fiber membrane, and in the areas of the cell membrane adjacent to the end plate, AP occurs, which propagates in both directions at a speed of approximately 3-5 m/s at a temperature of 36 oC. Thus, AP generation is the first stage of muscle contraction.

The second stage is the spread of AP inside the muscle fiber along the transverse system of tubules, which serves as a link between the surface membrane and the contractile apparatus of the muscle fiber. The T-system is in close contact with the terminal cisterns of the sarcoplasmic reticulum of two adjacent sarcomeres. Electrical stimulation of the contact site leads to the activation of enzymes located at the site of contact and the formation of inositol triphosphate. Inositol triphosphate activates calcium channels in the membranes of the terminal cisterns, which leads to the release of Ca2+ ions from the cisterns and an increase in the intracellular Ca2+ concentration from 107 to 105 M. The totality of processes leading to an increase in the intracellular Ca2+ concentration is the essence of the third stage of muscle contraction. Thus, at the first stages, the electrical signal of AP is converted into a chemical signal - an increase in the intracellular Ca2+ concentration, i.e., an electrochemical transformation.

With an increase in the intracellular concentration of Ca2+ ions, tropomyosin shifts into the groove between the actin filaments, while the actin filaments open areas with which myosin cross-bridges can interact. This displacement of tropomyosin is due to a change in the conformation of the troponin protein molecule upon Ca2+ binding. Therefore, the participation of Ca2+ ions in the mechanism of interaction between actin and myosin is mediated through troponin and tropomyosin.

The essential role of calcium in the mechanism of muscle contraction was proved in experiments with the use of the protein aequorin, which, when interacting with calcium, emits light. After injection of aequorin, the muscle fiber was subjected to electrical stimulation and simultaneously measured muscle tension in the isometric mode and aequorin luminescence. Both curves were fully correlated with each other (Fig. 2.21). Thus, the fourth stage of electromechanical coupling is the interaction of calcium with troponin.

The next, fifth, stage of electromechanical coupling is the attachment of the head of the cross bridge to the actin filament to the first of several successively located stable centers. In this case, the myosin head rotates around its axis, since it has several active centers that sequentially interact with the corresponding centers on the actin filament. The rotation of the head leads to an increase in the elastic elastic traction of the neck of the transverse bridge and an increase in stress. At each specific moment in the process of contraction development, one part of the heads of the transverse bridges is in connection with the actin filament, the other is free, i.e., there is a sequence of their interaction with the actin filament. This ensures the smoothness of the reduction process. At the fourth and fifth stages, chemomechanical transformation takes place.

The successive reaction of connecting and disconnecting the heads of the transverse bridges with the actin filament leads to the sliding of thin and thick filaments relative to each other and a decrease in the size of the sarcomere and the total length of the muscle, which is the sixth stage. The totality of the described processes is the essence of the theory of sliding threads

It was originally believed that Ca2+ ions serve as a cofactor for the ATPase activity of myosin. Further research disproved this assumption. In a resting muscle, actin and myosin have practically no ATPase activity. Attachment of the myosin head to actin causes the head to acquire ATPase activity.

Hydrolysis of ATP in the ATPase center of the myosin head is accompanied by a change in the conformation of the latter and its transfer to a new, high-energy state. The reattachment of the myosin head to a new center on the actin filament again leads to the rotation of the head, which is provided by the energy stored in it. In each cycle of connection and disconnection of the myosin head with actin, one ATP molecule is split per bridge. The speed of rotation is determined by the rate of splitting of ATP. Obviously, fast phasic fibers consume significantly more ATP per unit time and store less chemical energy during tonic loading than slow fibers. Thus, in the process of chemomechanical transformation, ATP ensures the separation of the myosin head and the actin filament and provides energy for further interaction of the myosin head with another section of the actin filament. These reactions are possible at calcium concentrations above 106M.

The described mechanisms of shortening of the muscle fiber suggest that relaxation primarily requires a decrease in the concentration of Ca2+ ions. It has been experimentally proven that the sarcoplasmic reticulum has a special mechanism - a calcium pump, which actively returns calcium to the cisterns. The activation of the calcium pump is carried out by inorganic phosphate, which is formed during the hydrolysis of ATP, and the energy supply of the calcium pump is also due to the energy generated during the hydrolysis of ATP. Thus, ATP is the second most important factor, absolutely necessary for the relaxation process. For some time after death, the muscles remain soft due to the cessation of the tonic influence of motor neurons (see Chapter 4). Then the ATP concentration decreases below a critical level, and the possibility of separation of the myosin head from the actin filament disappears. There is a phenomenon of rigor mortis with severe rigidity of skeletal muscles.

Muscle modes

The mechanical work (A) performed by the muscle is measured by the product of the lifted weight (P) and the distance (h): A = P * h kgm. When recording the work of an isolated frog muscle, it can be seen that the greater the load, the lower the height to which the muscle lifts it. There are 3 modes of muscle work: isotonic, isometric and auxotonic.

An isotonic regime (a regime of constant muscle tone) is observed when there is no load on the muscle, when the muscle is fixed at one end and contracts freely. The voltage in it does not change. This occurs when an isolated frog muscle, fixed at one end on a tripod, is stimulated. Since under these conditions P = 0, the mechanical work of the muscle is also zero (A = 0). In this mode, only one muscle works in the human body - the muscle of the tongue. (In modern literature, the term isotonic regime is also found in relation to such contraction of a muscle with a load, in which, as the length of the muscle changes, its tension remains unchanged, but in this case, the mechanical work of the muscle is not equal to a bullet, i.e., it does external work) .

Isometric mode (constant muscle length mode) is characterized by muscle tension in conditions when it is fixed at both ends or when the muscle cannot lift too much load. In this case, h \u003d 0 and, accordingly, mechanical work is also equal to zero (A \u003d 0). This mode is observed while maintaining a given posture and when performing static work. In this case, the processes of appearance and destruction of bridges between actin and myosin still occur in the muscle fiber, i.e., energy is spent on these processes, but there is no mechanical reaction of movement of actin filaments along myosin. The physiological characteristic of such work is to assess the magnitude of the load and the duration of work.

Auxotonic mode (mixed mode) is characterized by a change in the length and tone of the muscle, during the contraction of which the load is moved. In this case, the mechanical work of the muscle is performed (A \u003d P? h). This mode is manifested when performing dynamic muscle work even in the absence of an external load, since the muscles overcome the force of gravity acting on the human body. There are 2 varieties of this mode of muscle work: overcoming (concentric) and yielding (eccentric) mode.

Vertebrates and humans have three types of muscles: the striated muscles of the skeleton, the striated muscle of the heart - the myocardium and smooth muscles, forming the walls of hollow internal organs and blood vessels.

The anatomical and functional unit of skeletal muscle is neuromotor unit- a motor neuron and the group of muscle fibers innervated by it. The impulses sent by the motor neuron activate all the muscle fibers that form it.

Skeletal muscles are made up of a large number of muscle fibers. The fiber of the striated muscle has an elongated shape, its diameter is from 10 to 100 microns, the length of the fiber is from several centimeters to 10-12 cm. The muscle cell is surrounded by a thin membrane - the sarcolemma, contains sarcoplasm (protoplasm) and numerous nuclei. The contractile part of the muscle fiber is long muscle filaments - myofibrils, consisting mainly of actin, passing inside the fiber from one end to the other, having a transverse striation. Myosin in smooth muscle cells is in a dispersed state, but contains a lot of protein that plays an important role in maintaining a long tonic contraction.

During the period of relative rest, the skeletal muscles do not completely relax and retain a moderate degree of tension, i.e. muscle tone.

The main functions of muscle tissue:

motor - ensuring movement

static - ensuring fixation, including in a certain position

receptor - in the muscles there are receptors that allow you to perceive your own movements

storage - water and some nutrients are stored in the muscles.

Physiological properties of skeletal muscles:

Excitability. Lower than the excitability of the nervous tissue. Excitation spreads along the muscle fiber.

Conductivity. Less conduction of the nervous tissue.

Refractory period muscle tissue is more durable than nervous tissue.

Lability muscle tissue is much lower than nervous tissue.

Contractility- the ability of a muscle fiber to change its length and degree of tension in response to stimulation of a threshold force.

With isotonic contraction, the length of the muscle fiber changes without changing the tone. With isometric contraction, the tension of the muscle fiber increases without changing its length.

Depending on the conditions of stimulation and the functional state of the muscle, a single, continuous (tetanic) contraction or contracture of the muscle may occur.

Single muscle contraction. When a muscle is irritated by a single current pulse, a single muscle contraction occurs.

The amplitude of a single muscle contraction depends on the number of myofibrils contracted at that moment. The excitability of individual groups of fibers is different, so the threshold current strength causes a contraction of only the most excitable muscle fibers. The amplitude of such a reduction is minimal. With an increase in the strength of the irritating current, less excitable groups of muscle fibers are also involved in the excitation process; the amplitude of contractions is summed up and grows until there are no fibers left in the muscle that are not covered by the excitation process. In this case, the maximum amplitude of the contraction is recorded, which does not increase, despite a further increase in the strength of the irritating current.

tetanic contraction. Under natural conditions, muscle fibers receive not single, but a series of nerve impulses, to which the muscle responds with a prolonged, tetanic contraction, or tetanus. Only skeletal muscles are capable of tetanic contraction. Smooth muscle and striated muscle of the heart are not capable of tetanic contraction due to the long refractory period.

Tetanus results from the summation of single muscle contractions. For tetanus to occur, the action of repeated stimuli (or nerve impulses) on the muscle is necessary even before its single contraction ends.

If the irritating impulses are close and each of them falls at the moment when the muscle has just begun to relax, but has not yet had time to fully relax, then a serrated type of contraction occurs (serrated tetanus).

If the irritating impulses are so close that each subsequent one falls at a time when the muscle has not yet had time to move to relaxation from the previous irritation, that is, it occurs at the height of its contraction, then a long continuous contraction occurs, called a smooth tetanus.

smooth tetanus- the normal working state of the skeletal muscles is determined by the receipt of nerve impulses from the central nervous system with a frequency of 40-50 per 1 s.

Serrated tetanus occurs at a frequency of nerve impulses up to 30 per 1 s. If a muscle receives 10-20 nerve impulses per second, then it is in a state muscle tone, i.e. moderate tension.

Muscle fatigue. With prolonged rhythmic stimulation, fatigue develops in the muscle. Its signs are a decrease in the amplitude of contractions, an increase in their latent periods, a lengthening of the relaxation phase, and, finally, the absence of contractions with continued irritation.

Another type of prolonged muscle contraction is contracture. It continues even when the stimulus is removed. Muscle contracture occurs when there is a metabolic disorder or a change in the properties of the contractile proteins of muscle tissue. The causes of contracture can be poisoning with certain poisons and drugs, metabolic disorders, fever and other factors that lead to irreversible changes in muscle tissue proteins.

Physiological features of smooth muscles.

Smooth muscles form the walls (muscle layer) of internal organs and blood vessels. There is no transverse striation in myofibrils of smooth muscles. This is due to the chaotic arrangement of contractile proteins. Smooth muscle fibers are relatively shorter.

Smooth muscles less excitable than striated ones. Excitation propagates along them at a low speed - 2-15 cm / s. Excitation in smooth muscles can be transmitted from one fiber to another, in contrast to nerve fibers and striated muscle fibers.

The contraction of smooth muscles occurs more slowly and for a long time.

The refractory period in smooth muscle is longer than in skeletal muscle.

An important property of smooth muscle is its large plastic, i.e. the ability to maintain the length given by stretching without changing the stress. This property is essential, since some organs of the abdominal cavity (uterus, bladder, gallbladder) are sometimes significantly stretched.

A characteristic feature of smooth muscles is their ability for automatic activity, which is provided by nerve elements embedded in the walls of smooth muscle organs.

An adequate stimulus for smooth muscles is their rapid and strong stretching, which is of great importance for the functioning of many smooth muscle organs (ureter, intestines and other hollow organs)

A feature of smooth muscles is also their high sensitivity to certain biologically active substances(acetylcholine, adrenaline, norepinephrine, serotonin, etc.).

Smooth muscles are innervated by sympathetic and parasympathetic autonomic nerves, which, as a rule, have the opposite effect on their functional state.

Basic properties of the heart muscle.

The wall of the heart consists of 3 layers. The middle layer (myocardium) consists of the striated muscle. The cardiac muscle, like skeletal muscles, has the property of excitability, the ability to conduct excitation and contractility. The physiological features of the heart muscle include an extended refractory period and automatism.

Excitability of the heart muscle. Cardiac muscle is less excitable than skeletal muscle. For the occurrence of excitation in the heart muscle, a stronger stimulus is needed than for the skeletal muscle.

Conductivity. Excitation along the fibers of the heart muscle is carried out at a lower speed than along the fibers of the skeletal muscle.

Contractility. The reaction of the heart muscle does not depend on the strength of the applied stimuli. The cardiac muscle contracts as much as possible both to the threshold and to the stronger irritation.

Refractory period. The heart, unlike other excitable tissues, has a significantly pronounced and prolonged refractory period. It is characterized by a sharp decrease in tissue excitability during the period of its activity. Due to this, the heart muscle is not capable of tetanic (long-term) contraction and performs its work as a single muscle contraction.

Automatism of the heart. Outside the body, under certain conditions, the heart is able to contract and relax, maintaining the correct rhythm. The ability of the heart to contract rhythmically under the influence of impulses that arise in itself is called automatism.

Electromyography (from the Greek mys, myos - muscle, grapho - I write down) - registration of electrical potentials; skeletal muscles. Electromyography is used as a method for studying the normal and impaired function of the locomotor system in humans and animals. Electromyography includes techniques for studying the electrical activity of muscles at rest, with arbitrary, involuntary, and contractions caused by artificial stimulation.
With the help of electromyography, the functional state and functional features of muscle fibers, motor units, neuromuscular transmission, nerve trunks, segmental apparatus of the spinal cord, as well as suprasegmental structures are studied; they study the coordination of movements, the development of motor skills in various types of work and sports exercises, the restructuring of the work of transplanted muscles, and fatigue. Based on electromyography, a method for controlling muscle biocurrents has been created, which has found practical application in controlling the so-called bioelectric prostheses (see Prosthetics).
An electromyogram is a curve obtained on photographic paper, photographic film or on paper when registering the electrical potentials of skeletal muscles. It can be recorded using a special device called an electromyograph, or other devices used to register biopotentials. The device, as a rule, has at least two recording channels. Each channel includes discharge electrodes, a biopotential amplifier and a recording device. Most electromyographs provide a device for visual and auditory control (Fig. 1).

Rice. 1. Scheme of the device for electromyography.

The main source of fluctuations in the electrical potential of the muscles is the process of excitation spreading through the muscle fibers. However, since the electromyogram is recorded in the region of motor points (see Electrodiagnostics), part of the electrical potential is the potential that occurs when the end plates are excited. Skeletal muscle electrical potentials can be tapped intracellularly or extracellularly.
Intracellular assignment of electrical potentials of individual muscle fibers in humans allows us to determine those characteristics that were previously studied in microelectrode studies on animals or preparations: the magnitude of the membrane potentials of muscle fibers, depolarization and hyperpolarization of membranes, etc. (see Bioelectric phenomena). A number of authors call the registration of intracellular potentials of skeletal muscles intracellular electromyography.
Extracellular assignment of electrical potentials is carried out by two methods:
1) using electrodes with a relatively small discharge surface (hundredths of a square millimeter), immersed in the muscle by means of needles (Fig. 2, 1-3); in all cases, except for the unipolar lead, both discharge electrodes are at a small distance from each other (usually less than 0.5 mm); 2) using electrodes with a relatively large discharge surface (30-100 mm 2), usually placed on the skin above the muscle at a relatively large distance from each other (1-2 cm) (Fig. 2, 4-6). In the first case, it is customary to talk about "local", in the second - about "global" assignment. "Local" assignment allows you to study the electrical potentials that arise in a small amount of muscle tissue: the potentials of individual motor units, the total potentials of a small number of motor units, in pathological conditions - the potentials of individual muscle fibers. The main object of study is the motor unit. This concept originally meant a set of muscle fibers innervated by one motor neuron.

The modern understanding of the structure and function of the CNS is based on neural theory.

The nervous system is built of two types of cells: nerve and glial, the number of the latter being 8–9 times greater than the number of nerve cells. However, it is neurons that provide the whole variety of processes associated with the transmission and processing of information.

A neuron, a nerve cell, is the structural and functional unit of the CNS. Individual neurons, unlike other body cells that act in isolation, "work" as a whole. Their function is to transmit information (in the form of signals) from one part of the nervous system to another, in the exchange of information between the nervous system and different parts of the body. In this case, the transmitting and receiving neurons are combined into nerve networks and circuits.

The most complex information processing processes take place in nerve cells. With their help, the body's responses (reflexes) to external and internal stimuli are formed.

Neurons have a number of features common to all body cells. Regardless of its location and functions, any neuron, like any other cell, has a plasma membrane that defines the boundaries of an individual cell. When a neuron interacts with other neurons, or detects changes in the local environment, it does so with the help of the membrane and the molecular mechanisms contained within it. It is worth noting that the neuron membrane has a much higher strength than other cells in the body.

Everything inside the plasma membrane (except the nucleus) is called the cytoplasm. It contains the cytoplasmic organelles necessary for the existence of the neuron and the performance of its work. Mitochondria provide the cell with energy, using sugar and oxygen to synthesize special high-energy molecules that are consumed by the cell as needed. Microtubules - thin support structures - help the neuron maintain a certain shape. The network of internal membrane tubules, through which the cell distributes the chemicals necessary for its functioning, is called the endoplasmic reticulum.

There are two types of endoplasmic reticulum: "rough" and "smooth". Rough (granular) membranes are dotted with ribosomes necessary for cells to synthesize the proteins secreted by it. The abundance of elements of the "rough" reticulum in neurons characterizes them as cells with very intense activity. Another type of plasma reticulum - smooth, also called the Golgi apparatus, "packs" the substances synthesized by the cell into special "bags" built from the membranes of the smooth reticulum. The task of this neuron organelle is to transfer secrets to the cell surface.

In the center of the cytoplasm is the nucleus, which, like all cells with nuclei, contains genetic information encoded in the chemical structure of genes. In accordance with this information, a fully formed cell synthesizes specific substances that determine the form, chemistry and function of this cell. However, unlike most other body cells, mature neurons cannot divide. Therefore, the genetically determined chemical elements of any neuron must ensure the preservation and change of its functions throughout its life. In large neurons, 1/3-1/4 of the size of their body is the nucleus. The nucleoli included in its composition are involved in supplying the cell with ribonucleic acids and proteins (in motor neurons, for example, during the motor activity of the animal, the nucleoli increase significantly in size).

At the same time, neurons, unlike other cells of the body, have an essential feature, they, in addition to the body (soma), are equipped with processes. Numerous short tree-like branched processes - dendrites (translated from Greek - tree) serve as a kind of neuron inputs through which signals enter the nerve cell. They have a rough surface, created by small thickenings - spines, like beads, strung on a dendrite. Due to this, the surface of the neuron increases and the collection of information is maximized.

The output of the neuron is a long, smooth process extending from the gene - an axon (from the Greek axis - axis), which transmits nerve impulses further to another nerve cell or working organ (Fig. 1). The axons of many neurons are covered with a myelin sheath. It is formed by Schwann cells, repeatedly (up to 10 or more layers) "wrapped" like an insulating tape around the axon trunk. However, the Schwann cell sleeves worn on the axon do not touch each other. Narrow gaps remain between them - the intercepts of Ranvier. Only here the nerve fiber is in direct contact with the extracellular fluid. Therefore, in the nervous system of mammals, a wave of a propagating nerve impulse does not run smoothly, but moves in jumps (saltatorically) from one intercept to another, which greatly speeds up the process of impulse propagation.

As for the initial part of the axon at the place of its exit from the cell body (the area of ​​the "axon mound"), it is devoid of the myelin sheath. The membrane of this nonmyelinated part of the neuron, the so-called initial segment, is highly excitable. Therefore, it is called the starting zone, since it is from here that the excitation of the neuron begins.

There is no need to say that even for intracerebral connections, very long processes are needed, not to mention axons that extend beyond the CNS - to muscles, glands, and internal organs. Collected in bundles, they form nerves.

If a neuron forms output connections with a large member of other cells, then its axon can branch many times so that signals can reach each of them, the number of such branches (thermipoles) is huge and ranges from 1000 to 10000 or more. In addition, the axon is able to give off additional branches - collaterals, along which excitation goes far away from the main path. The processes separated from the cell body cannot exist for a long time and die. The cell body, on the other hand, regenerates them. Of course, this applies only to the central part of the process. Sometimes processes of regeneration of shoots proceed at a tremendous speed: up to 30 microns per minute.

It should be noted that it was precisely because of the presence of processes that neurons, like cells, were discovered later than other cells of the human and animal body. This is understandable, since the neuron with all its processes could not fit in the field of view of the microscope. Therefore, initially the cells themselves were not given due importance, considering them as a thickening among many processes.

The shape of the nerve cell, its size and the location of the processes are varied and depend on the functional purpose of the neuron (Fig. 2).

Each individual neuron is unique and unequal to its own kind, unlike other cells of the body. The size of neurons is very variable: the largest ones are tens and hundreds of times larger than the smallest ones. For example, the size of the diameter of the granular cells of the cerebellum is 7.0 microns, and the motor neurons of the spinal cord - 70.0.

The density of neurons in some parts of the CNS is very high. So, in the cerebral cortex, it is equal to 40,000 cells in 1 mm3. No one can accurately answer the question of how many neurons the brain of a person and highly organized animals contains, but it is believed that their number is measured in approximately tens of billions.

unmyelinated nerve fibers- one layer of Schwann cells, between them - slit-like spaces. The cell membrane is in contact with the environment throughout. When irritation is applied, excitation occurs at the site of action of the stimulus. Unmyelinated nerve fibers have electrogenic properties (the ability to generate nerve impulses) throughout.

myelinated nerve fibers- covered with layers of Schwann cells, which in places form nodes of Ranvier (areas without myelin) every 1 mm. The duration of the interception of Ranvier is 1 µm. The myelin sheath performs trophic and insulating functions (high resistance). The areas covered with myelin do not have electrogenic properties. They have the interceptions of Ranvier. Excitation occurs in the interception of Ranvier closest to the site of action of the stimulus. In the intercepts of Ranvier, there is a high density of Na-channels, therefore, in each interception of Ranvier, an increase in nerve impulses occurs.

Interceptions of Ranvier act as repeaters (generate and amplify nerve impulses).

The mechanism of conduction of excitation along the nerve fiber

1885 - L. German - circular currents arise between the excited and unexcited sections of the nerve fiber.

Under the action of an irritant, there is a potential difference between the outer and inner surfaces of the tissue (areas that carry different charges). Between these areas, an electric current arises (the movement of Na + ions). Inside the nerve fiber, a current arises from the positive pole to the negative pole, i.e., the current is directed from the excited area to the unexcited one. This current exits through the unexcited region and causes it to recharge. On the outer surface of the nerve fiber, the current flows from the unexcited area to the excited area. This current does not change the state of the excited area, since it is in a state of refractoriness.

Evidence of the presence of circular currents: the nerve fiber is placed in a NaCl solution and the speed of excitation is recorded. Then the nerve fiber is placed in oil (resistance increases) - the conduction speed decreases by 30%. After that, the nerve fiber is left in the air - the rate of excitation is reduced by 50%.

Features of the conduction of excitation along myelinated and unmyelinated nerve fibers:

1) myelin fibers - have a sheath with high resistance, electrogenic properties only in the nodes of Ranvier. Under the action of the stimulus, excitation occurs in the nearest intercept of Ranvier. Neighbor intercept in polarization state. The resulting current causes depolarization of the adjacent intercept. The nodes of Ranvier have a high density of Na-channels, therefore, in each next node, a slightly larger (in amplitude) action potential arises, due to this, the excitation propagates without a decrement and can jump over several nodes. This is Tasaki's saltatory theory. Proof of the theory - drugs were injected into the nerve fiber that blocked several intercepts, but the conduction of excitation was recorded after that. This is a highly reliable and profitable method, since minor damage is eliminated, the speed of excitation is increased, and energy costs are reduced;

2) non-myelinated fibers - the surface has electrogenic properties throughout. Therefore, small circular currents occur at a distance of a few micrometers. The excitation has the form of a constantly traveling wave. This method is less profitable: high energy costs (for the operation of the Na-K pump), a lower rate of excitation.

The mechanism of conduction of excitation along the nerve fibers depends on their type. There are two types of nerve fibers: myelinated and unmyelinated.

Metabolic processes in unmyelinated fibers do not provide a quick compensation for energy expenditure. The spread of excitation will go with a gradual attenuation - with a decrement. The decremental behavior of excitation is characteristic of a low-organized nervous system. The excitation is propagated by small circular currents that occur inside the fiber or in the liquid surrounding it. Between excitation

Introduction

The basis of all types of muscle contraction is the interaction of actin and myosin. In skeletal muscle, myofibrils are responsible for contraction (about two-thirds of the muscle's dry weight). Myofibrils are structures 1-2 µm thick, consisting of sarcomeres - structures about 2.5 µm long, consisting of actin and myosin (thin and thick) filaments and Z-disks connected to actin filaments. The contraction occurs with an increase in the concentration of Ca 2+ ions in the cytoplasm as a result of the sliding of myosin filaments relative to actin filaments. The energy source of contraction is ATP. The efficiency of the muscle cell is about 50%.

Sliding of myosin relative to actin

Myosin heads break down ATP and, due to the released energy, change their conformation, sliding along actin filaments. The cycle can be divided into 4 stages:

1. The free head of myosin binds to ATP and hydrolyzes it to ADP and phosphate and remains bound to them. (A reversible process - the energy released as a result of hydrolysis is stored in an altered conformation of myosin).
2. The heads bind weakly to the next actin subunit, the phosphate is released, and this leads to a strong binding of the myosin head to the actin filament. This reaction is already irreversible.
3. The head undergoes a conformational change that pulls the thick filament toward the Z-disk (or, equivalently, the free ends of the thin filaments toward each other).
4. ADP is separated, due to this, the head is separated from the actin filament. A new ATP molecule joins.

Then the cycle is repeated until the concentration of Ca 2+ ions decreases or the ATP supply is depleted (as a result of cell death). The speed of myosin sliding along actin is ≈15 µm/sec. There are many (about 500) myosin molecules in the myosin filament and, therefore, during contraction, the cycle is repeated by hundreds of heads at once, which leads to a fast and strong contraction. It should be noted that myosin behaves like an enzyme - actin-dependent ATPase. Since each repetition of the cycle is associated with ATP hydrolysis, and, consequently, with a positive change in free energy, the process is unidirectional. Myosin moves along actin only towards the plus end.

successive stages

Energy source for contraction

The energy of ATP hydrolysis is used to contract the muscle, but the muscle cell has an extremely efficient system for regenerating the ATP reserve, so that in a relaxed and working muscle, the ATP content is approximately equal. The enzyme phosphocreatine kinase catalyzes the reaction between ADP and creatine phosphate, the products of which are ATP and creatine. Creatine phosphate contains more stored energy than ATP. Thanks to this mechanism, during a burst of activity in the muscle cell, the content of creatine phosphate falls, and the amount of the universal energy source - ATP - does not change. The mechanisms for regenerating the ATP reserve may differ depending on the partial pressure of oxygen in the surrounding tissues (see Anaerobic Organisms).

Regulation mechanism

Mostly neurons are involved in the regulation of muscle activity, but there are cases when hormones (for example, adrenaline and oxytocin) also control smooth muscle contraction. The reduction signal can be divided into several stages:

From cell membrane to sarcoplasmic reticulum

Exposure to a mediator released from a motor neuron causes an action potential on the cell membrane of a muscle cell, which is transmitted further using special membrane invaginations called T-tubules, which extend from the membrane into the cell. From the T-tubules, the signal is transmitted to the sarcoplasmic reticulum - a special compartment of flattened membrane vesicles (the endoplasmic reticulum of the muscle cell) surrounding each myofibril. This signal causes the opening of Ca 2+ channels in the reticulum membrane. Back, Ca 2+ ions enter the reticulum with the help of membrane calcium pumps - Ca 2+ -ATPase.

From the release of Ca 2+ ions to the contraction of myofibrils

The mechanism of muscle contraction, taking into account troponin and tropomyosin

In order to control contraction, the protein tropomyosin and a complex of three proteins, troponin, are attached to the actin filament (the subunits of this complex are called troponins T, I and C). Troponin C is a close homologue of another protein, calmodulin. There is only one troponin complex every seven actin subunits. The binding of actin to troponin I moves tropomyosin to a position that interferes with myosin binding to actin. Troponin C binds to four Ca 2+ ions and weakens the effect of troponin I on actin, and tropomyosin occupies a position that does not prevent actin from binding to myosin.

Major proteins of myofibrils

Protein	Proportion of protein %	His prayer. mass, kDa	Its function
Myosin	44	510	The main component of thick filaments. Forms bonds with actin. Moves along actin due to ATP hydrolysis.
Actin	22	42	The main component of thin filaments. During muscle contraction, myosin moves along it.
Titin	9	2500	Large flexible protein that forms a chain for binding myosin to the Z-disk.
Troponin	5	78	A complex of three proteins that regulates contraction when bound to Ca 2+ ions.
Tropomyosin	5	64	A rod-shaped protein associated with actin filaments that blocks the movement of myosin.
Nebulin	3	600	A long, inextensible protein associated with the Z-disc and running parallel to actin filaments.

Literature

• B. Alberts, D. Bray, J. Lewis, M. Reff, K. Roberts, J. Watson, Molecular Biology of the Cell - In 3 volumes - Per. from English. - T.2. - M.: Mir, 1994. - 540 p.
• M. B. Berkinblit, S. M. Glagolev, V. A. Furalev, General biology - In 2 hours - Part 1. - M.: MIROS, 1999. - 224 p.: ill.

see also

Muscular system

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See what "Muscle Contraction" is in other dictionaries:

MUSCLE CONTRACTION- shortening or tension of the muscles in response to irritation caused by the discharge of the motor. neurons. The M. s model was adopted, according to the swarm, when the surface of the muscle fiber membrane is excited, the action potential first propagates through the system ... ... Biological encyclopedic dictionary

muscle contraction- raumens susitraukimas statusas T sritis Kūno kultūra ir sportas apibrėžtis Raumens ilgio kitimas cheminei energijai virstant mechanine ir šilumine. Skiriamas auksotoninis, izometrinis, izotoninis, maksimalusis ir pavienis raumens… … Sporto terminų žodynas

muscle contraction- raumens susitraukimas statusas T sritis Kūno kultūra ir sportas apibrėžtis Raumenį sudarančių skaidulų patrumpėjimas arba įtampos kitimas atliekant judesį (veiksmą). atitikmenys: engl. muscle contraction vok. Muskelkontraktion, f rus. muscle ... ... Sporto terminų žodynas

muscle contraction- see abbreviation... Big Medical Dictionary

muscle contraction- shortening of the muscle, as a result of which it performs mechanical work. M. s. provides the ability of animals and humans to voluntary movements. The most important component of muscle tissue (See Muscle tissue) proteins (16.5 ... ...

MUSCLE CONTRACTION- the main function of muscle tissue is the shortening or tension of muscles in response to irritation caused by the discharge of motor neurons. M. s. underlies all movements of the human body. Distinguish M. with. isometric, when the muscle develops an effort ... ... Psychomotor: Dictionary Reference

Isometric muscle contraction- contraction of a muscle, expressed in an increase in its tension with a constant length (for example, contraction of a muscle of a limb, both ends of which are fixed motionless). In an organism to And. the tension developed by the muscle when trying is approaching ... ... Great Soviet Encyclopedia

When the muscle is in an uncontracted (relaxed) state, the actin and myosin filaments are only partially advanced relative to each other, and each myosin filament is opposed by several actin filaments surrounding it.

The contraction of the muscle fiber, and, consequently, the muscle as a whole, is due to the fact that thin actin protofibrils are drawn into the depths of the gaps between myosin ones. This theory about the mechanism of muscle contraction, based on modern electron microscopic data, is called slip theories: actin filaments, drawn into the depths of anisotropic disks, seem to glide between myosin filaments. Under an electron microscope, one can see that during muscle contraction, the width of the anisotropic (dark) disks does not change, while the width of the isotropic (light) ones decreases. With a significant reduction, these disks completely disappear - they are completely drawn into the anisotropic ones (Fig. 2).

Skeletal muscle innervation. The muscle contracts only when excitation occurs in it - after a number of processes (electrical, chemical) have occurred that make it possible for it to perform a specific function - shortening or tension development. And the muscle is excited in a reflex way - under the influence of impulses that are transmitted to it from the central nervous system through efferent (i.e., centrifugal nerves.) Various sections of the central nervous system are involved in the organization of motor acts, but its lower sections have a direct connection with the skeletal muscles. In the so-called anterior horns of the spinal cord are the body motoneurons, long processes of which - axons go to the muscles of the trunk and limbs and end in them with their branches (Fig. 3). Axons of nerve cells form those nerve fibers through which peripheral organs are connected with the central nervous system. Through motor neurons and their axons, muscles are also influenced by the overlying parts of the central nervous system - various parts of the brain.

In addition to efferent (motor) nerves, the muscle is innervated and afferent, or sensitive, nerves. Their endings are associated with sensitive formations - proprioceptors. The latter are excited by changes in the state of muscle fibers - their contraction and stretching. Impulses from them are transmitted by afferent nerves to the central nervous system and thus inform the corresponding nerve centers about the length of the muscle and the tension it develops.

Nerve-muscular synapse. The endings of the efferent nerve fiber are connected to muscle fibers through specially arranged formations - synapses. Each synapse (Fig. 4) consists of a nerve ending membrane, a muscle fiber membrane, and a narrow gap between them. This shell, visible only under an electron microscope, is called synoptic gap; nerve ending membrane is called presynaptic(it is located in front of the synapse), and the muscle fiber membrane is postsynaptic(located behind the synapse). Excitation from the nerve ending to the muscle fiber is transmitted through a chemical substance. Nerve endings are able, as it turned out, to produce certain substances (similar to how glandular cells do), called mediators- mediators between the nerve and the organ innervated by it. The mediator of motor neuron endings in muscle fibers is acetylcholine. high sensitivity to this impact.

motor unit. As mentioned above, the axon of the motor neuron, approaching the muscle, branches, forming many endings. Each of these endings forms a synapse on one muscle fiber. Thus, each motor neuron supplies a whole group of muscle fibers with its endings. When a motor neuron is excited, all the muscle fibers that it

innervates, and this whole group of fibers works as a whole. In this regard, the motor neuron and the group of muscle fibers innervated by it are called motor unit(Fig.5). The muscle fiber can be called the structural unit of the muscle, and the motor unit is its functional unit.

Motor neurons differ in size and give rise to a different number of terminal branches. In this regard, motor units include a different number of muscle fibers. In some muscles, richly innervated and capable of performing very finely regulated movements, there are from 3-6 to several dozen muscle fibers per motor neuron. Such, for example, are the external muscles of the eye, the muscles of the fingers of the hand. In other muscles, for example, in the large muscles of the trunk and limbs, one motor neuron innervates a very large number of muscle fibers - hundreds and even thousands. Each muscle and its nerve includes a different (sometimes several thousand) number of motor units.

Nerve and muscle stimulation . From the provisions discussed above, it is clear that the muscle contracts only when excitation occurs in it, that the impulses that cause this process in it come from the central nervous system, that the immediate “departure station” is the motor neurons, along the long processes of which - the efferent motor nerve fibers - these impulses are conducted to muscle fibers and transmitted through neuromuscular synapses. It is necessary to find out in more detail what the process of excitation is.

Excitation- it is a very complex biological process, which underlies the activity of organs, tissues and cells of the body.a. When excited, each organ performs its own specific function: for example, the glands of the digestive tract produce enzymes, the endocrine glands produce hormones, and the muscles produce a contractile act. Along with these completely different specific reactions, there are common features in the excitation of various organs. This applies primarily to electrical phenomena- the first and main processes in which excitation is manifested.

Electrical properties of nerve and muscle cells at rest. Membrane potential. At rest, the cell has a certain electrical charge. The sarcolemma is positively charged on the outside and negatively charged on the inside. The occurrence of this double electric charge is associated with special properties membranes. She possesses selective permeability for various ions. So, it relatively easily passes positively charged ions (cations) of potassium (K +) and almost does not pass sodium cations (Na +). Cannot pass through the membrane and large molecules of protein anions. If it were permeable to all these substances, then their content inside and outside the cell would become the same. Due to the selective and limited permeability of the membrane, different concentration various ions inside the cell and in its environment - in the interstitial fluid. Potassium inside the cell contains 30-40 times more than outside, and sodium - 10-12 times less. Due to the difference in concentrations, K + cations leave the cell to the outside (the membrane is permeable to them), while Na + ions cannot penetrate inside (the membrane at rest is almost impermeable to them). They cannot go outside through the membrane and anions. In connection with the diffusion of K + outward and under the influence of electrostatic forces of attraction between oppositely charged ions, anions are concentrated at the surface of the membrane from the inside, and cations - from the outside, thus forming a double electric layer on the membrane, i.e. polarizing her. The potential difference between the outer and inner surfaces of the membrane, called the resting potential (RP), or membrane potential (MP), is equal to 70-90 mV.

Changes in the electrical state of the cell during excitation. action potential. When a cell is irritated, resting membrane potential change. The excited region of the membrane is externally charged negatively with respect to its inner surface. In other words, the membrane is recharged, the signs of the charges change - the resting potential is inverted. This process is due to changes in membrane permeability under the influence of irritation. For some short time, it becomes much more permeable to Na+ ions than to K+ ions. Na + ions, which, as already mentioned, are 10-12 times more in the interstitial fluid than inside the cell, begin to penetrate inside. Neutralizing the negative charge of the inner surface of the membrane in the irritated area, they thereby reduce the potential difference that existed at rest, i.e., lead to depolarization membranes. It is interesting that this process reinforces itself: the beginning depolarization increases the permeability of the membrane for Na + ions; diffusion of Na + cations deepens depolarization; in connection with this, the permeability for these ions becomes even more significant, etc. As a result of this process, not only does the membrane depolarize, but also recharge: its inner surface in the irritated area becomes positively charged, and the outer one in relation to it becomes negatively charged. When measuring the potential difference between the outer and inner surfaces of the cell, it turns out that instead of a charge of - 90 mV inside, which was noted at rest, a charge of + 30-40 mV is found there. The penetration of positively charged Na + ions into the cell led to the development of an electrical process characterized by a voltage of 120-130 mV (from -90 to +30 mV). This process - oscillation of the resting potential - is called action potential (AP). It characterizes the occurrence of excitation in a nerve or muscle cell.

In contrast to the resting potential, which is characterized by constancy, the action potential is a fast-flowing process. It consists of two phases - depolarization phases, which leads to membrane recharge, and repolarization phases, consisting in the restoration of the initial electrical state of the cell - the membrane potential (Fig. 6). The repolarization phase begins at the moment when the action potential reaches its peak - the peak, i.e., the amplitude of 120-130 mV. The permeability of the membrane for Na+ ions in this case decreases sharply, and their further entry into the interior almost stops. At this moment, the permeability of the membrane for K+ ions is much higher. In connection with the above-mentioned difference in the concentration of potassium inside and outside the cell, these cations begin to intensively leave the cell. This process is joined by the extraction of Na + ions that have penetrated into the cell - as if "pumping out" them. All this leads to the restoration of the initial state of the cell - a positive charge on the outer surface of the membrane and a negative charge on the inner one. The depolarization phase of the action potential (ascending phase - see Fig. 6) lasts about 1 ms, in some cells - 0.5 ms; the repolarization phase (descending phase) is much longer than the first.

Conduction of excitation along nerve and muscle fibers. The action potential is associated with the conduction of excitation along the nerve and muscle fibers. When an action potential occurs between the excited site and neighboring, which are in a state of rest, there is potential difference. The outer surface of the excited portion of the membrane, as noted above, turns out to be negatively charged, and the adjacent one, which is at rest, is positively charged. Due to the potential difference between these adjacent sections, an electric current arises - the so-called local action current. This current is irritant section of the fiber that was at rest until that time. Under the influence of irritation in this area, the processes described above begin - depolarization, an increase in sodium permeability, etc., i.e., an action potential arises. Then the next section of the fiber is excited, and so on. Thus, the conduction of the impulse consists, in essence, in successive, one after the other, excitation of sections of the fiber.

Transmission of excitation through synapses occurs, as mentioned above, through chemicals - mediators, produced by terminal branches of axons. Excitation is transmitted chemically both in the synapses of the central nervous system, where the branches of some axons form synapses on the body and dendrites of others, and in neuromuscular, or mioneural, synapse. The mediator of motor neuron endings in muscle fibers is acetylcholine. When the nerve endings are excited, the synaptic vesicles containing the mediator burst, acetylcholine penetrates through the presynaptic membrane into the synaptic one. gap and causes excitation of the postsynaptic membrane. The latter has high sensitivity to this impact. Under the influence of acetylcholine, its permeability for Na + and K + ions increases, depolarization occurs and postsynaptic potential. From the postsynaptic membrane, excitation is transmitted to other (extrasynaptic) sections of the muscle fiber membrane again by electrical means (see Fig. 4).

The value of the functional state of the neuromuscular apparatus for the development of the excitation process. Excitability. The ability of a living tissue to develop excitation in response to a stimulus is called excitability. Thus, excitability is one of the main properties of living tissue that ensures the interaction of the organism with the environment. Different tissues have different excitability. The level of excitability of the same tissue is also changeable Moderate effects on the tissue increase its excitability, excessive in strength or duration reduce it. So, under the influence of a warm-up, the excitability of the central nervous system and the neuromuscular apparatus increases, while it decreases during tedious work.

The change in excitability occurs naturally during the course of each wave of excitation. When an action potential occurs (during the depolarization phase), the tissue becomes unexcitable: she is unable to respond to new irritation. This so-called absolute refractory phase. Gradually, the excitability of the tissue is restored to its original level, and then becomes for some time even higher than it.

Excitability can be measured. The higher the excitability of the tissue, the easier it is to cause its excitation - a response. The minimum force of irritation, which is needed to cause tissue excitation, characterizes the so-called excitability threshold this tissue is called threshold power. The level of excitability is an important indicator of the functional state of the tissue.

Functional mobility (lability). One of the important factors on which the activity of excitable tissues (such as nerve cells, synapses, neuromuscular apparatus) depends is speed flow of excitation, called lability(N. E. Vvedensky). In some formations, the excitation wave develops and decays at a high rate, in others much more slowly. The frequency of impulses that the tissue can develop per unit time depends on the rate of excitation. The lability of nerve fibers is the highest, the lability of muscle fibers, nerve cells and especially synapses is much lower.

Lability, like tissue excitability, not constant. Moderate influences increase the speed of the flow of the excitation wave, excessive influences decrease it. Under the influence of a warm-up, for example, the lability of the central nervous formations and the neuromuscular apparatus increases, and decreases with fatigue.

The level of lability can be judged by various indicators. N.E. Vvedensky proposed to measure it the maximum number of excitation waves, which can occur in the tissue per unit time (in 1 sec).

Biochemical processes in the muscle during excitation. Electrical manifestations of excitation, i.e., action potentials arising on the membrane of the muscle fiber, lead to a number of chemical processes that culminate in the mechanical reaction of the fiber - reduction.

The connection between the electrical processes occurring on the membrane and the mechanical reaction of myofibrils is provided through calcium ions(Ca++). At rest, these ions are predominantly inside the system. tubules And cavities, which penetrates the fiber along (between myofibrils) and across (between separate parts - sarcomeres of myofibrils). These tubules and cavities also have their own semi-permeable membranes, through which Ca ++ ions almost do not penetrate at rest. When a muscle fiber membrane is excited, its action potentials cause depolarization of the membranes of the tubules and cavities and increase their permeability. Ca++ ions come out (due to the fact that their concentration inside this system is much higher than outside) and are very close to the myofibrils.

Ca++ ions affect the protein myosin. When considering the structure of muscle fiber myofibrils, it has already been noted that they consist of protofibrils - thin (actin) and relatively thicker (myosin), alternating with each other in the transverse direction. Myosin, as it turned out, is not only a contractile muscle protein, but also has properties enzyme. It is able to break down a very energy-rich substance - adenosine triphosphate (ATP). When the fiber is at rest, myosin is inactive as an enzyme. When exposed to Ca ++ ions, the enzymatic properties of myosin are activated and it starts breaking down ATP. Due to the chemical energy that is released in this case, there is reduction myofibrils, i.e. retraction (sliding) of actin protofibrils into the spaces between myosin ones. Relaxation muscle fiber is associated with the removal of Ca ++ from the contractile apparatus. Special studies have shown that Ca ++ ions, after exposure to myosin, which led to ATP splitting and fiber contraction, are, as it were, “pumped out” from the sphere of the contractile apparatus into the system where they were before excitation of the muscle fiber.

Muscle work. In the process of muscle contraction, potential chemical energy is converted into potential mechanical energy of tension and kinetic energy of movement. Distinguish between internal and external work. Internal work is associated with friction in the muscle fiber during its contraction. External work is manifested when moving one's own body, cargo, individual parts of the body (dynamic work) in space. It is characterized by the coefficient of performance (COP) of the muscular system, i.e. the ratio of the work done to the total energy costs (for human muscles, the efficiency is 15-20%, for physically developed trained people this figure is slightly higher).

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The mechanism of muscle contraction

The transfer of excitation from the motor neuron to the muscle fiber occurs with the help of the mediator acetylcholine (ACh). The interaction of ACh with the end plate cholinergic receptor leads to the activation of ACh-sensitive channels and the appearance of an end plate potential that can reach 60 mV. In this case, the area of ​​the end plate becomes a source of irritating current for the muscle fiber membrane, and in the areas of the cell membrane adjacent to the end plate, an action potential (AP) arises, which propagates in both directions at a speed of approximately 3-5 m/s at a temperature of 36 ° WITH.

The second stage is the spread of AP inside the muscle fiber along the transverse system of tubules, which serves as a link between the surface membrane and the contractile apparatus of the muscle fiber. The T-system is in close contact with the terminal cisterns of the sarcoplasmic reticulum of two neighboring sarcomeres. Electrical stimulation of the contact site leads to the activation of enzymes located at the contact site and the formation of inositol triphosphate. Inositol triphosphate activates calcium channels in the membranes of the terminal cisterns, which leads to the release of Ca 2+ ions from the cisterns and an increase in the intracellular Ca 2+ concentration from 107 to 105 M. The totality of processes leading to an increase in the intracellular Ca 2+ concentration constitutes the essence of the third stage of muscle contraction. Thus, at the first stages, the electrical signal of AP is converted into a chemical signal - an increase in the intracellular Ca 2+ concentration, i.e., an electrochemical transformation.

With an increase in the intracellular concentration of Ca 2+ ions, tropomyosin shifts into the groove between actin filaments, while actin filaments open areas with which myosin cross-bridges can interact. This displacement of tropomyosin is due to a change in the conformation of the troponin protein molecule upon Ca 2+ binding. Consequently, the participation of Ca 2 ~ ions in the mechanism of interaction between actin and myosin is mediated through troponin and tropomyosin.

The next step in electromechanical coupling is the attachment of the head of the cross bridge to the actin filament, to the first of several consecutive stable centers. In this case, the myosin head rotates around its axis, since it has several active centers that sequentially interact with the corresponding centers on the actin filament. The rotation of the head leads to an increase in the elastic elastic traction of the neck of the transverse bridge and an increase in stress. At each specific moment in the process of contraction development, one part of the heads of the transverse bridges is in connection with the actin filament, the other is free, i.e., there is a sequence of their interaction with the actin filament. This ensures the smoothness of the reduction process. At the fourth and fifth stages, xmomechanical transformation takes place.

The successive reaction of connecting and disconnecting the heads of the transverse bridges with the actin filament leads to the sliding of thin and thick filaments relative to each other and a decrease in the size of the sarcomere and the total length of the muscle, which is the sixth stage. The totality of the processes described is the essence of the theory of sliding threads.

The mechanism of muscle relaxation

The described mechanisms of muscle fiber shortening suggest that for relaxation, first of all, it is necessary to lower the concentration of Ca 2+ ions. It has been experimentally proven that the sarcoplasmic reticulum has a special mechanism - a calcium pump that actively returns calcium to the cisterns. The activation of the calcium pump is carried out by inorganic phosphate, which is formed during the hydrolysis of ATP, and the energy supply of the calcium pump also occurs due to the energy generated during the hydrolysis of ATP. Thus, ATP is the second most important factor, absolutely necessary for the relaxation process.

In addition, after muscle contractions, thin protofibrils tend to return to their previous position due to their elastic properties.

For some time after death, the muscles remain soft due to the cessation of the tonic influence of motor neurons. Then the concentration

ATP decreases below a critical level and the possibility of separation of the myosin head from the actin filament disappears. There is a phenomenon of rigor mortis with severe rigidity of skeletal muscles.

Features of the structure of smooth muscles

The smooth muscles of the internal organs differ significantly from the skeletal ones in the nature of innervation, excitation and contraction. Waves of excitation and contraction proceed in smooth muscles at a very slow pace. The development of a state of "untiring" tone of smooth muscles is associated, as in tonic skeletal fibers, with a slowdown in contractile waves that merge with each other even with rare rhythmic stimuli. Smooth muscles are also characterized by the ability to automatism, i.e., to activities that are not associated with the entry of nerve impulses into the muscles from the central nervous system. It has been established that not only the nerve cells present in smooth muscles, but also the smooth muscle cells themselves have the ability for rhythmic spontaneous excitation and contraction.

The peculiarity of the contractile function of the smooth muscles of vertebrates is determined not only by the peculiarities of their innervation and histological structure, but also by the specifics of their chemical composition: a lower content of contractile proteins (actomyosin), macroergic compounds, in particular ATP, low ATP-ase activity of myosin, the presence of water-soluble modification of actomyosin - tonoactomyosin, etc.

The ability of smooth muscles to change length without increasing tension (filling hollow organs, such as the bladder, stomach, etc.) is essential for the body.