It forms the skeletal muscles of humans and animals, designed to perform various actions: body movements, contraction of the vocal cords, breathing. Muscles are made up of 70-75% water.
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The structure of the muscle cell
The structure of skeletal striated muscles
Contraction of muscle fibers
Subtitles
We examined the mechanism of muscle contraction at the molecular level. Now let's talk about the structure of the muscle itself and how it is connected with the surrounding tissues. I'll draw the biceps. Like this... Contracting biceps... Here is the elbow, here is the hand. Here is such a person's biceps during contraction. Probably, you have all seen drawings of muscles, at least schematically, the muscle is attached to the bones on both sides. I'll mark the bones. Schematically ... The muscle on both sides is attached to the bone with the help of tendons. Here we have a bone. And here too. And with white color I will designate the tendons. They attach muscles to bones. And this is a tendon. The muscle is attached to two bones; when contracted, it moves part of the skeletal system. Today we are talking about skeletal muscles. Skeletal… Other types include smooth muscle and cardiac muscle. Cardiac muscles, as you understand, are in our heart; and smooth muscles contract involuntarily and slowly, they form, for example, the digestive tract. I will make a video about them. But in most cases, the word “muscles” refers to the skeletal muscles that move the bones and make it possible to walk, talk, chew, and the like. Let's look at these muscles in more detail. If you look at the biceps muscle in cross section... cross section of the muscle... I'll make the drawing bigger. Let's draw the biceps... No, let it be just an abstract muscle. Let's take a look at it in cross section. Now we will find out what the muscle has inside. The muscle passes into the tendon. Here is the tendon. And the muscle has a shell. There is no clear boundary between the shell and the tendon; the sheath of the muscle is called the epimysium. This is connective tissue. It surrounds the muscle, performs some protective functions, reduces the friction of the muscle on the bone and other tissues, in our example, the tissue of the hand. There is also connective tissue inside the muscle. I'll take another color. Orange. This is a connective tissue sheath; it surrounds bundles of muscle fibers of different thicknesses. It's called the perimysium, it's the connective tissue inside the muscle. Perimisium... And each of these bundles is surrounded by perimysium... If we consider it in more detail... Here is one such bundle of muscle fibers surrounded by perimysium... Let's take this bundle. It is surrounded by a sheath called the perimysium. This is such a "smart" word for connective tissue. There, of course, there are other tissues - nerve fibers, capillaries, because blood and nerve impulses need to be supplied to the muscle. So there, in addition to the connective tissue, there are other tissues that ensure the life of muscle cells. Each of these groups of fibers - and these are large groups of muscle fibers - is called a bundle. It's a bundle... A bundle. There is also connective tissue inside such a bundle; it is called endomysium. Now I will mark it. Endomysium. I repeat: in the composition of the connective tissue there are nerve fibers, capillaries - everything necessary to ensure contact with muscle cells. We are looking at the structure of the muscle. This is the endomysium. The green color indicates the connective tissue, which is called the endomysium. Endomysium. But such a “fiber”, surrounded by endomysium, is a muscle cell. Muscle cell. I'll mark it with a different color. Here is such an elongated cage. I'll "pull" it out a bit. Muscle cell. Let's look inside it and see how myosin and actin filaments are located there. So, here is a muscle cell or muscle fiber. Muscle fiber… You will often see two prefixes; the first is "myo", derived from the Greek word for "muscle"; And the second is “sarco”, for example, in the words “sarcolemma”, “sarcoplasmic network”, which comes from the Greek word “meat”, “flesh”. It was preserved in a number of words, for example, "sarcophagus". "Sarco" means flesh, "myo" means muscle. So, here is the muscle fiber. Or a muscle cell. Let's look at it in more detail. Now I'll draw it bigger. Muscle cell, otherwise called muscle fiber. "Fiber" - because it is much longer in length than in width; it has an elongated shape. Now I will draw. Here is my muscle cell ... Let's consider it in cross section. Muscle fiber ... They are relatively short - a few hundred micrometers - and very long, at least by cellular standards. Let's have a few centimeters. Imagine such a cage! It is very long, so it has several cores. And to indicate the nuclei, I'll tweak my drawing. I will add such tubercles on the cell membrane - under them there will be nuclei. Let me remind you that this is just one muscle cell; such cells are very long, so they have several nuclei. Here is the cross section. As I said, there are several nuclei in a cell. Imagine that the membrane is transparent; here is one core, here is another, here is the third, and the fourth. Many nuclei are needed in order not to waste time on overcoming long distances by proteins; let's say from this nucleus to this part of the cell. In a multinucleated cell, DNA information is always nearby. If I'm not mistaken, there are an average of thirty cores in one millimeter of muscle tissue. I don’t know how many nuclei are in our cell, but they are located directly under the membrane - and you remember what it is called from the last lesson. The membrane of the muscle cell is called the sarcolemma. Let's write down. Sarcolemma. Emphasis on the third syllable. Here are the cores. The nucleus... And if we look at the cross section, we will see even finer structures, they are called myofibrils. These are the thread-like structures inside the cell. I will draw one of them in the picture. Here is one of those threads. This is a myofibril. Myofibril... If you look at it through a microscope, you can see the grooves. These are the grooves... Here, here and here... And a couple of thin ones... Inside the myofibrils, the interaction of myosin and actin filaments takes place. Let's zoom in even more. So we will increase until we reach the molecular level. So, myofibril; it is located inside the muscle cell or muscle fiber. A muscle fiber is a muscle cell. A myofibril is a filamentous structure within a muscle cell. It is myofibrils that provide muscle contraction. I will draw the myofibril on a larger scale. Something like this... There are stripes on it... This is called striation. Narrow stripes. More ... There are wider stripes. I'll try to draw as accurately as possible. Here is another strip here ... And then everything repeats. Each of these repeating regions is called a sarcomere. This is a sarcomere. Sarcomere... Such areas are located between the so-called Z-lines. The terms were coined when researchers first saw these lines under a microscope. We will talk about how they are related to myosin and actin very soon. This zone is usually called Disk A or A-disk. But this zone here and here - disk I or I-disk. In a couple of minutes, we will find out how they are related to the mechanisms, molecules, which we talked about in the last lesson. If we look inside the myofibrils, we make a cross section of it, we divide it into sections parallel to the screen we look at, that's what we see. So, here's one Z-line. Z-line… Next Z-line. I draw one sarcomere on a large scale. Neighboring Z-line. And here we go to the molecular level, as I promised. Here are the actin filaments. I'll mark them with wavy lines. Let there be three ... I will sign them ... Actin filaments ... And between the actin filaments - myosin. I'll draw them in a different color ... Remember, there are two heads on myosin fibers. Each of them has two heads that slide or "crawl" along the actin fibers. I'll name a few... Here they are attached... Now we'll see what happens when a muscle contracts. Let's draw more myosin fibers. In fact, there are incomparably more myosin heads, but we have a schematic drawing. These are the filaments of the myosin protein, they are twisted, as we saw in the last lesson; here is another one. I will outline it schematically ... You can immediately notice that the myosin filaments are in the A-disk. This is the area of the A-disk. The A-disk... Sections of the actin and myosin filaments overlap each other, but the I-disk is the area where there is no myosin, only actin. I-disc... Myosin filaments are held by titin; it is a resilient, elastic protein. I will color it in a different color. These spirals... Myosin filaments are held by titin. It connects myosin to the Z-zone. So what's going on? When a neuron is excited... Let's draw the terminal branch of the neuron, more precisely, the terminal branch of the axon. This is a motor neuron. He gives the command to the myofibril to contract. The action potential propagates along the membrane in all directions. And in the membrane, we remember, there are T-tubules. The action potential passes through them into the cell and continues to spread. The sarcoplasmic reticulum releases calcium ions. Calcium ions bind to troponin, which attaches to actin filaments, tropomyosin shifts, and myosin can interact with actin. Myosin heads can use the energy of ATP and glide along actin filaments. Remember this "workflow"? This can be thought of as a movement of the actin filaments to the right (away from us) or as a movement of the myosin head to the left (away from us); It's a mirror movement, right? Look, the myosin stays in place, and the actin filaments are attracted to each other. To each other. This is how the muscle contracts. So, we have gone from the general view of the muscle to the processes occurring at the molecular level, which we talked about in previous lessons. These processes occur in all myofibrils inside the cell, because the sarcoplasmic reticulum releases calcium into the cytoplasm, another name for which is myoplasm, because we are talking about a muscle cell, the whole cell. Calcium enters all myofibrils. There is enough calcium ion to bind to all - or most - of the troponin proteins on the actin filaments, and the entire muscle contracts. Individual muscle fibers, muscle cells, probably have a small contractile force. By the way, when one or more fibers contract, you feel twitches. But when they are all working, their strength is enough to do the work, move our bones, lift weights. I hope the lesson was useful.
Histogenesis
The source of development of skeletal muscles are myotome cells - myoblasts. Some of them are differentiated in the places of formation of the so-called autochthonous muscles. Others migrate from myotomes to mesenchyme; at the same time, they are already determined, although outwardly they do not differ from other cells of the mesenchyme. Their differentiation continues in the places of laying of other muscles of the body. In the course of differentiation, 2 cell lines arise. The cells of the first merge, forming symplasts - muscle tubes (myotubes). The cells of the second group remain independent and differentiate into myosatellites (myosatellitocytes).
In the first group, differentiation of specific organelles of myofibrils occurs, gradually they occupy most of the lumen of the myotube, pushing the nuclei cells to the periphery.
The cells of the second group remain independent and are located on the surface of the myotubes.
Structure
The structural unit of muscle tissue is the muscle fiber. It consists of myosymplast and myosatellitocytes (satellite cells) covered by a common basement membrane. The length of the muscle fiber can reach several centimeters with a thickness of 50-100 micrometers.
Skeletal muscles are attached to bones or to each other by strong, flexible tendons.
The structure of the myosymplast
Myosymplast is a collection of fused cells. It has a large number of nuclei located along the periphery of the muscle fiber (their number can reach tens of thousands). Like the nuclei, on the periphery of the symplast there are other organelles necessary for the functioning of the muscle cell - the endoplasmic reticulum (sarcoplasmic reticulum), mitochondria, etc. The central part of the symplast is occupied by myofibrils. The structural unit of the myofibril is the sarcomere. It consists of actin and myosin molecules, it is their interaction that provides a change in the length of the muscle fiber and, as a result, muscle contraction. The composition of the sarcomere also includes many auxiliary proteins - titin, troponin, tropomyosin, and other motor neurons. The number of muscle fibers that make up one IU varies in different muscles. For example, where fine control of movements is required (in the fingers or in the muscles of the eye), the motor units are small, containing no more than 30 fibers. And in the calf muscle, where fine control is not needed, there are more than 1000 muscle fibers in the IU.
The motor units of one muscle can be different. Depending on the speed of contraction, motor units are divided into slow (slow (S-ME)) and fast (fast (F-ME)). And F-ME, in turn, is divided according to resistance to fatigue into fast-fatigue-resistant (FR-ME)) and fast-fatigue (fast-fatigable (FF-ME)).
The ME motor neurons innervating these data are subdivided accordingly. There are S-motor neurons (S-MN), FF-motor neurons (F-MN) and FR-motoneurons (FR-MN) S-ME are characterized by a high content of myoglobin protein, which is able to bind oxygen (O2). Muscles predominantly composed of this type of ME are called red because of their dark red color. Red muscles perform the function of maintaining a person's posture. The ultimate fatigue of such muscles occurs very slowly, and the restoration of functions occurs, on the contrary, very quickly.
This ability is due to the presence of myoglobin and a large number of mitochondria. Red muscle IUs tend to contain large amounts of muscle fibers. FR-MEs are muscles that can perform fast contractions without noticeable fatigue. FR-ME fibers contain a large number of mitochondria and are able to form ATP through oxidative phosphorylation.
As a rule, the number of fibers in FR-ME is less than in S-ME. FF-ME fibers are characterized by a lower content of mitochondria than in FR-ME, and also by the fact that ATP is formed in them due to glycolysis. They lack myoglobin, which is why muscles composed of this type of ME are called white. White muscles develop a strong and rapid contraction, but tire rather quickly.
Function
This type of muscle tissue provides the ability to perform voluntary movements. A contracting muscle acts on the bones or skin to which it attaches. In this case, one of the points of attachment remains motionless - the so-called fixation point(lat. púnctum fíxsum), which in most cases is considered as the initial section of the muscle. The moving piece of muscle is called moving point, (lat. púnctum móbile), which is the place of its attachment. However, depending on the function performed, punctum fixum can act as punctum mobile, and vice versa.
Skeletal muscles built from striated skeletal muscle tissue. They are arbitrary, i.e. their reduction is carried out consciously and depends on our desire. In total, there are 639 muscles in the human body, 317 of them are paired, 5 are unpaired.
Skeletal muscle is an organ that has a characteristic shape and structure, typical architectonics of blood vessels and nerves, built mainly from striated muscle tissue, covered on the outside with its own fascia, with the ability to contract.
Principles muscle classification. The classification of the skeletal muscles of the human body is based on various signs: body area, origin and shape of muscles, function, ana-
tomo-topographic relationships, the direction of muscle fibers, the relationship of muscles to joints. In relation to the areas of the human body, the muscles of the trunk, head, neck and limbs are distinguished. The muscles of the trunk, in turn, are divided into the muscles of the back, chest and abdomen. muscles
The upper limb, according to the existing parts of the skeleton, is divided into the muscles of the girdle of the upper limb, the muscles of the shoulder, forearm and hand. Homologous sections are characteristic of the muscles of the lower limb - the muscles of the girdle of the lower limb (muscles of the pelvis), the muscles of the thigh, lower leg and foot.
By shape muscles can be simple or complex. Simple muscles include long, short and wide. Many-headed (biceps, triceps, quadriceps), multitendinous, digastric muscles are considered complex. Muscles of a certain geometric shape are also complex: round, square, deltoid, trapezoid, rhomboid, etc.
By function distinguish between flexor and extensor muscles; adductor and abductor muscles; rotating (rotators); sphincters (constrictors) and dilators (dilators). Rotating muscles in
Depending on the direction of movement, they are divided into pronators and arch supports (rotating inward and outward). It also provides for their division into synergists and antagonists. Synergists These are muscles that perform the same function and at the same time reinforce each other. Antagonists- these are muscles that perform opposite functions, i.e. producing opposite movements.
By location- superficial and deep; external and internal; medial and lateral.
In the direction of the muscle fibers- with parallel, oblique, circular and transverse course of muscle fibers.
Muscle structure. Skeletal muscle as an organ includes the actual muscle and tendon parts, the system of connective tissue membranes, its own vessels and nerves. The middle, thickened part of the muscle is called the abdomen. At both ends of the muscle, in most cases, there are tendons with which it is attached to the bones. Structural and functional unit of the proper muscle part is striated muscle fiber.
In the process of muscle contraction, actin filaments are drawn into the spaces between myosin filaments, change their configuration, and interlock with each other. The energy supply for these processes occurs due to the splitting of ATP molecules in the mitochondria.
The functional unit of a muscle is mion- a set of striated muscle fibers innervated by one motor nerve fiber. Auxiliary apparatus of skeletal muscles are fascia, fibrous and bone-fibrous channels, synovial sheaths, synovial bags, muscle blocks and sesamoid bones. Fascia are connective tissue membranes that limit the subcutaneous fatty tissue, covering the muscles and some internal organs.
The first includes the entire human skeletal muscles, which provide the ability to perform voluntary movements, the muscles of the tongue, the upper third of the esophagus, and some others, the muscle of the heart (myocardium), which has its own characteristics (composition of proteins, the nature of contraction, etc.). Smooth muscles include the muscle layers of the internal organs and walls of human blood vessels, which provide the ability to perform a number of important physiological functions.
Structural elements of all types of muscles are muscle fibers. Striated muscle fibers in skeletal muscles form bundles connected to each other by layers of connective tissue. At their ends, muscle fibers are intertwined with tendon fibers, through which muscle traction is transmitted to the bones of the skeleton. Striated muscle fibers are giant multinucleated cells, the diameter of which varies from 10 to 100 microns, and the length often corresponds to the length of the muscles, reaching, for example, 12 cm in some human muscles. The fiber is covered with an elastic membrane - sarcolemma and consists of sarcoplasm, the structural elements of which are organelles such as mitochondria, ribosomes, tubules and vesicles of the sarcoplasmic reticulum and the so-called T-systems, various inclusions, etc. In the sarcoplasm, usually in the form of bundles, there are many filamentous formations with a thickness of 0.5 to several microns - myofibrils that have , like the entire fiber as a whole, is transversely striated. Each myofibril is divided into several hundred sections 2.5-3 microns long, called sarcomeres. Each sarcomere, in turn, consists of alternating sections - disks that have unequal optical density and give the myofibrils and muscle fiber as a whole a characteristic transverse striation, clearly visible when observed in a phase-contrast microscope. Darker discs have the ability to birefringence and are called anisotropic, or discs A. Lighter discs do not have this ability and are called isotropic, or discs I. The middle part of disc A is occupied by a zone of weaker birefringence - zone H. Disc I is divided into 2 equal parts by a dark Z-plate delimiting one sarcomere from another. Each sarcomere has two types of filaments (filaments) consisting of muscle proteins: thick myosin and thin actin. Smooth muscle fibers have a slightly different structure. They are spindle-shaped mononuclear cells, devoid of transverse striation. Their length usually reaches 50-250 microns (in the uterus - up to 500 microns), width - 4-8 microns; myofilaments in them are usually not combined into separate myofibrils, but are located along the length of the fiber in the form of many single actin filaments. There is no ordered system of myosin filaments in smooth muscle cells. In the smooth muscles of mollusks, paramyosin fibers (tropomyosin A) seem to play the most important role in the obturator function.
The chemical composition of muscles varies depending on the type and functional state of the muscle and a number of other factors. The main substances that make up the human striated muscles and their content (in% of wet weight) are presented below:
- Water 72-80
- dense substances 20-28
Including:
- Squirrels 16,5-20,9
- Glycogen 0,3-3,0
- Phosphatides 0,4-1,0
- Cholesterol 0,06-0,2
- Creatine + Creatine Phosphate 0,2-0,55
- Creatinine 0,003-0,005
- ATP 0,25-0,4
- Carnosine 0,2-0,3
- Carnitine 0,02-0,05
- Anzerin 0,09-0,15
- Free amino acids 0,1-0,7
- Lactic acid 0,01-0,02
- Ash 1,0-1,5
On average, about 75% of raw muscle mass is water. The main amount of dense substances falls on the share of proteins. There are myofibrillar (contractile) proteins - myosin, actin and their complex - actomyosin, tropomyosin and a number of so-called minor proteins (a and b-actinins, troponin, etc.), and sarcoplasmic - globulins X, myogens, respiratory pigments, in particular myoglobin , nucleoproteins and enzymes involved in muscle metabolism. Of the other compounds, the most important are the extractive ones, which take part in the metabolism and exercise of the contractile function of the muscles: ATP, phosphocreatine, carnosine, anserine, etc.; phospholipids, which play an important role in the formation of cellular microstructures and in metabolic processes; nitrogen-free substances: glycogen and its decay products (glucose, lactic acid, etc.), neutral fats, cholesterol, etc.; minerals - salts of K, Na, Ca, Mg. Smooth muscles significantly differ in chemical composition from striated ones (lower content of contractual proteins - actomyosin, macroergic compounds, dipeptides, etc.).
Functional features of striated muscles. The striated muscles are richly supplied with various nerves, with the help of which the regulation of muscle activity is carried out by the nerve centers. The most important of them are: motor nerves, which conduct impulses to the muscles, causing its excitation and contraction; sensory nerves, through which information about its condition comes from the muscle to the nerve centers, and, finally, adaptive-trophic fibers of the sympathetic nervous system, which affect metabolism and slow down the development of muscle fatigue.
Each branch of the motor nerve, which innervates a whole group of muscle fibers that form the so-called motor unit, reaches a separate muscle fiber. All muscle fibers that make up such a unit contract almost simultaneously when excited. Under the influence of a nerve impulse, a mediator, acetylcholine, is released at the endings of the motor nerve, which interacts with the cholinergic receptor of the postsynaptic membrane (synapses). As a result, there is an increase in the permeability of the membrane for Na and K ions, which, in turn, causes its depolarization (the appearance of a postsynaptic potential). After that, an excitation wave (electronegativity wave) arises in neighboring sections of the muscle fiber membrane, which propagates along the skeletal muscle fiber, usually at a speed of several meters per 1 second. As a result of excitation, the muscle changes its elastic properties. If the attachment points of the muscle are not fixed motionless, it shortens (contracts). In this case, the muscle performs a certain mechanical work. If the points of attachment of the muscle are motionless, tension develops in it. Between the appearance of excitation and the appearance of a wave of contraction or a wave of tension, some time passes, called the latent period. The contraction of the muscle is accompanied by the release of heat, which continues for a certain time and after their relaxation.
In human muscles, the existence of “slow” muscle fibers (they include “red”, containing the respiratory pigment myoglobin) and “fast” (“white”, without myoglobin), differing in the speed of the contraction wave and its duration, has been established. In "slow" fibers, the duration of the contraction wave is approximately 5 times longer, and the conduction velocity is 2 times less than in "fast" fibers. Almost all skeletal muscles are of mixed type, i.e. contain both "fast" and "slow" fibers. Depending on the nature of the irritation, either a single - phase - contraction of muscle fibers occurs, or a long - tetanic. Tetanus occurs when a series of stimuli enters the muscle with such a frequency that each subsequent stimulus still finds the muscle in a state of contraction, as a result of which the summation of contractile waves occurs. NOT. Vvedensky found that an increase in the frequency of stimulation causes an increase in tetanus, but only up to a certain limit, which he calls "optimum". Further increase in stimulation reduces tetanic contraction (pessimum). The development of tetanus is of great importance in the contraction of "slow" muscle fibers. In muscles with a predominance of "fast" fibers, the maximum contraction is usually the result of the summation of contractions of all motor units, into which nerve impulses arrive, as a rule, not simultaneously, asynchronously.
In striated muscles, the existence of so-called purely tonic fibers has also been established. Tonic fibers are involved in maintaining "untiring" muscle tone. A tonic contraction is a slowly developing continuous contraction that can be maintained for a long time without significant energy costs and is expressed in an "untiring" resistance to external forces that seek to stretch the muscular organ. Tonic fibers react to a nerve impulse with a wave of contraction only locally (at the site of irritation). However, due to the large number of terminal motor plaques, the tonic fiber can be excited and contracted as a whole. The contraction of such fibers develops so slowly that even at very low frequencies of stimulation, individual contraction waves overlap and merge into a long-term sustained shortening. Long-term resistance of tonic fibers, as well as slow phase fibers, to tensile forces is provided not only by elastic stress, but also by an increase in the viscosity of muscle proteins.
To characterize the contractile function of muscles, the concept is used "absolute power", which is a quantity proportional to muscle section, directed perpendicular to its fibers, and is expressed in kg/cm2. So, for example, the absolute strength of the human biceps muscle is 11.4, the gastrocnemius is 5.9 kg/cm2.
Systematic enhanced work of muscles (training) increases their mass, strength and performance. However, excessive work leads to the development of fatigue, i.e. to a decrease in muscle performance. Muscle inactivity leads to their atrophy.
Functional features of smooth muscles
The smooth muscles of the internal organs differ significantly from the skeletal muscles 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 muscle 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 ability of smooth muscles to change length without increasing tension (filling hollow organs, such as the bladder, stomach, etc.) is essential for the body.
Human skeletal muscles
The skeletal muscles of a person, different in shape, size, position, make up over 40% of the mass of his body. During contraction, the muscle is shortened, which can reach 60% of their length; the longer the muscle (the longest muscle of the tailor's body reaches 50 cm), the greater the range of motion. Contraction of the domed muscle (for example, the diaphragm) causes its flattening, contraction of the annular muscles (sphincters) is accompanied by a narrowing or closing of the hole. The muscles of the radial direction, on the contrary, cause the expansion of the holes during contraction. If the muscles are located between the bone protrusions and the skin, their contraction causes a change in the skin relief.
All skeletal, or somatic (from the Greek soma - body), muscles, according to the topographic and anatomical principle, can be divided into the muscles of the head, among which there are mimic and chewing muscles that affect the lower jaw, muscles of the neck, trunk and limbs. The muscles of the trunk cover the chest, make up the walls of the abdominal cavity, as a result of which they are divided into the muscles of the chest, abdomen and back. The dismemberment of the skeleton of the limbs serves as the basis for isolating the corresponding muscle groups: for the upper limb, these are the muscles of the shoulder girdle, shoulder, forearm and hand; for the lower limb - the muscles of the pelvic girdle, thigh, lower leg, foot.
A person has about 500 muscles associated with the skeleton. Among them, some are large (for example, the quadriceps femoris), others are small (for example, short back muscles). Joint work of muscles is carried out according to the principle of synergy, although individual functional muscle groups work as antagonists when performing certain movements. So, in front of the shoulder are the biceps and brachial muscles, which perform flexion of the forearm in the elbow joint, and behind is the triceps muscle of the shoulder, the contraction of which causes the opposite movement - extension of the forearm.
In the joints of a spherical shape, simple and complex movements occur. For example, in the hip joint, flexion of the hip is caused by the lumboiliac muscle, extension - by the gluteus maximus. The thigh is abducted by contraction of the gluteus medius and minimus muscles, and is adducted with the help of five muscles of the medial thigh group. Along the circumference of the hip joint, there are also muscles that cause the rotation of the thigh in and out.
The most powerful muscles are located on the torso. These are the muscles of the back - the rectifier of the body, the muscles of the abdomen, which make up a special formation in a person - the abdominal press. In connection with the vertical position of the body, the muscles of the lower limb of a person have become stronger, since, in addition to participating in locomotion, they provide support for the body. The muscles of the upper limb in the process of evolution, on the contrary, have become more dexterous, guaranteeing the performance of fast and precise movements.
Based on the analysis of the spatial position and functional activity of the muscles, modern science also uses the following combination of them: a muscle group that performs movements of the trunk, head and neck; a group of muscles that performs movements of the shoulder girdle and free upper limb; muscles of the lower limb. Within these groups, smaller ensembles are distinguished.
Muscle pathology
Violations of the contractile function of muscles and their ability to develop and maintain tone are observed with hypertension, myocardial infarction, myodystrophy, atony of the uterus, intestines, bladder, with various forms of paralysis (for example, after suffering poliomyelitis), etc. Pathological changes in the functions of muscle organs can occur in connection with disorders of the nervous or humoral regulation, damage to individual muscles or their sections (for example, with myocardial infarction) and, finally, at the cellular and subcellular levels. In this case, there may be a metabolic disorder (primarily the enzymatic system of regeneration of high-energy compounds - mainly ATP) or a change in the protein contractile substrate. These changes may be due to insufficient formation of muscle proteins on the basis of a violation of the synthesis of the corresponding informational, or matrix, RNA, i.e. birth defects in the DNA structure of the chromosomal apparatus of cells. The last group of diseases, therefore, belongs to the number of hereditary diseases.
Sarcoplasmic proteins of skeletal and smooth muscles are of interest not only from the point of view of their possible participation in the development of a viscous aftereffect. Many of them have enzymatic activity and are involved in cellular metabolism. When muscle organs are damaged, for example, with myocardial infarction or impaired permeability of the surface membranes of muscle fibers, enzymes (creatine kinase, lactate dehydrogenase, aldolase, aminotransferases, etc.) can be released into the blood. Thus, the determination of the activity of these enzymes in blood plasma in a number of diseases (myocardial infarction, myopathy, etc.) is of serious clinical interest.
Lecture 6. ODA. MUSCULAR SYSTEM
1. Structure and functions of skeletal muscles
2. Classification of skeletal muscles
4. Muscles of the human body
The structure and function of skeletal muscles
Skeletal muscles are the active part of the musculoskeletal system. These muscles are built from striated (striated) muscle fibers. Muscles are attached to the bones of the skeleton and, with their contraction (shortening), set the bone levers in motion. Muscles hold the position of the body and its parts in space, move the bone levers when walking, running and other movements, perform chewing, swallowing and breathing movements, participate in the articulation of speech and facial expressions, and generate heat.
There are about 600 muscles in the human body, most of which are paired. The mass of skeletal muscles in an adult reaches 30-40% of body weight. In newborns and children, muscles account for up to 20-25% of body weight. In the elderly and senile age, the mass of muscle tissue does not exceed 20-30%.
Each muscle is made up of a large number of muscle fibers. Each fiber has a thin shell - endomysium, formed by a small amount of connective tissue fibers. The bundles of muscle fibers are surrounded by loose fibrous connective tissue, called the internal perimysium, which separates the muscle bundles from each other. Outside, the muscle also has a thin connective tissue sheath - the outer perimysium, closely fused with the inner perimysium by bundles of connective tissue fibers penetrating into the muscle. The connective tissue fibers surrounding the muscle fibers and their bundles, going beyond the muscle, form a tendon.
In each muscle, a large number of blood vessels branch out, through which blood brings nutrients and oxygen to the muscle fibers, and carries away metabolic products. The source of energy for muscle fibers is glycogen. During its breakdown, adenosine triphosphoric acid (ATP) is produced, which is used for muscle contraction. The nerves entering the muscle contain sensory and motor fibers.
Skeletal muscles have properties such as excitability, conductivity, and contractility. Muscles are able, under the influence of nerve impulses, to be excited, to come into a working (active) state. In this case, the excitation quickly spreads (conducted) from the nerve endings (effectors) to contractile structures - muscle fibers. As a result, the muscle contracts, shortens, sets the bone levers in motion.
In muscles, there is a contractile part (abdomen), built from striated muscle fibers, and tendon ends (tendons), which are attached to the bones of the skeleton. In some muscles, the tendons are woven into the skin (mimic muscles), attached to the eyeball or to neighboring muscles (in the muscles of the perineum). Tendons are formed from formed dense fibrous connective tissue and are very durable. In the muscles located on the limbs, the tendons are narrow and long. Many ribbon-like muscles have wide tendons, called aponeuroses.
Classification of skeletal muscles
Currently, muscles are classified according to their shape, structure, location and function.
Muscle shape. The most common muscles are fusiform and ribbon-shaped (Fig. 30). Fusiform muscles are located mainly on the limbs, where they act on long bony levers. Ribbon-like muscles have different widths, they are usually involved in the formation of the walls of the trunk, abdominal, chest cavities. Fusiform muscles can have two bellies, separated by an intermediate tendon (bigastric muscle), two, three and four initial parts - heads (biceps, triceps, quadriceps). There are muscles long and short, straight and oblique, round and square.
Muscle structure. Muscles can have a pinnate structure, when muscle bundles are attached to the tendon from one, two or more sides. These are single-feathered, double-feathered, multi-feathered muscles. The pennate muscles are built from a large number of short muscle bundles and have considerable strength. These are strong muscles. However, they can only shrink to a small length. At the same time, muscles with a parallel arrangement of long muscle bundles are not very strong, but they are able to shorten up to 50% of their length. These are dexterous muscles, they are present where movements are performed on a large scale.
According to the function performed and the effect on the joints, flexor and extensor muscles, adductors and abductors, constrictors (sphincters) and dilators are distinguished. Muscles are distinguished by their location in the human body: superficial and deep, lateral and medial, anterior and posterior.
3. Auxiliary apparatus of muscles
Muscles perform their functions with the help of auxiliary devices, which include fascia, fibrous and bone-fibrous channels, synovial bags, blocks.
Fascia are connective tissue sheaths of muscles. They divide the muscles into muscle partitions, eliminate the friction of the muscles one against the other.
Channels (fibrous and osteofibrous) are present in those places where the tendons are thrown over several joints (on the hand, foot). The channels serve to hold the tendons in a certain position during muscle contraction.
Synovial sheaths formed by a synovial membrane (membrane), one plate of which lines the walls of the canal, and the other surrounds the tendon and fuses with it. Both plates grow together at their ends, form a closed narrow cavity, which contains a small amount of fluid (synovia) and wets the synovial plates sliding one against the other.
Synovial (mucous) bags perform a function similar to synovial sheaths. Bags are closed sacs filled with synovial fluid or mucus, located at places where the tendon is thrown over a bony prominence or over the tendon of another muscle.
Blocks called bone protrusions (condyles, epicondyles), through which the muscle tendon is thrown. As a result, the angle of attachment of the tendon to the bone increases. This increases the force of the muscle on the bone.
Muscle work and strength
Muscles act on bone levers, set them in motion or hold parts of the body in a certain position. Each movement usually involves several muscles. Muscles that act in one direction are called synergists, those that act in different directions are called antagonists.
Muscles act on the bones of the skeleton with a certain force and perform work - dynamic or static. During dynamic work, bone levers change their position, move in space. During static work, the muscles tense up, but their length does not change, the body (or parts of it) is held in a certain fixed position. Such contraction of muscles without changing their length is called isometric contraction. Muscle contraction accompanied by a change in its length is called isotonic contraction.
Taking into account the place of application of muscle force to the bone lever and their other characteristics, in biomechanics, levers of the first kind and levers of the second order are distinguished (Fig. 32). For a lever of the first kind, the point of application of muscle force and the point of resistance (weight of the body, weight of the load) are located on opposite sides of the fulcrum (from the joint). An example of a lever of the first kind is the head, which rests on the atlas (fulcrum). The severity of the head (its front part) is located on one side of the axis of the atlantooccipital articulation, and the place of application of the strength of the occipital muscles to the occipital bone is on the other side of the axis. The balance of the head is achieved under the condition that the torque of the applied force (the product of the force of the occipital muscles and the length of the shoulder, equal to the distance from the fulcrum to the place of application of the force) will correspond to the torque of the gravity of the front of the head (the product of the force of gravity and the length of the shoulder, equal to the distance from fulcrum to the point of application of gravity).
For a lever of the second kind, both the point of application of muscle force and the point of resistance (gravity) are on the same side of the fulcrum (joint axis). In biomechanics, there are two types of lever of the second kind. In the first type of lever of the second kind, the leverage for application of muscle force is longer than the leverage for resistance. For example, a human foot. The shoulder for applying the force of the triceps muscle of the lower leg (the distance from the calcaneal tuber to the fulcrum - the heads of the metatarsal bones) is longer than the shoulder for applying the force of gravity of the body (from the axis of the ankle joint to the fulcrum). In this lever, there is a gain in the applied muscle force (the lever is longer) and a loss in the speed of movement of the body's gravity (the lever is shorter). In the second type of lever of the second kind, the shoulder for applying muscle force will be shorter than the shoulder for resistance (applying gravity). The shoulder from the elbow joint to the insertion of the biceps tendon is shorter than the distance from this joint to the hand, where gravity is applied. In this case, there is a gain in both the range of movement of the hand (long arm) and a loss in the force acting on the bone lever (short arm of force application).
Force of the muscle is determined by the mass (weight) of the load that this muscle can lift to a certain height with its maximum contraction. This force is called the lifting force of the muscle. The lifting force of a muscle depends on the number and thickness of its muscle fibers. In humans, muscle strength is 5-10 kg per 1 sq. see the physiological diameter of the muscle. For the morphological and functional characteristics of muscles, there is the concept of their anatomical and physiological cross sections (Fig. 33). The physiological diameter of a muscle is the sum of the cross section (areas) of all muscle fibers of a given muscle. The anatomical diameter of a muscle is the size (area) of its cross section at its widest point. In a muscle with longitudinally arranged fibers (ribbon-like, fusiform muscles), the anatomical and physiological diameters will be the same. With an oblique orientation of a large number of short muscle bundles, as is the case with pennate muscles, the physiological diameter will be greater than the anatomical one.
The rotating force of a muscle depends not only on its physiological or anatomical diameter, or lifting force, but also on the angle of attachment of the muscle to the bone. The greater the angle at which a muscle attaches to a bone, the greater the effect it can have on that bone. Blocks are used to increase the angle of attachment of muscles to the bone.
Muscles of the human body
Depending on the location in the body and for the convenience of studying, the muscles of the head, neck, torso are distinguished; muscles of the upper and lower limbs.
Muscles located in different areas of the human body not only perform different functions, but also have their own structural features. On the extremities, with their long bony levers adapted for movement, grasping and holding various objects, the muscles are, as a rule, spindle-shaped, with a longitudinal or oblique arrangement of muscle fibers, and narrow and long tendons. In the region of the trunk, in the formation of its walls, ribbon-shaped muscles with wide flat tendons participate. Such wide tendons are called aponeuroses. In the head region, the masticatory muscles at one end begin on the fixed bones of the base of the skull, and at the other end they are attached to the only movable part of the skull - the lower jaw. The mimic muscles begin on the bones of the skull and attach to the skin. With the contraction of facial muscles, the relief of the skin of the face changes, and facial expressions are formed.
The main element of skeletal muscle is the muscle cell. Due to the fact that the muscle cell in relation to its cross section (0.05-0.11 mm) is relatively long (biceps fibers, for example, have a length of up to 15 cm), it is also called a muscle fiber.
Skeletal muscle consists of a large number of these structural elements, which make up 85-90% of its total mass. For example, the biceps contains more than one million fibers.
Between the muscle fibers is a thin network of small blood vessels (capillaries) and nerves (approximately 10% of the total muscle mass). From 10 to 50 muscle fibers are connected into a bundle. The bundles of muscle fibers form the skeletal muscle. Muscle fibers, bundles of muscle fibers and muscles are shrouded in connective tissue.
Muscle fibers at their ends pass into tendons. Through the tendons attached to the bones, muscle force acts on the bones of the skeleton. Tendons and other elastic elements of the muscle, in addition, have elastic properties. With a high and sharp internal load (muscle traction force) or with a strong and sudden external force action, the elastic elements of the muscle stretch and thereby soften the force effects, distributing them over a longer period of time.
Therefore, after a good warm-up in the muscles, ruptures of muscle fibers and separations from the bones rarely occur. Tendons have a much higher tensile strength (about 7000 N/sq cm) than muscle tissue (about 60 N/sq cm), where N is a Newton, so they are much thinner than the muscle belly. Muscle fiber contains a basic substance called sarcoplasm. The sarcoplasm contains mitochondria (30-35% of the fiber mass), in which metabolic processes take place and energy-rich substances, such as phosphates, glycogen and fats, accumulate. Thin muscle filaments (myofibrils) are immersed in the sarcoplasm, lying parallel to the long axis of the muscle fiber.
Myofibrils together make up approximately 50% of the mass of the fiber, their length is equal to the length of the muscle fibers, and they are, in fact, the contractile elements of the muscle. They consist of small, sequentially included elementary blocks, called sarcomeres (Fig. 33).
Rice. 33. Skeletal muscle diagram: muscle (up to 5 cm), bundle of muscle fibers (0.5 mm), muscle fiber (0.05-0.1 mm), myofibril (0.001-0.003 mm). The numbers in brackets indicate the approximate size of the cross-section of the building elements of the muscle.
Since the length of the sarcomere at rest is approximately only 0.0002 mm, in order, for example, to form chains of links of biceps myofibrils 10-15 cm long, it is necessary to “connect” a huge number of sarcomeres. The thickness of muscle fibers depends mainly on the number and cross section of myofibrils.
In skeletal muscle myofibrils, there is a regular alternation of lighter and darker areas. Therefore, often skeletal muscles are called striated. The myofibril is made up of identical repeating elements, the so-called sarcomeres. The sarcomere is bounded on both sides by Z-discs. Thin actin filaments are attached to these discs on both sides. Actin filaments have a low density and therefore appear more transparent or lighter under a microscope. These transparent, bright areas, located on both sides of the Z-disk, are called isotropic zones (or I-zones).
In the middle of the sarcomere is a system of thick filaments built primarily from another contractile protein, myosin. This part of the sarcomere is denser and forms a darker anisotropic zone (or A-zone). During contraction, myosin becomes able to interact with actin and begins to pull the actin filaments towards the center of the sarcomere. As a result of this movement, the length of each sarcomere and the entire muscle as a whole decreases. It is important to note that with such a system of motion generation, called the sliding filament system, the length of the filaments (neither actin filaments nor myosin filaments) changes. Shortening is a consequence of only the movement of the threads relative to each other. The signal for the start of muscle contraction is an increase in Ca 2+ concentration inside the cell. The concentration of calcium in the cell is regulated by special calcium pumps built into the outer membrane and the membrane of the sarcoplasmic reticulum, which wraps around the myofibrils.
motor unit(DE) - a group of muscle fibers innervated by one motor neuron. The muscle and its nerve drive consist of a large number of parallel DUs (Fig. 34).
Rice. 34. The structure of the motor unit: 1 - spinal cord; 2 - motoneurons; 3 - axons; 4 - muscle fibers
Under normal conditions, the DE works as a whole: the impulses sent by the motor neuron activate all the muscle fibers that make up it. Due to the fact that the muscle consists of many MUs (in large muscles up to several hundred), it can work not with the whole mass, but in parts. This property is used in the regulation of the strength and speed of muscle contraction. Under natural conditions, the frequency of impulses sent by motoneurons to the MU is in the range of 5–35 impulses/s, only with maximum muscular effort can it be possible to register a discharge frequency above 50 impulses/s.
DE components have different lability: axon - up to 1000 imp./s, muscle fiber - 250-500, myoneural synapse - 100-150, motor neuron body - up to 50 imp./s. The fatigue of a component is the higher, the lower its lability.
Distinguish fast And slow DE. Fast ones have great strength and speed of contraction in a short time, high activity of glycolytic processes, slow ones work under conditions of high activity of oxidative processes for a long time, with less force and speed of contraction. The former are quickly tired, contain a lot of glycogen, the latter are hardy - they have a lot of mitochondria. Slow MUs are active with any muscle tension, while fast MUs are active only with strong muscle tension.
Based on the analysis of muscle fiber enzymes, they are classified into three types: type I, type IIa, type IIb.
Depending on the rate of contraction, aerobic and anaerobic capacity, the concepts are used: slow-twitch, oxidative type (MO), fast-twitch, oxidative-glycolytic type (GOD) and fast-twitch, glycolytic type (BG).
There are other classifications of DE. So, based on two parameters - a decrease in intermittent tetanus and resistance to fatigue - MUs are divided into three groups (Burke, 1981): slowly twitching, immune to fatigue (type S); fast twitch fatigue resistant (FR type) and fast twitch fatigue susceptible (FF type).
Type I fibers correspond to MO type fibers, type IIa fibers correspond to BOG type fibers, and type IIb fibers correspond to BG type fibers. Muscle fibers of the MO type belong to the S type MU, GOD type fibers to the FR type MU, and BG type fibers to the FF type MU.
Each human muscle contains a combination of all three types of fibers. DE type FF is characterized by the greatest force of contraction, the shortest duration of contraction and the greatest susceptibility to fatigue.
Speaking about the proportions of various muscle fibers in humans, it should be noted that both men and women have slightly more slow fibers (according to various authors -
from 52 to 55%).
There is a strict relationship between the number of slow and fast twitch fibers in muscle tissue and athletic performance at sprint and long distance distances.
The calf muscles of world marathon champions contain 93-99% slow fibers, while the strongest sprinters in the world have more fast fibers in these muscles (92%).
In an untrained person, the number of motor units that can be mobilized at maximum power stress usually does not exceed 25–30%, and in people well trained for power loads, the number of motor units involved in the work can exceed 80–90%. This phenomenon is based on the adaptation of the central nervous system, which leads to an increase in the ability of motor centers to mobilize a greater number of motor neurons and to an improvement in intermuscular coordination (Fig. 35).
Rice. 35. Characteristics of motor units
