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Schematic structure of a motor neuron. What is a neuron? body of the nerve cell

Schematic structure of a motor neuron. What is a neuron? body of the nerve cell

17.10.2019

Each structure in the human body consists of specific tissues inherent in the organ or system. In the nervous tissue - a neuron (neurocyte, nerve, neuron, nerve fiber). What are brain neurons? This is a structural and functional unit of the nervous tissue, which is part of the brain. In addition to the anatomical definition of a neuron, there is also a functional one - it is a cell excited by electrical impulses that is capable of processing, storing and transmitting information to other neurons using chemical and electrical signals.

The structure of the nerve cell is not so complicated, in comparison with the specific cells of other tissues, it also determines its function. neurocyte consists of a body (another name is soma), and processes - an axon and a dendrite. Each element of the neuron performs its function. The soma is surrounded by a layer of adipose tissue that allows only fat-soluble substances to pass through. Inside the body is the nucleus and other organelles: ribosomes, endoplasmic reticulum and others.

In addition to the neurons themselves, the following cells predominate in the brain, namely: glial cells. They are often referred to as brain glue for their function: glia serve as a support function for neurons, providing an environment for them. Glial tissue allows the nervous tissue to regenerate, nourish and helps in creating a nerve impulse.

The number of neurons in the brain has always been of interest to researchers in the field of neurophysiology. Thus, the number of nerve cells ranged from 14 billion to 100. The latest research by Brazilian experts found that the number of neurons averages 86 billion cells.

offshoots

The tools in the hands of the neuron are the processes, thanks to which the neuron is able to perform its function as a transmitter and store of information. It is the processes that form a wide nervous network, which allows the human psyche to unfold in all its glory. There is a myth that a person’s mental abilities depend on the number of neurons or on the weight of the brain, but this is not so: those people whose fields and subfields of the brain are highly developed (several times more) become geniuses. Due to this, the fields responsible for certain functions will be able to perform these functions more creatively and faster.

axon

An axon is a long process of a neuron that transmits nerve impulses from the soma of the nerve to other similar cells or organs innervated by a certain section of the nerve column. Nature endowed vertebrates with a bonus - myelin fiber, in the structure of which there are Schwann cells, between which there are small empty areas - Ranvier's intercepts. Along them, like a ladder, nerve impulses jump from one area to another. This structure allows you to speed up the transfer of information at times (up to about 100 meters per second). The speed of movement of an electrical impulse along a fiber that does not have myelin averages 2-3 meters per second.

Dendrites

Another type of processes of the nerve cell - dendrites. Unlike a long and unbroken axon, a dendrite is a short and branched structure. This process is not involved in the transmission of information, but only in its receipt. So, excitation comes to the body of a neuron with the help of short branches of dendrites. The complexity of the information a dendrite is able to receive is determined by its synapses (specific nerve receptors), namely its surface diameter. Dendrites, due to the huge number of their spines, are able to establish hundreds of thousands of contacts with other cells.

Metabolism in a neuron

A distinctive feature of nerve cells is their metabolism. Metabolism in the neurocyte is distinguished by its high speed and the predominance of aerobic (oxygen-based) processes. This feature of the cell is explained by the fact that the work of the brain is extremely energy-intensive, and its need for oxygen is great. Despite the fact that the weight of the brain is only 2% of the weight of the entire body, its oxygen consumption is approximately 46 ml / min, which is 25% of the total body consumption.

The main source of energy for brain tissue, in addition to oxygen, is glucose where it undergoes complex biochemical transformations. Ultimately, a large amount of energy is released from sugar compounds. Thus, the question of how to improve the neural connections of the brain can be answered: eat foods containing glucose compounds.

Functions of a neuron

Despite the relatively simple structure, the neuron has many functions, the main of which are the following:

perception of irritation;
stimulus processing;
impulse transmission;
formation of a response.

Functionally, neurons are divided into three groups:

Afferent(sensitive or sensory). The neurons of this group perceive, process and send electrical impulses to the central nervous system. Such cells are anatomically located outside the CNS, but in the spinal neuronal clusters (ganglia), or the same clusters of cranial nerves.

Intermediaries(Also, these neurons that do not extend beyond the spinal cord and brain are called intercalary). The purpose of these cells is to provide contact between neurocytes. They are located in all layers of the nervous system.

Efferent(motor, motor). This category of nerve cells is responsible for the transmission of chemical impulses to the innervated executing organs, ensuring their performance and setting their functional state.

In addition, another group is functionally distinguished in the nervous system - inhibitory (responsible for inhibiting cell excitation) nerves. Such cells counteract the propagation of electrical potential.

Classification of neurons

Nerve cells are diverse as such, so neurons can be classified based on their different parameters and attributes, namely:

Body shape. In different parts of the brain, neurocytes of different soma shapes are located:
- stellate;
- spindle-shaped;
- pyramidal (Betz cells).
By the number of shoots:
- unipolar: have one process;
- bipolar: two processes are located on the body;
- multipolar: three or more processes are located on the soma of such cells.
Contact features of the neuron surface:
- axo-somatic. In this case, the axon contacts the soma of the neighboring cell of the nervous tissue;
- axo-dendritic. This type of contact involves the connection of an axon and a dendrite;
- axo-axonal. The axon of one neuron has connections with the axon of another nerve cell.

Types of neurons

In order to carry out conscious movements, it is necessary that the impulse formed in the motor convolutions of the brain be able to reach the necessary muscles. Thus, the following types of neurons are distinguished: central motor neuron and peripheral one.

The first type of nerve cells originates from the anterior central gyrus, located in front of the largest sulcus of the brain - namely, from Betz's pyramidal cells. Further, the axons of the central neuron deepen into the hemispheres and pass through the inner capsule of the brain.

Peripheral motor neurocytes are formed by motor neurons of the anterior horns of the spinal cord. Their axons reach various formations, such as plexuses, spinal nerve clusters, and, most importantly, the performing muscles.

Development and growth of neurons

A nerve cell originates from a precursor cell. Developing, the first begin to grow axons, dendrites mature somewhat later. At the end of the evolution of the neurocyte process, a small, irregularly shaped densification is formed near the soma of the cell. This formation is called a growth cone. It contains mitochondria, neurofilaments and tubules. The receptor systems of the cell gradually mature and the synaptic regions of the neurocyte expand.

Conducting paths

The nervous system has its spheres of influence throughout the body. With the help of conductive fibers, the nervous regulation of systems, organs and tissues is carried out. The brain, thanks to a wide system of pathways, completely controls the anatomical and functional state of any structure of the body. Kidneys, liver, stomach, muscles and others - all this is inspected by the brain, carefully and painstakingly coordinating and regulating every millimeter of tissue. And in the event of a failure, it corrects and selects the appropriate behavior model. Thus, thanks to the pathways, the human body is distinguished by autonomy, self-regulation and adaptability to the external environment.

Pathways of the brain

The pathway is a collection of nerve cells whose function is to exchange information between different parts of the body.

Associative nerve fibers. These cells connect various nerve centers that are located in the same hemisphere.
commissural fibers. This group is responsible for the exchange of information between similar centers of the brain.
Projective nerve fibers. This category of fibers articulates the brain with the spinal cord.
exteroceptive pathways. They carry electrical impulses from the skin and other sense organs to the spinal cord.
Proprioceptive. This group of pathways carry signals from tendons, muscles, ligaments, and joints.
Interoceptive pathways. The fibers of this tract originate from the internal organs, vessels and intestinal mesentery.

Interaction with neurotransmitters

Neurons of different locations communicate with each other using electrical impulses of a chemical nature. So, what is the basis of their education? There are so-called neurotransmitters (neurotransmitters) - complex chemical compounds. On the surface of the axon is a nerve synapse - a contact surface. On one side is the presynaptic cleft, and on the other is the postsynaptic cleft. There is a gap between them - this is the synapse. On the presynaptic part of the receptor, there are sacs (vesicles) containing a certain amount of neurotransmitters (quantum).

When the impulse approaches the first part of the synapse, a complex biochemical cascade mechanism is initiated, as a result of which the sacs with mediators are opened, and the quanta of mediator substances smoothly flow into the gap. At this stage, the impulse disappears and reappears only when the neurotransmitters reach the postsynaptic cleft. Then biochemical processes are activated again with the opening of the gate for mediators, and those, acting on the smallest receptors, are converted into an electrical impulse, which goes further into the depths of the nerve fibers.

Meanwhile, different groups of these same neurotransmitters are distinguished, namely:

Inhibitory neurotransmitters are a group of substances that have an inhibitory effect on excitation. These include:
- gamma-aminobutyric acid (GABA);
- glycine.
Excitatory mediators:
- acetylcholine;
- dopamine;
- serotonin;
- norepinephrine;
- adrenalin.

Do nerve cells recover

For a long time it was thought that neurons were incapable of dividing. However, such a statement, according to modern research, turned out to be false: in some parts of the brain, the process of neurogenesis of the precursors of neurocytes occurs. In addition, brain tissue has an outstanding capacity for neuroplasticity. There are many cases when a healthy part of the brain takes over the function of a damaged one.

Many experts in the field of neurophysiology wondered how to restore brain neurons. Recent research by American scientists revealed that for the timely and proper regeneration of neurocytes, you do not need to use expensive drugs. To do this, you just need to make the right sleep schedule and eat right with the inclusion of B vitamins and low-calorie foods in the diet.

If there is a violation of the neural connections of the brain, they are able to recover. However, there are serious pathologies of nerve connections and pathways, such as motor neuron disease. Then it is necessary to turn to specialized clinical care, where neurologists can find out the cause of the pathology and make the right treatment.

People who have previously used or used alcohol often ask the question of how to restore brain neurons after alcohol. The specialist would answer that for this it is necessary to systematically work on your health. The complex of activities includes a balanced diet, regular exercise, mental activity, walks and travel. It has been proven that the neural connections of the brain develop through the study and contemplation of information that is categorically new to a person.

In the conditions of a glut of unnecessary information, the existence of a fast food market and a sedentary lifestyle, the brain is qualitatively amenable to various damages. Atherosclerosis, thrombotic formation on the vessels, chronic stress, infections - all this is a direct path to clogging the brain. Despite this, there are drugs that restore brain cells. The main and popular group is nootropics. Preparations of this category stimulate the metabolism in neurocytes, increase resistance to oxygen deficiency and have a positive effect on various mental processes (memory, attention, thinking). In addition to nootropics, the pharmaceutical market offers drugs containing nicotinic acid, vascular wall strengthening agents, and others. It should be remembered that the restoration of neural connections in the brain when taking various drugs is a long process.

The effect of alcohol on the brain

Alcohol has a negative effect on all organs and systems, and especially on the brain. Ethyl alcohol easily penetrates the protective barriers of the brain. The metabolite of alcohol, acetaldehyde, is a serious threat to neurons: alcohol dehydrogenase (an enzyme that processes alcohol in the liver) pulls more fluid, including water, from the brain during processing by the body. Thus, alcohol compounds simply dry the brain, pulling water out of it, as a result of which brain structures atrophy and cell death occurs. In the case of a single use of alcohol, such processes are reversible, which cannot be said about chronic alcohol intake, when, in addition to organic changes, stable pathocharacterological features of an alcoholic are formed. More detailed information about how "The Effect of Alcohol on the Brain" happens.

In this article we will talk about the neurons of the brain. The neurons of the cerebral cortex is the structural and functional unit of the entire general nervous system.

Such a cell has a very complex structure, high specialization, and if we talk about its structure, then the cell consists of a nucleus, a body and processes. There are approximately 100 billion of these cells in the human body.

Functions

Any cells that are located in the human body are necessarily responsible for one or another of its functions. Neurons are no exception.

They, like other brain cells, are required to maintain their own structure and some functions, as well as adapt to possible changes in conditions, and, accordingly, carry out regulatory processes on cells that are in close proximity.

The main function of neurons is the processing of important information, namely its receipt, conduction, and then transmission to other cells. Information comes through synapses that have receptors for sensory organs or some other neurons.

Also, in some situations, the transfer of information can occur directly from the external environment with the help of so-called specialized dendrites. Information is carried through axons, and its transmission is carried out by synapses.

Structure

Cell body. This part of the neuron is considered the most important and consists of the cytoplasm and the nucleus, which create the protoplasm, outside it is limited to a kind of membrane consisting of a double layer of lipids.

In turn, such a layer of lipids, which is also commonly called the biolipid layer, consists of hydrophobic tails and the same heads. It should be noted that such lipids are tails to each other, and thus create a kind of hydrophobic layer that is able to pass through itself only substances that dissolve in fats.

On the surface of the membrane are proteins that are in the form of globules. On such membranes there are outgrowths of polysaccharides, with the help of which the cell has a good opportunity to perceive irritations of external factors. Integral proteins are also present here, which actually penetrate the entire surface of the membrane through and through, and in them, in turn, ion channels are located.

Neuronal cells of the cerebral cortex consist of bodies, the diameter ranges from 5 to 100 microns, which contain a nucleus (having many nuclear pores), as well as some organelles, including a fairly strongly developing rough ER with active ribosomes .

Also, processes are included in each individual cell of a neuron. There are two main types of processes - axon and dendrites. A feature of the neuron is that it has a developed cytoskeleton, which is actually able to penetrate into its processes.

Thanks to the cytoskeleton, the necessary and standard shape of the cell is constantly maintained, and its threads act as a kind of "rails" through which organelles and substances are transported, which are packed into membrane vesicles.

Dendrites and axon. The axon looks like a rather long process, which is perfectly adapted to the processes aimed at excitation of a neuron from the human body.

Dendrites look completely different, if only because their length is much shorter, and they also have overly developed processes that play the role of the main site where inhibitory synapses begin to appear, which can thus affect the neuron, which within a short period of time human neurons are excited.

Typically, a neuron is made up of more dendrites at a time. As there is only one axon. One neuron has connections with many other neurons, sometimes there are about 20,000 such connections.

Dendrites divide in a dichotomous way, in turn, axons are able to give collaterals. Almost every neuron contains several mitochondria at the branch nodes.

It is also worth noting the fact that dendrites do not have any myelin sheath, while axons can have such an organ.

A synapse is a place where contact is made between two neurons or between an effector cell that receives a signal and the neuron itself.

The main function of such a component neuron is the transmission of nerve impulses between different cells, while the frequency of the signal may vary depending on the rate and types of transmission of this signal.

It should be noted that some synapses are able to cause neuron depolarization, while others, on the contrary, hyperpolarize. The first type of neurons are called excitatory, and the second - inhibitory.

As a rule, in order for the process of excitation of a neuron to begin, several excitatory synapses must act as stimuli at once.

Classification

According to the number and localization of dendrites, as well as the location of the axon, brain neurons are divided into unipolar, bipolar, axon-free, multipolar and pseudo-unipolar neurons. Now I would like to consider each of these neurons in more detail.

Unipolar neurons have one small process, and are most often located in the sensory nucleus of the so-called trigeminal nerve, located in the middle part of the brain.

Axonless neurons they are small in size and localized in the immediate vicinity of the spinal cord, namely in the intervertebral galls and have absolutely no division of processes into axons and dendrites; all processes have almost the same appearance and there are no serious differences between them.

bipolar neurons consist of one dendrite, which is located in special sensory organs, in particular in the eye grid and the bulb, as well as only one axon;

Multipolar neurons have several dendrites and one axon in their own structure, and are located in the central nervous system;

Pseudo-unipolar neurons are considered peculiar in their own way, since at first only one process departs from the main body, which is constantly divided into several others, and such processes are found exclusively in the spinal ganglia.

There is also a classification of neurons according to the functional principle. So, according to such data, efferent neurons, afferent, motor, and also interneurons are distinguished.

Efferent neurons have in their composition non-ultimatum and ultimatum subspecies. In addition, they include the primary cells of human sensitive organs.

Afferent neurons. Neurons of this category include both primary cells of sensitive human organs and pseudo-unipolar cells that have dendrites with free endings.

Associative neurons. The main function of this group of neurons is the implementation of communication between afferent efferent types of neurons. Such neurons are divided into projection and commissural.

Development and growth

Neurons begin to develop from a small cell, which is considered its predecessor and stops dividing even before the first own processes are formed.

It should be noted that at the present time, scientists have not yet fully studied the issue of the development and growth of neurons, but they are constantly working in this direction.

In most cases, axons develop first, followed by dendrites. At the very end of the process, which begins to develop steadily, a thickening of a shape specific and unusual for such a cell is formed, and thus a path is paved through the tissue surrounding the neurons.

This thickening is commonly called the growth cone of nerve cells. This cone consists of some flattened part of the process of the nerve cell, which in turn is made up of a large number of rather thin spines.

Microspines have a thickness of 0.1 to 0.2 micromicrons, and in length they can reach 50 microns. Speaking directly about the flat and wide area of the cone, it should be noted that it tends to change its own parameters.

There are some gaps between the microspikes of the cone, which are completely covered by a folded membrane. The microspines move on a permanent basis, due to which, in case of damage, the neurons are restored and acquire the necessary shape.

I would like to note that each individual cell moves in its own way, so if one of them lengthens or expands, the second one can deviate in different directions or even stick to the substrate.

The growth cone is completely filled with membranous vesicles, which are characterized by too small size and irregular shape, as well as connections with each other.

In addition, the growth cone contains neurofilaments, mitochondria, and microtubules. Such elements have the ability to move at great speed.

If we compare the speeds of movement of the elements of the cone and the cone itself, it should be emphasized that they are approximately the same, and therefore it can be concluded that neither assembly nor any disturbances of microtubules are observed during the growth period.

Probably, new membrane material starts to be added already at the very end of the process. The growth cone is a site of rather rapid endocytosis and exocytosis, which is confirmed by the large number of vesicles that are located here.

As a rule, the growth of dendrites and axons is preceded by the moment of migration of neuron cells, that is, when immature neurons actually settle and begin to exist in the same permanent place.

Nerve cell Not to be confused with neutron.

Pyramidal cells of neurons in the mouse cerebral cortex

Neuron(nerve cell) is the structural and functional unit of the nervous system. This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure. There are over one hundred billion neurons in the human body.

Review

The complexity and diversity of the nervous system depends on the interaction between neurons, which, in turn, are a set of different signals transmitted as part of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions, which generate an electrical charge that travels along the neuron.

Structure

cell body

The neuron consists of a body with a diameter of 3 to 100 microns, containing a nucleus (with a large number of nuclear pores) and other organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), and processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). In the body of the neuron, a developed synthetic apparatus is revealed, the granular ER of the neuron stains basophilically and is known as the "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon.

A distinction is made between anterograde (away from the body) and retrograde (towards the body) axon transport.

Dendrites and axon

Diagram of the structure of a neuron

Synapse

Synapse- the place of contact between two neurons or between a neuron and an effector cell receiving a signal. It serves to transmit a nerve impulse between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. Some synapses cause neuron depolarization, others hyperpolarization; the former are excitatory, the latter are inhibitory. Usually, to excite a neuron, stimulation from several excitatory synapses is necessary.

Classification

Structural classification

Based on the number and arrangement of deindrites and axons, neurons are divided into non-axonal, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.

Axonless neurons- small cells, grouped near the spinal cord in the intervertebral ganglia, which do not have anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of axonless neurons is poorly understood.

Unipolar neurons- neurons with a single process, are present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain.

bipolar neurons- neurons with one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;

Multipolar neurons- Neurons with one axon and several dendrites. This type of nerve cells predominates in the central nervous system.

Pseudo-unipolar neurons- are unique in their kind. One sharp point leaves the body, which immediately divides in a T-shape. This entire single tract is covered with a myelin sheath and structurally represents an axon, although along one of the branches, excitation goes not from, but to the body of the neuron. Structurally, dendrites are ramifications at the end of this (peripheral) process. The trigger zone is the beginning of this branching (that is, it is located outside the cell body).

Functional classification

By position in the reflex arc, afferent neurons (sensitive neurons), efferent neurons (some of them are called motor neurons, sometimes this is not a very accurate name applies to the entire group of efferents) and interneurons (intercalary neurons) are distinguished.

Afferent neurons(sensitive, sensory or receptor). Neurons of this type include primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free endings.

Efferent neurons(effector, motor or motor). Neurons of this type include final neurons - ultimatum and penultimate - non-ultimatum.

Associative neurons(intercalary or interneurons) - this group of neurons communicates between efferent and afferent, they are divided into commissural and projection (brain).

Morphological classification

Nerve cells are stellate and spindle-shaped, pyramidal, granular, pear-shaped, etc.

Development and growth of a neuron

A neuron develops from a small precursor cell that stops dividing even before it releases its processes. (However, the issue of neuronal division is currently debatable. (Russian)) As a rule, the axon begins to grow first, and dendrites form later. At the end of the developing process of the nerve cell, an irregularly shaped thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the growth cone of the nerve cell. It consists of a flattened part of the process of the nerve cell with many thin spines. The microspines are 0.1 to 0.2 µm thick and can be up to 50 µm in length; the wide and flat area of the growth cone is about 5 µm wide and long, although its shape may vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others elongate, deviate in different directions, touch the substrate and can stick to it.

The growth cone is filled with small, sometimes interconnected, irregularly shaped membranous vesicles. Directly under the folded areas of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments found in the body of the neuron.

Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axon transport in a mature neuron. Since the average rate of advance of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs at the far end of the neuron process during the growth of the neuron process. New membrane material is added, apparently, at the end. The growth cone is an area of rapid exocytosis and endocytosis, as evidenced by the many vesicles found here. Small membrane vesicles are transported along the process of the neuron from the cell body to the growth cone with a stream of fast axon transport. Membrane material, apparently, is synthesized in the body of the neuron, transferred to the growth cone in the form of vesicles, and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell.

The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons settle and find a permanent place for themselves.

Structure and functions of neurons

Neuron(nerve cell) is a structural and functional unit.

The anatomical structure and properties of the neuron ensure its implementation main functions: implementation of metabolism, obtaining energy, perception of various signals and their processing, formation or participation in responses, generation and conduction of nerve impulses, combining neurons into neural circuits that provide both the simplest reflex reactions and higher integrative functions of the brain.

Neurons consist of a body of a nerve cell and processes - an axon and dendrites.

Rice. 2. Structure of a neuron

body of the nerve cell

Body (pericaryon, soma) The neuron and its processes are covered throughout by a neuronal membrane. The membrane of the cell body differs from the membrane of the axon and dendrites in the content of various receptors, the presence on it.

In the body of a neuron, there is a neuroplasm and a nucleus delimited from it by membranes, a rough and smooth endoplasmic reticulum, the Golgi apparatus, and mitochondria. The chromosomes of the nucleus of neurons contain a set of genes encoding the synthesis of proteins necessary for the formation of the structure and implementation of the functions of the body of the neuron, its processes and synapses. These are proteins that perform the functions of enzymes, carriers, ion channels, receptors, etc. Some proteins perform functions while in the neuroplasm, while others are embedded in the membranes of organelles, soma and processes of the neuron. Some of them, for example, enzymes necessary for the synthesis of neurotransmitters, are delivered to the axon terminal by axonal transport. In the cell body, peptides are synthesized that are necessary for the vital activity of axons and dendrites (for example, growth factors). Therefore, when the body of a neuron is damaged, its processes degenerate and collapse. If the body of the neuron is preserved, and the process is damaged, then its slow recovery (regeneration) and the restoration of the innervation of denervated muscles or organs occur.

The site of protein synthesis in the bodies of neurons is the rough endoplasmic reticulum (tigroid granules or Nissl bodies) or free ribosomes. Their content in neurons is higher than in glial or other cells of the body. In the smooth endoplasmic reticulum and the Golgi apparatus, proteins acquire their characteristic spatial conformation, are sorted and sent to transport streams to the structures of the cell body, dendrites or axon.

In numerous mitochondria of neurons, as a result of oxidative phosphorylation processes, ATP is formed, the energy of which is used to maintain the vital activity of the neuron, the operation of ion pumps, and to maintain the asymmetry of ion concentrations on both sides of the membrane. Consequently, the neuron is in constant readiness not only to perceive various signals, but also to respond to them - the generation of nerve impulses and their use to control the functions of other cells.

In the mechanisms of perception of various signals by neurons, molecular receptors of the cell body membrane, sensory receptors formed by dendrites, and sensitive cells of epithelial origin take part. Signals from other nerve cells can reach the neuron through numerous synapses formed on the dendrites or on the gel of the neuron.

Dendrites of a nerve cell

Dendrites neurons form a dendritic tree, the nature of branching and the size of which depend on the number of synaptic contacts with other neurons (Fig. 3). On the dendrites of a neuron there are thousands of synapses formed by the axons or dendrites of other neurons.

Rice. 3. Synaptic contacts of the interneuron. The arrows on the left show the flow of afferent signals to the dendrites and the body of the interneuron, on the right - the direction of propagation of the efferent signals of the interneuron to other neurons

Synapses can be heterogeneous both in function (inhibitory, excitatory) and in the type of neurotransmitter used. The dendritic membrane involved in the formation of synapses is their postsynaptic membrane, which contains receptors (ligand-dependent ion channels) for the neurotransmitter used in this synapse.

Excitatory (glutamatergic) synapses are located mainly on the surface of dendrites, where there are elevations, or outgrowths (1-2 microns), called spines. There are channels in the membrane of the spines, the permeability of which depends on the transmembrane potential difference. In the cytoplasm of dendrites in the region of spines, secondary messengers of intracellular signal transduction were found, as well as ribosomes, on which protein is synthesized in response to synaptic signals. The exact role of the spines remains unknown, but it is clear that they increase the surface area of the dendritic tree for synapse formation. Spines are also neuron structures for receiving input signals and processing them. Dendrites and spines ensure the transmission of information from the periphery to the body of the neuron. The dendritic membrane is polarized in mowing due to the asymmetric distribution of mineral ions, the operation of ion pumps, and the presence of ion channels in it. These properties underlie the transfer of information across the membrane in the form of local circular currents (electrotonically) that occur between the postsynaptic membranes and the areas of the dendrite membrane adjacent to them.

Local currents, when they propagate along the dendrite membrane, attenuate, but turn out to be sufficient in magnitude to transmit signals to the membrane of the neuron body that have arrived through the synaptic inputs to the dendrites. No voltage-gated sodium and potassium channels have yet been found in the dendritic membrane. It does not have excitability and the ability to generate action potentials. However, it is known that the action potential arising on the membrane of the axon hillock can propagate along it. The mechanism of this phenomenon is unknown.

It is assumed that dendrites and spines are part of the neural structures involved in memory mechanisms. The number of spines is especially high in the dendrites of neurons in the cerebellar cortex, basal ganglia, and cerebral cortex. The area of the dendritic tree and the number of synapses are reduced in some areas of the cerebral cortex of the elderly.

neuron axon

axon - a branch of a nerve cell that is not found in other cells. Unlike dendrites, the number of which is different for a neuron, the axon of all neurons is the same. Its length can reach up to 1.5 m. At the exit point of the axon from the body of the neuron, there is a thickening - the axon mound, covered with a plasma membrane, which is soon covered with myelin. The area of the axon hillock that is not covered by myelin is called the initial segment. The axons of neurons, up to their terminal branches, are covered with a myelin sheath, interrupted by intercepts of Ranvier - microscopic non-myelinated areas (about 1 micron).

Throughout the entire length of the axon (myelinated and unmyelinated fiber) is covered with a bilayer phospholipid membrane with protein molecules embedded in it, which perform the functions of ion transport, voltage-gated ion channels, etc. Proteins are distributed evenly in the membrane of the unmyelinated nerve fiber, and they are located in the membrane of the myelinated nerve fiber predominantly in the intercepts of Ranvier. Since there is no rough reticulum and ribosomes in the axoplasm, it is obvious that these proteins are synthesized in the body of the neuron and delivered to the axon membrane through axonal transport.

Properties of the membrane covering the body and axon of a neuron, are different. This difference primarily concerns the permeability of the membrane for mineral ions and is due to the content of various types. If the content of ligand-dependent ion channels (including postsynaptic membranes) prevails in the membrane of the body and dendrites of the neuron, then in the axon membrane, especially in the region of the nodes of Ranvier, there is a high density of voltage-dependent sodium and potassium channels.

The membrane of the initial segment of the axon has the lowest polarization value (about 30 mV). In areas of the axon more distant from the cell body, the value of the transmembrane potential is about 70 mV. The low value of polarization of the membrane of the initial segment of the axon determines that in this area the membrane of the neuron has the greatest excitability. It is here that the postsynaptic potentials that have arisen on the membrane of the dendrites and the cell body as a result of the transformation of information signals received by the neuron in the synapses are propagated along the membrane of the neuron body with the help of local circular electric currents. If these currents cause depolarization of the axon hillock membrane to a critical level (E k), then the neuron will respond to signals from other nerve cells coming to it by generating its own action potential (nerve impulse). The resulting nerve impulse is then carried along the axon to other nerve, muscle or glandular cells.

On the membrane of the initial segment of the axon there are spines on which GABAergic inhibitory synapses are formed. The arrival of signals along these lines from other neurons can prevent the generation of a nerve impulse.

Classification and types of neurons

Classification of neurons is carried out both according to morphological and functional features.

By the number of processes, multipolar, bipolar and pseudo-unipolar neurons are distinguished.

According to the nature of connections with other cells and the function performed, they distinguish touch, plug-in And motor neurons. Touch neurons are also called afferent neurons, and their processes are centripetal. Neurons that carry out the function of transmitting signals between nerve cells are called intercalary, or associative. Neurons whose axons form synapses on effector cells (muscle, glandular) are referred to as motor, or efferent, their axons are called centrifugal.

Afferent (sensory) neurons perceive information with sensory receptors, convert it into nerve impulses and conduct it to the brain and spinal cord. The bodies of sensory neurons are found in the spinal and cranial. These are pseudounipolar neurons, the axon and dendrite of which depart from the body of the neuron together and then separate. The dendrite follows the periphery to the organs and tissues as part of sensory or mixed nerves, and the axon as part of the posterior roots enters the dorsal horns of the spinal cord or as part of the cranial nerves into the brain.

Insertion, or associative, neurons perform the functions of processing incoming information and, in particular, ensure the closure of reflex arcs. The bodies of these neurons are located in the gray matter of the brain and spinal cord.

Efferent neurons also perform the function of processing the information received and transmitting efferent nerve impulses from the brain and spinal cord to the cells of the executive (effector) organs.

Integrative activity of a neuron

Each neuron receives a huge amount of signals through numerous synapses located on its dendrites and body, as well as through molecular receptors in plasma membranes, cytoplasm and nucleus. Many different types of neurotransmitters, neuromodulators, and other signaling molecules are used in signaling. Obviously, in order to form a response to the simultaneous receipt of multiple signals, the neuron must be able to integrate them.

The set of processes that ensure the processing of incoming signals and the formation of a neuron response to them is included in the concept integrative activity of the neuron.

The perception and processing of signals arriving at the neuron is carried out with the participation of dendrites, the cell body, and the axon hillock of the neuron (Fig. 4).

Rice. 4. Integration of signals by a neuron.

One of the options for their processing and integration (summation) is the transformation in synapses and the summation of postsynaptic potentials on the membrane of the body and processes of the neuron. The perceived signals are converted in the synapses into fluctuations in the potential difference of the postsynaptic membrane (postsynaptic potentials). Depending on the type of synapse, the received signal can be converted into a small (0.5-1.0 mV) depolarizing change in potential difference (EPSP - synapses are shown in the diagram as light circles) or hyperpolarizing (TPSP - synapses are shown in the diagram as black circles). Many signals can simultaneously arrive at different points of the neuron, some of which are transformed into EPSPs, while others are transformed into IPSPs.

These oscillations of the potential difference propagate with the help of local circular currents along the neuron membrane in the direction of the axon hillock in the form of waves of depolarization (in the white diagram) and hyperpolarization (in the black diagram), overlapping each other (in the diagram, gray areas). With this superimposition of the amplitude of the waves of one direction, they are summed up, and the opposite ones are reduced (smoothed out). This algebraic summation of the potential difference across the membrane is called spatial summation(Fig. 4 and 5). The result of this summation can be either depolarization of the axon hillock membrane and generation of a nerve impulse (cases 1 and 2 in Fig. 4), or its hyperpolarization and prevention of the occurrence of a nerve impulse (cases 3 and 4 in Fig. 4).

In order to shift the potential difference of the axon hillock membrane (about 30 mV) to Ek, it must be depolarized by 10-20 mV. This will lead to the opening of the voltage-gated sodium channels present in it and the generation of a nerve impulse. Since the depolarization of the membrane can reach up to 1 mV upon receipt of one AP and its transformation into EPSP, and all propagation to the axon colliculus is attenuated, generation of a nerve impulse requires simultaneous delivery of 40-80 nerve impulses from other neurons to the neuron through excitatory synapses and summation the same amount of EPSP.

Rice. 5. Spatial and temporal summation of EPSP by a neuron; (a) EPSP to a single stimulus; and — EPSP to multiple stimulation from different afferents; c — EPSP for frequent stimulation through a single nerve fiber

If at this time a neuron receives a certain number of nerve impulses through inhibitory synapses, then its activation and generation of a response nerve impulse will be possible with a simultaneous increase in the flow of signals through excitatory synapses. Under conditions when signals coming through inhibitory synapses cause hyperpolarization of the neuron membrane, equal to or greater than the depolarization caused by signals coming through excitatory synapses, depolarization of the axon colliculus membrane will be impossible, the neuron will not generate nerve impulses and become inactive.

The neuron also performs time summation EPSP and IPTS signals coming to it almost simultaneously (see Fig. 5). The changes in the potential difference caused by them in the near-synaptic regions can also be algebraically summed up, which is called temporal summation.

Thus, each nerve impulse generated by a neuron, as well as the period of silence of a neuron, contains information received from many other nerve cells. Usually, the higher the frequency of signals coming to the neuron from other cells, the more frequently it generates response nerve impulses that are sent along the axon to other nerve or effector cells.

Due to the fact that there are sodium channels (albeit in a small number) in the membrane of the body of the neuron and even its dendrites, the action potential arising on the membrane of the axon hillock can spread to the body and some part of the dendrites of the neuron. The significance of this phenomenon is not clear enough, but it is assumed that the propagating action potential momentarily smooths out all the local currents on the membrane, nullifies the potentials, and contributes to a more efficient perception of new information by the neuron.

Molecular receptors take part in the transformation and integration of signals coming to the neuron. At the same time, their stimulation by signal molecules can lead through changes in the state of ion channels initiated (by G-proteins, second mediators), transformation of the perceived signals into fluctuations in the potential difference of the neuron membrane, summation and formation of a neuron response in the form of generation of a nerve impulse or its inhibition.

The transformation of signals by the metabotropic molecular receptors of the neuron is accompanied by its response in the form of a cascade of intracellular transformations. The response of the neuron in this case may be an acceleration of the overall metabolism, an increase in the formation of ATP, without which it is impossible to increase its functional activity. Using these mechanisms, the neuron integrates the received signals to improve the efficiency of its own activity.

Intracellular transformations in a neuron, initiated by the received signals, often lead to an increase in the synthesis of protein molecules that perform the functions of receptors, ion channels, and carriers in the neuron. By increasing their number, the neuron adapts to the nature of the incoming signals, increasing sensitivity to the more significant of them and weakening to the less significant ones.

The receipt by a neuron of a number of signals may be accompanied by the expression or repression of certain genes, for example, those controlling the synthesis of neuromodulators of a peptide nature. Since they are delivered to the axon terminals of the neuron and used in them to enhance or weaken the action of its neurotransmitters on other neurons, the neuron, in response to the signals it receives, can, depending on the information received, have a stronger or weaker effect on other nerve cells controlled by it. Considering that the modulating action of neuropeptides can last for a long time, the influence of a neuron on other nerve cells can also last for a long time.

Thus, due to the ability to integrate various signals, a neuron can subtly respond to them with a wide range of responses that allow it to effectively adapt to the nature of incoming signals and use them to regulate the functions of other cells.

neural circuits

CNS neurons interact with each other, forming various synapses at the point of contact. The resulting neural foams greatly increase the functionality of the nervous system. The most common neural circuits include: local, hierarchical, convergent and divergent neural circuits with one input (Fig. 6).

Local neural circuits formed by two or more neurons. In this case, one of the neurons (1) will give its axonal collateral to the neuron (2), forming an axosomatic synapse on its body, and the second one will form an axonome synapse on the body of the first neuron. Local neural networks can act as traps in which nerve impulses are able to circulate for a long time in a circle formed by several neurons.

The possibility of long-term circulation of an excitation wave (nerve impulse) that once occurred due to transmission but a ring structure was experimentally shown by Professor I.A. Vetokhin in experiments on the nerve ring of the jellyfish.

Circular circulation of nerve impulses along local neural circuits performs the function of excitation rhythm transformation, provides the possibility of prolonged excitation after the cessation of signals coming to them, and participates in the mechanisms of storing incoming information.

Local circuits can also perform a braking function. An example of it is recurrent inhibition, which is realized in the simplest local neural circuit of the spinal cord, formed by the a-motoneuron and the Renshaw cell.

Rice. 6. The simplest neural circuits of the CNS. Description in text

In this case, the excitation that has arisen in the motor neuron spreads along the branch of the axon, activates the Renshaw cell, which inhibits the a-motoneuron.

convergent chains are formed by several neurons, on one of which (usually efferent) the axons of a number of other cells converge or converge. Such circuits are widely distributed in the CNS. For example, the axons of many neurons in the sensory fields of the cortex converge on the pyramidal neurons of the primary motor cortex. The axons of thousands of sensory and intercalary neurons of various levels of the CNS converge on the motor neurons of the ventral horns of the spinal cord. Convergent circuits play an important role in the integration of signals by efferent neurons and in the coordination of physiological processes.

Divergent chains with one input are formed by a neuron with a branching axon, each of whose branches forms a synapse with another nerve cell. These circuits perform the functions of simultaneously transmitting signals from one neuron to many other neurons. This is achieved due to the strong branching (formation of several thousand branches) of the axon. Such neurons are often found in the nuclei of the reticular formation of the brainstem. They provide a rapid increase in the excitability of numerous parts of the brain and the mobilization of its functional reserves.

The structural unit of the nervous system is the nerve cell, or neuron. Neurons differ from other body cells in many ways. First of all, their population, numbering from 10 to 30 billion (and perhaps even more *) cells, is almost completely “staffed” by the time of birth, and not a single neuron, if it dies, is replaced by a new one. It is generally accepted that after a person passes the period of maturity, about 10 thousand neurons die in him every day, and after 40 years this daily figure doubles.

* The assumption that the nervous system consists of 30 billion neurons was made by Powell et al. (1980), who showed that in mammals, regardless of species, there are about 146,000 nerve cells per 1 mm2 of nervous tissue. The total surface area of the human brain is 22 dm 2 (Changeux, 1983, p. 72).

Another feature of neurons is that, unlike other cell types, they do not produce, secrete, or structure anything; their sole function is to conduct neural information.

Structure of a neuron

There are many types of neurons, the structure of which varies depending on the functions they perform in the nervous system; a sensory neuron differs in structure from a motor neuron or a neuron in the cerebral cortex (Fig. A.28).

Rice. A.28. Various types of neurons.

But whatever the function of a neuron, all neurons are made up of three main parts: the cell body, the dendrites, and the axon.

Body neuron, like any other cell, it consists of a cytoplasm and a nucleus. The cytoplasm of the neuron, however, is especially rich in mitochondria, responsible for generating the energy needed to maintain high cell activity. As already noted, accumulations of neuron bodies form nerve centers in the form of a ganglion, in which the number of cell bodies is in the thousands, nuclei, where there are even more of them, or, finally, a cortex, consisting of billions of neurons. The bodies of neurons form the so-called Gray matter.

Dendrites serve as an antenna for the neuron. Some neurons have many hundreds of dendrites that receive information from receptors or other neurons and conduct it to the cell body and its single process of another type. - axon.

axon is a part of a neuron responsible for transmitting information to the dendrites of other neurons, muscles or glands. In some neurons, the length of the axon reaches a meter, in others the axon is very short. As a rule, the axon branches, forming the so-called terminal tree; at the end of each branch synoptic plaque. It is she who forms the connection (synapse) of a given neuron with the dendrites or bodies of other neurons.

Most nerve fibers (axons) are covered by a sheath consisting of myelin- a white fat-like substance that acts as an insulating material. Myelin sheath at regular intervals of 1-2 mm is interrupted by constrictions - interceptions of Ranvier, which increase the speed of the nerve impulse along the fiber, allowing it to "jump" from one interception to another, instead of gradually spreading along the fiber. Hundreds and thousands of bundled axons form nerve pathways, which, thanks to myelin, look like white matter.

nerve impulse

Information enters the nerve centers, is processed there and then transmitted to the effectors in the form nerve impulses, running along the neurons and the neural pathways connecting them.

Regardless of what information is transmitted by nerve impulses running through billions of nerve fibers, they are no different from each other. Why, then, does the impulses coming from the ear convey information about sounds, and the impulses from the eye - about the shape or color of the object, and not about sounds or about something completely different? Yes, simply because the qualitative differences between nerve signals are determined not by these signals themselves, but by the place where they come: if it is a muscle, it will contract or stretch; if it is a gland, it will secrete, reduce or stop the secretion; if this is a certain area of the brain, a visual image of an external stimulus will be formed in it, or the signal will be decoded in the form, for example, of sounds. Theoretically, it would be enough to change the course of nerve pathways, for example, part of the optic nerve to the area of the brain responsible for decoding sound signals, to make the body "hear with the eyes."

Resting potential and action potential

Nerve impulses are transmitted through dendrites and axons not by the external stimulus itself as such, and not even by its energy. An external stimulus only activates the corresponding receptors, and this activation is converted into energy. electrical potential, which is created at the tips of the dendrites that form contacts with the receptor.

The resulting nerve impulse can be roughly compared to a fire running along a Fickford cord and setting fire to a cartridge of dynamite located in its path; The "fire" is thus propagated towards the final target by small successive explosions. The transmission of a nerve impulse, however, differs fundamentally from this in that almost immediately after the passage of the discharge, the potential of the nerve fiber is restored.

A nerve fiber at rest can be likened to a small battery; there is a positive charge on the outside of its membrane, and a negative charge on the inside (Fig. A.29), and this resting potential is converted into electric current only when both poles are closed. This is exactly what happens during the passage of a nerve impulse, when the fiber membrane becomes permeable for a moment and depolarizes. Following this depolarization there comes a period refractoriness, during which the membrane repolarizes and restores the ability to conduct a new impulse*. So due to successive depolarizations, this spreads. action potential(i.e., a nerve impulse) at a constant speed, ranging from 0.5 to 120 meters per second, depending on the type of fiber, its thickness, and the presence or absence of myelin sheath.

* During the refractory period, which lasts about a thousandth of a second, nerve impulses cannot pass through the fiber. Therefore, in one second, the nerve fiber is able to conduct no more than 1000 impulses.

Rice. A.29. action potential. The development of the action potential, accompanied by a change in electrical voltage (from -70 to + 40 mV), is due to the restoration of equilibrium between positive and negative ions on both sides of the membrane, the permeability of which increases for a short time.

The law of everything or nothing". Since each nerve fiber has a certain electrical potential, the impulses propagating along it, regardless of the intensity or any other properties of the external stimulus, always have the same characteristics. This means that an impulse in a neuron can only occur if its activation, caused by stimulation of the receptor or an impulse from another neuron, exceeds a certain threshold, below which activation is ineffective; but, if the threshold is reached, a "full-dimensional" pulse immediately arises. This fact is known as the all-or-nothing law.

synaptic transmission

Synapse. A synapse is the area of connection between the end of the axon of one neuron and the dendrites or body of another. Each neuron can form up to 800-1000 synapses with other nerve cells, and the density of these contacts in the gray matter of the brain is more than 600 million per 1 mm 3 (Fig. A.30) *.

*This means that if 1000 synapses are counted in one second, then it will take from 3 to 30 thousand years to fully count them (Changeux, 1983, p. 75).

Rice. A.30. Synaptic connection of neurons (in the middle - the synapse area at higher magnification). The terminal plaque of the presynaptic neuron contains vesicles with a supply of neurotransmitter and mitochondria, which supply the energy necessary for the transmission of the nerve signal.

The place of transition of a nerve impulse from one neuron to another is, in fact, not a point of contact, but rather a narrow gap called synoptic gap. We are talking about a gap with a width of 20 to 50 nanometers (millionths of a millimeter), which, on the one hand, is limited by the membrane of the presynaptic plaque of the neuron that transmits the impulse, and, on the other hand, by the postsynaptic membrane of the dendrite or body of another neuron that receives the nerve signal and then transmits it further.

Neurotransmitters. It is in the synapses that processes occur, as a result of which chemicals released by the presynaptic membrane transmit a nerve signal from one neuron to another. These substances, called neurotransmitters(or simply mediators), - a kind of "brain hormones" (neurohormones) - accumulate in the vesicles of synaptic plaques and are released when a nerve impulse comes here along the axon.

After that, mediators diffuse into the synaptic cleft and attach to specific receptor sites postsynaptic membrane, i.e., to such areas to which they "fit like a key to a lock." As a result, the permeability of the postsynaptic membrane changes, and thus the signal is transmitted from one neuron to another; mediators can also block the transmission of nerve signals at the level of the synapse, reducing the excitability of the postsynaptic neuron.

Having completed their function, mediators are split or neutralized by enzymes or absorbed back into the presynaptic ending, which leads to the restoration of their stock in the vesicles by the time the next impulse arrives (Fig. A.31).

Rice. A.31. la. Mediator A, whose molecules are released from the terminal plaque of neuron I, binds to specific receptors on the dendrites of neuron II. X molecules that do not fit these receptors in their configuration cannot occupy them and therefore do not cause any synaptic effects.

1b. M molecules (for example, molecules of some psychotropic drugs) are similar in their configuration to neurotransmitter A molecules and therefore can bind to receptors for this neurotransmitter, thus preventing it from performing its functions. For example, LSD prevents serotonin from inhibiting the conduction of sensory signals.

2a and 2b. Some substances, called neuromodulators, are able to act on the end of the axon, facilitating or inhibiting the release of the neurotransmitter.

The excitatory or inhibitory function of a synapse depends mainly on the type of mediator secreted by it and on the action of the latter on the postsynaptic membrane. Some mediators always have only an excitatory effect, others only have an inhibitory (inhibitory) effect, and still others play the role of activators in some parts of the nervous system, and inhibitors in others.

Functions of the main neurotransmitters. Currently, several dozen of these neurohormones are known, but their functions have not yet been studied enough. This, for example, applies to acetylcholine, which is involved in muscle contraction, causes slowing of the heart and respiratory rate and is inactivated by the enzyme acetylcholinesterase*. The functions of such substances from the group monoamines, as norepinephrine, which is responsible for the wakefulness of the cerebral cortex and increased heart rate, dopamine, present in the "pleasure centers" of the limbic system and some nuclei of the reticular formation, where it participates in the processes of selective attention, or serotonin, which regulates sleep and determines the amount of information circulating in sensory pathways. Partial inactivation of monoamines occurs as a result of their oxidation by the enzyme monoamine oxidase. This process, which usually returns brain activity to a normal level, in some cases can lead to its excessive decrease, which psychologically manifests itself in a person in a feeling of depression (depression).

* Apparently, the lack of acetylcholine in some nuclei of the diencephalon is one of the main causes of Alzheimer's disease, and the lack of dopamine in the putamen (one of the basal nuclei) can be the cause of Parkinson's disease.

Gamma-aminobutyric acid (GABA) is a neurotransmitter that performs approximately the same physiological function as monoamine oxidase. Its action consists mainly in reducing the excitability of brain neurons in relation to nerve impulses.

Along with neurotransmitters, there is a group of so-called neuromodulators, which are mainly involved in the regulation of the nervous response, interacting with mediators and modifying their effects. As an example one can name substance P And bradykinin, involved in the transmission of pain signals. The release of these substances in the synapses of the spinal cord, however, can be suppressed by secretion endorphins And enkephalin, which thus leads to a decrease in the flow of pain nerve impulses (Fig. A.31, 2a). The functions of modulators are also performed by substances such as factorS, which seems to play an important role in sleep processes, cholecystokinin, responsible for the feeling of satiety, angiotensin, regulating thirst, and other agents.

neurotransmitters and action of psychotropic substances. It is currently known that various psychotropic drugs act at the level of synapses and those processes in which neurotransmitters and neuromodulators participate.

The molecules of these drugs are similar in structure to the molecules of certain mediators, which allows them to "deceive" the various mechanisms of synaptic transmission. Thus, they disrupt the action of true neurotransmitters, either taking their place at the receptor sites, or preventing them from being absorbed back into the presynaptic endings or being destroyed by specific enzymes (Fig. A.31, 26).

It has been established, for example, that LSD, by occupying serotonin receptor sites, prevents serotonin from inhibiting the influx of sensory signals. In this way, LSD opens up consciousness to a wide variety of stimuli that continuously attack the senses.

Cocaine enhances the effects of dopamine, taking its place in the receptor sites. They operate in the same way morphine and other opiates, the instant effect of which is explained by the fact that they quickly manage to occupy the receptor sites for endorphins *.

* Accidents associated with drug overdose are explained by the fact that the binding of an excessive amount, for example, heroin, to ndorphin receptors in the nerve centers of the medulla oblongata leads to a sharp respiratory depression, and sometimes to a complete stop (Besson, 1988, Science et Vie, Hors series, n° 162).

Action amphetamines due to the fact that they suppress the reuptake of noradrenaline by presynaptic endings. As a result, the accumulation of an excess amount of neurohormone in the synaptic cleft leads to an excessive degree of wakefulness of the cerebral cortex.

It is generally accepted that the effects of the so-called tranquilizers(for example, Valium) are mainly due to their facilitating effect on the action of GABA in the limbic system, which leads to an increase in the inhibitory effects of this mediator. On the contrary, as antidepressants mainly enzymes that inactivate GABA, or drugs such as, for example, monoamine oxidase inhibitors, the introduction of which increases the amount of monoamines in synapses.

Death by some poison gases occurs due to suffocation. This effect of these gases is due to the fact that their molecules block the secretion of an enzyme that destroys acetylcholine. Meanwhile, acetylcholine causes muscle contraction and a slowing of the heart and respiratory rhythm. Therefore, its accumulation in synaptic spaces leads to inhibition and then complete blockade of cardiac and respiratory functions and a simultaneous increase in the tone of all muscles.

The study of neurotransmitters is just beginning, and it can be expected that hundreds, and perhaps thousands of these substances will soon be discovered, the diverse functions of which determine their primary role in the regulation of behavior.