The structure of the human nerve cell. Neurons of the brain - structure, classification and pathways. unmyelinated nerve fibers

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.

Structure nerve cell not so difficult, 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 complex 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 at the anterior central gyrus, located in front of the great furrow brain - namely, from the Betz 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 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, allocate different groups these same neurotransmitters, 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 you need to contact a 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.

The main structural and functional unit of the nervous system is the neuron (neurocyte). One long process (axon) departs from the body of the neuron in one direction, and short branching processes - dendrites - in the other.

Through the dendrites, nerve impulses flow to the body of the neuron (impulse conduction is afferent, cellulopetal), from its receptive regions. The axon conducts impulses afferently (cellulofugally) - from the cell body and dendrites.

When describing the axon and dendrites, they proceed from the possibility of conducting impulses in only one direction - the so-called law of dynamic polarization of a neuron (manifested in neural circuits).

In stained sections of the nervous tissue, the axon is recognized by the absence of the tigroid substance in it, while in the dendrites, at least in their initial part, it is detected.

Depending on the number of processes extending from the cell body, 3 types of neurons are distinguished

• unipolar (pseudo-unipolar)
• bipolar
• multipolar

Depending on the form, there are

• pyramidal cells
• spindle cells
• basket cells
• stellate cells (astrocytes)

Depending on the size, they are distinguished from very small to giant cells, for example, giant Betz cells in the motor cortex.

Most neurons in the CNS are bipolar cells with one axon and a large number of dichotomously branching dendrites. Such cells are characteristic of visual, auditory and olfactory systems- specialized sensor systems.

Unipolar (pseudo-unipolar) cells are found much less frequently. They are located in the mesencephalic nucleus trigeminal nerve and in the spinal nodes (ganglia of the posterior roots and sensory cranial nerves). These cells provide certain types of sensitivity - pain, temperature, tactile, as well as a sense of pressure, vibration, stereognosia and perception of the distance between the places of two point touches on the skin (two-dimensional-spatial feeling). Such cells, although called unipolar, actually have 2 processes (axon and dendrite) that merge near the cell body.

True unipolar cells are found only in the mesencephalic nucleus of the trigeminal nerve, which conducts proprioceptive impulses from the masticatory muscles to the cells of the thalamus.

Neurons are classified according to their functions.

• receptor (sensitive, vegetative)
• effector (motor, vegetative)
• associative (associative)

Communication between nerve cells occurs through synapses. [show] , in which excitation transmitters are involved - mediators.

Synapse - connection of nerve cells

Nerve cells are connected to each other only by contact - synapse (Greek synapsis - contact, grasping, connection). Synapses can be classified according to their location on the surface of the postsynaptic neuron. Distinguish

• axodendritic synapses - the axon ends at the dendrite;
• axosomatic synapses - a contact is formed between the axon and the body of the neuron;
• axo-axonal - contact is established between axons. In this case, an axon can only synapse on the unmyelinated portion of another axon. This is possible either in the proximal part of the axon, or in the region of the terminal button of the axon, since in these places the myelin sheath is absent.
• There are other variants of synapses: dendro-dendritic and dendrosomatic.

Approximately half of the entire surface of the body of a neuron and almost the entire surface of its dendrites are dotted with synaptic contacts from other neurons. However, not all synapses transmit nerve impulses. Some of them inhibit the reactions of the neuron with which they are associated (inhibitory synapses), while others, located on the same neuron, excite it (excitatory synapses). The total action of both types of synapses on one neuron leads to each this moment to a balance between two opposite kinds of synaptic effects.

Excitatory and inhibitory synapses have the same structure. Their opposite action is explained by the release of various chemical neurotransmitters in the synaptic endings, which have a different ability to change the permeability of the synaptic membrane for potassium, sodium and chlorine ions. In addition, excitatory synapses often form axodendritic contacts, while inhibitory synapses form axosomatic and axo-axonal ones.

The section of the neuron through which impulses enter the synapse is called the presynaptic ending, and the section that receives the impulses is called the postsynaptic ending. The cytoplasm of the presynaptic ending contains many mitochondria and synaptic vesicles containing the neurotransmitter. The axolemma of the presynaptic section of the axon, which comes close to the postsynaptic neuron, forms the presynaptic membrane in the synapse. The area of ​​the plasma membrane of a postsynaptic neuron that is closely adjacent to the presynaptic membrane is called the postsynaptic membrane. The intercellular space between the pre- and postsynaptic membranes is called the synaptic cleft.

Reflex arcs are built from a chain of such neurons. Each reflex is based on the perception of stimuli, its processing and transfer to the reacting organ - the performer. The set of neurons necessary for the implementation of the reflex is called the reflex arc. Its structure can be both simple and very complex, including both afferent and efferent systems.

Afferent systems are ascending conductors of the spinal cord and brain, which conduct impulses from all tissues and organs. A system that includes specific receptors, conductors from them and their projections in the cerebral cortex is defined as an analyzer. It performs the functions of analysis and synthesis of irritations, i.e., the primary decomposition of the whole into parts, units, and then the gradual addition of the whole from units, elements [Pavlov I.P., 1936].

Efferent systems originate from many parts of the brain: the cortex hemispheres, subcortical nodes, hypotuberous region, cerebellum, stem structures (in particular, from those departments reticular formation that affect the segmental apparatus of the spinal cord). Numerous descending conductors from these formations of the brain approach the neurons of the segmental apparatus of the spinal cord and then follow to the executive organs: striated muscles, endocrine glands, blood vessels, internal organs and skin.

Nerve cells have the ability to perceive, conduct and transmit nerve impulses. In addition, there are secretory neurons.

secretory neurons they synthesize mediators involved in their conduction (neurotransmitters), acetylcholine, catecholamines, indolamines, as well as lipids, carbohydrates and proteins. Some specialized nerve cells have the ability to neurocrinia (synthesize protein products - octa-peptides, such as antidiuretic hormone, vasopressin, oxytocin in the cells of the supraoptic and paraventricular nuclei of the hypothalamus). Other neurons that make up the basal parts of the hypothalamus produce so-called releasing factors that affect the function of the adenohypophysis.

body of the nerve cell has its own structural features, which are due to the specificity of their function. A nerve cell, like any somatic cell, has a membrane, a cell body, a nucleus, a central Golgi apparatus, mitochondria and cell inclusions. But besides this, it also contains some specific components: Nissl's tigroid substance and neurofibrils.

The body of the neuron, in addition to the outer shell, has a three-layer cytoplasmic membrane, consisting of two layers of phospholipids and proteins. The membrane performs a barrier function, protecting the cell from the ingress of foreign substances, and a transport one, ensuring the entry into the cell of the substances necessary for its vital activity. [show] .

There are passive and active transport of substances and ions through the membrane.

• Passive transport is the transfer of substances in the direction of decreasing electrochemical potential, along a concentration gradient (free diffusion through the lipid bilayer, facilitated diffusion - transport of substances through the membrane).
• Active transport - the transfer of substances against the gradient of the electrochemical potential using ion pumps.
• Cytosis is also distinguished - a mechanism for the transfer of substances through the cell membrane, which is accompanied by reversible changes in the structure of the membrane.

Through the plasma membrane, not only the entry and exit of substances is regulated, but also the exchange of information between the cell and the extracellular environment is carried out. Nerve cell membranes contain many receptors, the activation of which leads to an increase in the intracellular concentration of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), which regulate cellular metabolism.

Neuron nucleus [show] .

The nucleus of a neuron is the largest of the cellular structures visible under light microscopy. It has a spherical or bubble shape and, in most neurons, is located in the center of the cell body. It contains chromatin granules, which are a complex of deoxyribonucleic acid (DNA) with the simplest proteins (histones), non-histone proteins (nucleoproteins), protamines, lipids, etc. Chromosomes become visible only during mitosis.

In the center of the nucleus is the nucleolus, which contains a significant amount of RNA and proteins; ribosomal RNA (rRNA) is formed in it.

The genetic information contained in chromatin DNA is transcribed into messenger RNA (mRNA). Then the mRNA molecules penetrate through the pores of the nuclear membrane and enter the ribosomes and polyribosomes of the granular endoplasmic reticulum. There is a synthesis of protein molecules; in this case, amino acids brought by special transfer RNA (tRNA) are used. This process is called translation. Some substances (cAMP, hormones, etc.) can increase the rate of transcription and translation.

The nuclear envelope consists of two membranes - inner and outer. The pores through which the exchange between the nucleoplasm and cytoplasm takes place occupy 10% of the surface of the nuclear envelope. In addition, the outer nuclear membrane forms protrusions from which strands of the endoplasmic reticulum arise with ribosomes attached to them (granular reticulum). The nuclear membrane and the membrane of the endoplasmic reticulum are morphologically close to each other.

In the bodies and large dendrites of nerve cells, under light microscopy, clumps of basophilic substance (tigroid substance or Nissl substance) are clearly visible.

Tigroid substance was first discovered and studied by Nissl (1889), otherwise it is called lumps, or Nissl bodies, or chromatophilic substance. It is now established that Nissl bodies are ribosomes.

The size of lumps of basophilic granularity and their distribution in neurons of different types are different. It depends on the state of impulse activity of neurons, because. tigroid actively participates in metabolic processes. It continuously synthesizes new cytoplasmic proteins. These proteins include proteins involved in the construction and repair of cell membranes, metabolic enzymes, specific proteins involved in synaptic conduction, and enzymes that inactivate this process. Newly synthesized proteins in the cytoplasm of the neuron enter the axon (as well as the dendrites) to replace the spent proteins. The amount of chromatophilic substance in neurons decreases during their long-term functioning and is restored at rest.

Of all the morphological parts of the nerve cell, the chromatophilic substance is the most sensitive to various physiological and pathological factors.

Tigroid granules are found in the cell body, in dendrites and absent in axons.

If the axon of the nerve cell is cut not too close to the perikaryon (so as not to cause irreversible damage), then the basophilic substance is redistributed, reduced and temporarily disappears (chromatolysis), and the nucleus moves to the side. During axon regeneration in the body of a neuron, the basophilic substance moves towards the axon, the number of granular endoplasmic reticulum and the number of mitochondria increase, protein synthesis increases, and processes may appear at the proximal end of the cut axon.

Lamellar complex (Golgi apparatus) [show] .

The lamellar complex (Golgi apparatus) is a system of intracellular membranes, each of which is a row of flattened cisterns and secretory vesicles. This system of cytoplasmic membranes is called the agranular reticulum due to the absence of ribosomes attached to its cisterns and vesicles.

The lamellar complex takes part in the transport of certain substances from the cell, in particular proteins and polysaccharides. A significant part of the proteins synthesized in ribosomes on the membranes of the granular endoplasmic reticulum, having entered the lamellar complex, is converted into glycoproteins, which are packaged in secretory vesicles and then released into the extracellular environment. This indicates the presence of a close connection between the lamellar complex and the membranes of the granular endoplasmic reticulum.

Neurofilaments can be detected in most large neurons, where they are located in the basophilic substance, as well as in myelinated axons and dendrites. They are the thinnest threads located both in the cell body and in its processes, and in the cell body the fibrils in most cases have a mesh arrangement, while in the processes they pass in parallel bundles.

Neurofilaments in their structure are fibrillar proteins with a function that has not been fully elucidated. They are believed to play a major role in the transmission of nerve impulses, maintain the shape of the neuron, especially its processes, and participate in the axoplasmic transport of substances along the axon.

In relation to various hazards, neurofibrils turn out to be much more enduring than other elements of the nerve cell.

Lysosomes [show] .

are vesicles bounded by a simple membrane and providing phagocytosis of the cell. They contain a set of hydrolytic enzymes capable of hydrolyzing substances that have entered the cell. In the case of cell death, the lysosomal membrane breaks and the process of autolysis begins - hydrolases released into the cytoplasm break down proteins, nucleic acids and polysaccharides. A normally functioning cell is reliably protected by a lysosomal membrane from the action of hydrolases contained in lysosomes.

Mitochondria [show] .

Mitochondria are structures in which enzymes of oxidative phosphorylation are localized. Mitochondria have outer and inner membranes. They are located in the cell body, dendrites, axon, synapses. They are absent in the nucleus.

Mitochondria are a kind of energy stations of cells in which adenosine triphosphate (ATP) is synthesized - the main source of energy in a living organism.

Thanks to mitochondria, the process of cellular respiration is carried out in the body. The components of the tissue respiratory chain, as well as the ATP synthesis system, are localized in the inner membrane of mitochondria.

Among other various cytoplasmic inclusions (vacuoles, glycogen, crystalloids, iron-containing granules, etc.), a yellowish-brown pigment, lipofuscin, is often found. This pigment is deposited as a result of cell activity. In young people there is little lipofuscin in nerve cells, in old age there is a lot. There are also some black or dark brown pigments, similar to melanin (in the cells of the black substance, blue spot, gray wing, etc.). The role of pigments has not been completely elucidated. However, it is known that a decrease in the number of pigmented cells in the substantia nigra is associated with a decrease in the content of dopamine in its cells and the caudate nucleus, which leads to parkinsonism syndrome.

N E Y R O G L I A

Neuroglia are cells that surround neurons. She has great value in ensuring the normal functioning of neurons, tk. is in close metabolic relationship with them, taking part in the synthesis of proteins, nucleic acids and information storage. In addition, neuroglial cells are an internal support for the neurons of the central nervous system - they support the bodies and processes of neurons, ensuring their proper relative position. Thus, neuroglia performs supporting, delimiting, trophic, secretory and protective functions in the nervous tissue. Certain types of glia are also assigned special functions.

All neuroglial cells are divided into two genetically different types:

• gliocytes (macroglia)

Macroglia of the central nervous system include ependymocytes, astrocytes, and oligodendrocytes.

Ependymocytes. They form a dense layer of cellular elements lining the spinal canal and all the ventricles of the brain. They perform a proliferative, supporting function, participate in the formation of the choroid plexuses of the ventricles of the brain. In the choroid plexuses, a layer of ependyma separates the cerebrospinal fluid from the capillaries. The ependymal cells of the brain ventricles function as the blood-brain barrier. Some ependymocytes perform secretory function participating in the formation of cerebrospinal fluid and highlighting various active substances directly into the cavity of the cerebral ventricles or blood. For example, in the region of the posterior commissure of the brain, ependymocytes form a special "subcommissural organ" that secretes a secret, possibly involved in the regulation of water metabolism.

astrocytes. They form the supporting apparatus of the central nervous system. There are two types of astrocytes: protoplasmic and fibrous. Between them there are also transitional forms. Protoplasmic astrocytes lie predominantly in the gray matter of the central nervous system and carry out delimiting and trophic functions. Fibrous astrocytes are located mainly in the white matter of the brain and together form a dense network - the supporting apparatus of the brain. The processes of astrocytes on the blood vessels and on the surface of the brain, with their terminal extensions, form perivascular glial boundary membranes that play important role in the exchange of substances between neurons and circulatory system [show] .

In most parts of the brain, the surface membranes of the bodies of nerve cells and their processes (axons and dendrites) do not come into contact with the walls of blood vessels or the cerebrospinal fluid of the ventricles, the central canal, and the subarachnoid space. The exchange of substances between these components, as a rule, is carried out through the so-called blood-brain barrier. This barrier is no different from the endothelial cell barrier in general.

Substances transported in the blood must first of all pass through the cytoplasm of the endothelium of the vessel. Then they need to pass through the basement membrane of the capillary, the layer of astrocytic glia and, finally, through the surface membranes of the neurons themselves. The last two structures are believed to be the main components of the blood-brain barrier.

In other organs, the cells of the brain tissue are in direct contact with the basement membranes of the capillaries, and there is no intermediate layer similar to the cytoplasmic layer of astrocytic glia. Large astrocytes, which play an important role in the rapid intracellular transfer of metabolites into and out of neurons and ensure the selective nature of this transfer, probably constitute the main morphological substrate of the blood-brain barrier.

In certain structures of the brain - neurohypophysis, epiphysis, gray tubercle, supraoptic, subfornical and other areas - the metabolism is very fast. It is assumed that the blood-brain barrier in these brain structures does not function.

The main function of astrocytes is the support and isolation of neurons from external influences, which is necessary for the implementation of the specific activity of neurons.

Oligodendrocytes. This is the most numerous group of neuroglial cells. Oligodendrocytes surround the bodies of neurons in the central and peripheral nervous systems, are part of the sheaths of nerve fibers and in nerve endings. In different parts of the nervous system, oligodendrocytes have a different shape. The study by electron microscopy showed that, in terms of cytoplasm density, oligodendroglia cells approach nerve cells and differ from them in that they do not contain neurofilaments.

The functional significance of these cells is very diverse. They perform a trophic function, taking part in the metabolism of nerve cells. Oligodendrocytes play a significant role in the formation of membranes around cell processes, and they are called neurolemmocytes (lemmocytes - Schwann cells). In the process of degeneration and regeneration of nerve fibers, oligodendrocytes perform another very important function - they are involved in neuronophagy (from the Greek phagos - devouring), i.e. remove dead neurons by actively absorbing decay products.

The macroglia of the peripheral nervous system are

• Schwann cells are specialized oligodendrocytes that synthesize the myelin sheath of myelinated fibers. They differ from oligodendroglia in that they usually cover only one section of an individual axon. The length of such coverage does not exceed 1 mm. Peculiar borders are formed between individual Schwann cells, which are called nodes of Ranvier.
• satellite cells - encapsulate the neurons of the ganglia of the spinal and cranial nerves, regulating the microenvironment around these neurons in the same way as astrocytes do.

• microglia- These are small cells scattered in the white and gray matter of the nervous system. Microglial cells are glial macrophages and perform a protective function, taking part in a variety of reactions in response to damaging factors. In this case, microglial cells first increase in volume, then mitotically divide. Microglial cells altered by irritation are called granular balls.

N E R V N E F IB O C N A

The main component of the nerve fiber is the process of the nerve cell. The nerve process is surrounded by sheaths, together with which it forms the nerve fiber.

In different parts of the nervous system, the sheaths of nerve fibers differ significantly from each other in their structure, therefore, in accordance with the peculiarities of their structure, all nerve fibers are divided into two main groups - myelinated (fleshy fibers) and unmyelinated (fleshless) or, rather, myelin-poor ( finely myelinated fibers). Both consist of a process of a nerve cell, which lies in the center of the fiber and is therefore called an axial cylinder, and a sheath, formed by cells oligodendroglia, which are here called neurolemmocytes (Schwann cells).

In the central and peripheral nervous system, pulpy fibers predominate, in the autonomic nervous system - non-fleshy. In cutaneous nerves, the number of non-fleshy fibers can exceed the number of pulpy ones by 3-4 times. On the contrary, there are very few non-fleshy fibers in muscle nerves. AT vagus nerve pulpless fibers make up almost 95%.

unmyelinated nerve fibers

The cells of the oligodendroglia of the sheaths of non-myelinated nerve fibers, being dense, form strands, in which oval nuclei are visible at a certain distance from each other. In the nerve fibers of the internal organs, as a rule, not one, but several (10-20) axial cylinders belonging to various neurons are located in such a strand. They can, leaving one fiber, move into an adjacent one. Such fibers containing several axial cylinders are called cable-type fibers.

Electron microscopy of unmyelinated nerve fibers shows that as the axial cylinders sink into the strands of lemmocytes, the latter dress them like a clutch. At the same time, the membranes of lemmocytes bend, tightly cover the axial cylinders and, closing over them, form deep folds, at the bottom of which individual axial cylinders are located. The sections of the neurolemmocyte membrane close together in the fold area form a double membrane - mesaxon, on which, as it were, an axial cylinder is suspended (see Fig. B). Unmyelinated fibers of the autonomic nervous system are covered with a single helix of the lemmocyte membrane.

The membranes of neurolemmocytes are very thin, therefore, neither the mesaxon nor the boundaries of these cells can be seen under a light microscope, and the sheath of unmyelinated nerve fibers under these conditions is revealed as a homogeneous strand of cytoplasm, "clothing" the axial cylinders. From the surface, each nerve fiber is covered with a basement membrane.

myelinated nerve fibers

Myelinated nerve fibers are found in both the central and peripheral nervous systems. They are much thicker than unmyelinated nerve fibers. Their cross-sectional diameter ranges from 1 to 20 microns. They also consist of an axial cylinder, "dressed" by a sheath of neurolemmocytes, but the diameter of the axial cylinders of this type of fiber is much thicker, and the sheath is more complex. In the formed myelin fiber, it is customary to distinguish two layers of the membrane: the inner, thicker one, the myelin layer (see Fig. A) and the outer, thin one, consisting of the cytoplasm and nuclei of neurolemmocytes, the neurolemma.

Myelin sheaths contain cholesterol, phospholipids, some cerebrosides and fatty acid, as well as protein substances intertwined in the form of a network (neurokeratin). The chemical nature of the myelin of the peripheral nerve fibers and the myelin of the central nervous system is somewhat different. This is due to the fact that in the central nervous system myelin is formed by oligodendroglia cells, and in the peripheral nervous system by lemmocytes (Schwann cells). These two types of myelin also have different antigenic properties, which is revealed in the infectious-allergic nature of the disease.

The myelin sheath of the nerve fiber is interrupted in places, forming the so-called nodes of Ranvier. Interceptions correspond to the border of adjacent neurolemmocytes. The fiber segment enclosed between adjacent intercepts is called the internodal segment, and its sheath is represented by one glial cell. The myelin sheath provides the role of an electrical insulator. In addition, it is supposed to participate in the exchange processes of the axial cylinder.

Myelination of the peripheral nerve fiber is carried out by lemmocytes (oligodendrocytes in the central nervous system and Schwann cells in the peripheral). These cells form an outgrowth of the cytoplasmic membrane, which spirally wraps around the nerve fiber, forming a mesaxon. At further development mesaxon elongates, concentrically layered on the axial cylinder and forms around it a dense layered zone - the myelin layer. Up to 100 spiral layers of myelin with a regular lamellar structure can form (Fig.).

There are differences in the formation of the myelin sheath and the structure of the myelin of the CNS and the peripheral nervous system (PNS). During the formation of CNS myelin, one oligodendrogliocyte has connections with several myelin segments of several axons; at the same time, a process of oligodendrogliocyte located at some distance from the axon adjoins the axon, and the outer surface of myelin is in contact with the extracellular space.

The Schwann cell, during the formation of myelin, the PNS forms spiral plates of myelin and is only responsible for separate plot myelin sheath between nodes of Ranvier. The cytoplasm of the Schwann cell is forced out of the space between the spiral coils and remains only on the inner and outer surfaces of the myelin sheath. This zone, containing the cytoplasm of neurolemmocytes (Schwann cells) and their nuclei pushed here, is called the outer layer (neurolemma) and is the peripheral zone of the nerve fiber.

The myelin sheath provides isolated, non-decremental (without a drop in potential amplitude) and faster conduction of excitation along the nerve fiber (saltatory conduction of excitation, i.e., jumps, from one intercept of Ranvier to another). There is a direct relationship between the thickness of this shell and the speed of the pulses. Fibers with a thick layer of myelin conduct impulses at a speed of 70-140 m/s, while conductors with a thin myelin sheath at a speed of about 1 m/s and even more slowly - "fleshless" fibers (0.3-0.5 m/s). c), because in a non-myelinated (non-myelinated) fiber, the wave of membrane depolarization proceeds without interruption throughout the plasmalemma.

Axial cylinder of nerve fibers consists of neuroplasm - the cytoplasm of a nerve cell containing longitudinally oriented neurofilaments and neurotubules. The neuroplasm of the axial cylinder contains many filamentous mitochondria, axoplasmic vesicles, neurofilaments, and neurotubules. Ribosomes are very rare in the axoplasm. The granular endoplasmic reticulum is absent. This causes the body of the neuron to supply the axon with proteins; therefore, glycoproteins and a number of macromolecular substances, as well as some organelles, such as mitochondria and various vesicles, must move along the axon from the cell body. This process is called axonal, or axoplasmic, transport. [show] .

axon transport

The processes of intracellular transport can be most clearly demonstrated on the axon of a nerve cell. It is assumed that similar events occur in a similar way in most cells.

It has long been known that when any section of the axon undergoes constriction, the proximal portion of the axon expands. It looks like the centrifugal flow is blocked in the axon. Such a flow fast axon transport- can be demonstrated by the movement of radioactive markers in the experiment.

Radiolabeled leucine was injected into the dorsal root ganglion, and then, from the 2nd to the 10th hour, the radioactivity was measured in the sciatic nerve at a distance of 166 mm from the neuron bodies. For 10 hours, the peak of radioactivity at the injection site did not change significantly. But the wave of radioactivity propagated along the axon at a constant speed of about 34 mm in 2 hours, or 410 mm * day -1. It has been shown that in all neurons of homoiothermic animals, fast axon transport occurs at the same rate, and there are no noticeable differences between thin, unmyelinated fibers and the thickest axons, as well as between motor and sensory fibers. The type of radioactive marker also does not affect the rate of fast axonal transport; markers can be a variety of radioactive molecules, such as various amino acids included in the proteins of the neuron body.

If we analyze the peripheral part of the nerve to determine the nature of the carriers of the radioactivity transported here, then such carriers are found mainly in the protein fraction, but also in the composition of mediators and free amino acids. Knowing that the properties of these substances are different and especially the sizes of their molecules are different, we can explain the constant rate of transport only by the transport mechanism common to all of them.

The fast axon transport described above is anterograde, i.e. directed away from the cell body. It has been shown that some substances move from the periphery to the cell body using retrograde transport. For example, acetylcholinesterase is transported in this direction at a rate two times lower than the rate of fast axonal transport. A marker often used in neuroanatomy, horseradish peroxidase, also moves retrogradely. Retrograde transport probably plays an important role in the regulation of protein synthesis in the cell body.

A few days after axon transection, chromatolysis is observed in the cell body, which indicates a violation of protein synthesis. The time required for chromatolysis correlates with the duration of retrograde transport from the site of axon transection to the cell body. Such a result also suggests an explanation for this violation - the transmission from the periphery of the "signal substance" that regulates protein synthesis is disrupted.

Obviously, the main "vehicles" used for fast axonal transport are vesicles (vesicles) and organelles, such as mitochondria, containing the substances to be transported.

The movement of the largest vesicles or mitochondria can be observed using an in vio microscope. Such particles make short, quick movements in one direction, stop, often move slightly backwards or to the side, stop again, and then make a dash in the main direction. 410 mm * day -1 correspond to an average speed of anterograde movement of approximately 5 μm * s -1 ; the speed of each individual movement should therefore be much higher, and if we take into account the size of organelles, filaments and microtubules, then these movements are really very fast.

Rapid axon transport requires a significant concentration of ATP. Poisons such as microtubule-destroying colchicine also block fast axonal transport. It follows from this that in the transport process we are considering, vesicles and organelles move along microtubules and actin filaments; this movement is provided by small aggregates of dynein and myosin molecules, acting using the energy of ATP.

Fast axon transport may also be involved in pathological processes. Some neurotropic viruses (for example, herpes or polio viruses) penetrate the axon at the periphery and move with the help of retrograde transport to the neuron body, where they multiply and exert their toxic effect. Tetanus toxin, a protein produced by bacteria that enter the body when the skin is damaged, is captured by nerve endings and transported to the neuron body, where it causes characteristic muscle spasms.

Cases of toxic effects on the axon transport itself are known, for example, exposure to the industrial solvent acrylamide. In addition, it is believed that the pathogenesis of beriberi beriberi and alcoholic polyneuropathy includes a violation of fast axonal transport.

In addition to fast axon transport in the cell, there is also a rather intense slow axon transport. Tubulin moves along the axon at a speed of about 1 mm * day -1, and actin is faster - up to 3 mm * day -1. Other proteins also migrate with these components of the cytoskeleton; for example, enzymes appear to be associated with actin or tubulin.

The rates of movement of tubulin and actin are roughly consistent with the growth rates found for the mechanism described earlier when the molecules are incorporated into the active cone of a microtubule or microfilament. Therefore, this mechanism may underlie slow axonal transport. The rate of slow axon transport also approximately corresponds to the rate of axon growth, which, apparently, indicates the limitations imposed by the structure of the cytoskeleton on the second process.

Certain cytoplasmic proteins and organelles move along the axon in two streams at different speeds. One is a slow stream moving along the axon at a speed of 1-3 mm/day, moving lysosomes and some enzymes necessary for the synthesis of neurotransmitters in axon endings. The other flow is fast, also directed away from the cell body, but its speed is 5-10 mm/h (about 100 times higher than the speed of the slow flow). This stream transports the components necessary for synaptic function (glycoproteins, phospholipids, mitochondria, dopamine hydroxylase for the synthesis of adrenaline).

Dendrites usually much shorter than axons. Unlike the axon, dendrites branch dichotomously. In the CNS, dendrites do not have a myelin sheath. Large dendrites differ from the axon also in that they contain ribosomes and cisterns of the granular endoplasmic reticulum (basophilic substance); there are also many neurotubules, neurofilaments and mitochondria. Thus, dendrites have the same set of organelles as the body of the nerve cell. The surface of the dendrites is greatly enlarged by small outgrowths (spines) that serve as sites of synaptic contact.

All nerve fibers end in end apparatuses, which are called nerve endings.

CONNECTIVE TISSUE

Connective tissue is represented in the central nervous system by the membranes of the brain and spinal cord, vessels penetrating together with the pia mater into the substance of the brain, and the choroid plexus of the ventricles.

in the peripheral nerves connective tissue forms membranes that enclose the nerve trunk (epineurium), its individual bundles (perineurium) and nerve fibers (endoneurium). Vessels supplying the nerve pass through the membranes.

The importance of the vascular-connective tissue apparatus is especially great in protecting the nervous tissue from various hazards and in combating hazards that have already penetrated the central nervous system or the peripheral nerve.

The accumulation in the spinal cord and brain of the bodies of neurons and dendrites makes up the gray matter of the brain, and the processes of nerve cells form white matter brain. The bodies of nerve cells form clusters and are called nuclei in the central nervous system and ganglia (nerve nodes) in the peripheral.

In the cerebellum and in the cerebral hemispheres, cells form layered (stratified) structures called the cortex.

CELLULAR STRUCTURE (CYTOARCHITECTONICS) OF THE CROBES OF THE GREAT HEMISPHERES

The cortex covers the entire surface of the cerebral hemispheres. Its structural elements are nerve cells with processes extending from them - axons and dendrites - and neuroglial cells.

In the cerebral cortex of the human brain, there are about 12-18 billion nerve cells. Of these, 8 billion are large and medium-sized cells of the third, fifth and sixth layers, about 5 billion are small cells of various layers. [show]

The cerebral cortex has a different structure in different areas. This has been well known since the time of Vic d "Azire, a French anatomist who described in 1782 the stripes of white matter bearing his name, macroscopically visible in the cortex of the occipital lobe. The extremely uneven thickness of the gray matter of the cloak has also long attracted attention. The thickness of the cortex ranges from 4 .5 mm (in the region of the anterior central gyrus) to 1.2 mm (in the region of sulcus calcarinus).

In 1874 V.A. Betz discovered giant pyramidal cells (Betz cells) in the cortex of the human anterior central gyrus and in the motor area of ​​the animal cortex and emphasized the absence of these cells in areas of the cortex, stimulation of which by an electric current does not cause a motor effect.

A cytoarchitectonic study of the cerebral cortex of adults, human embryos, and the cerebral cortex of various animals made it possible to divide it into two areas: homogeneous and heterogeneous (according to Brodmann) or isocortex and allocortex (according to Vogt).

The homogeneous cortex (isocortex) in its development necessarily passes through the phase of a six-layer structure, while the heterogeneous cortex (allocortex) is formed without passing through this phase. Phylogenetic studies show that the isocortex corresponds to the new cortex - neocortex, which appears in more highly organized animals and reaches its greatest development in humans, while the allocortex corresponds to the old cortex, paleo- and archicortex. In the human brain, the allocortex occupies only 5% of the entire cortex, and 95% belongs to the isocortex.

Those areas of the isocortex that retain a six-layer structure in an adult also make up the homotypic cortex. Heterotypic cortex - a part of the isocortex that deviated from the six-layer structure in the direction of a decrease or increase in the number of layers.

In heterotypic regions of the isocortex, the six-layer structure of the cortex is disturbed. Distinguish

• agranular heterotypy

  The agranular regions of the human cortex are completely or almost completely devoid of the outer and inner granular layers. The place of granule cells was occupied by pyramidal cells of various sizes, which is why the agranular area is otherwise called the pyramidal cortex.

  Agranular heterotype characterizes mainly some motor areas of the cortex, especially the anterior central gyrus, where numerous giant Betz cells lie.

• granular heterotypy

  In the area of ​​granular heterotypy, the cerebral cortex presents the reverse picture. Here, the pyramidal cells of the third and fifth layers are mostly replaced by densely arranged small granule cells.

  Granular heterotypy is present in sensitive areas of the cortex.

The bulk of the cells of the cortex consists of elements of three genera:

• pyramidal cells
• spindle cells
• stellate cells

It is believed that pyramidal and fusiform cells with long axons represent predominantly efferent systems of the cortex, and stellate cells are predominantly afferent. It is believed that there are 10 times more neuroglial cells in the brain than ganglion (nerve) cells, that is, about 100-130 billion. The thickness of the cortex varies from 1.5 to 4 mm. The total surface of both hemispheres of the cortex in an adult is from 1450 to 1700 cm 2.

A feature of the structure of the cerebral cortex is the arrangement of nerve cells in six layers lying one above the other.

1. the first layer - lamina zonalis, zonal (marginal) layer or molecular - is poor in nerve cells and is formed mainly by a plexus of nerve fibers
2. the second - lamina granularis externa, the outer granular layer - is so called because of the presence of densely located small cells in it, 4-8 microns in diameter, which have the shape of round, triangular and polygonal grains on microscopic preparations
3. the third - lamina pyramidalis, the pyramidal layer - is thicker than the first two layers. It contains pyramidal cells of various sizes
4. the fourth - lamina dranularis interna, the inner granular layer - like the second layer, it consists of small cells. This layer in some areas of the cerebral cortex of an adult organism may be absent; so, for example, it is not in the motor cortex
5. the fifth - lamina gigantopyramidalis, a layer of large pyramids (giant Betz cells) - a thick process departs from the upper part of these cells - a dendrite, which branches many times in the surface layers of the cortex. Another long process - the axon - of large pyramidal marks goes into the white matter and goes to the subcortical nuclei or to the spinal cord.
6. sixth - lamina multiformis, polymorphic layer (multiform) - consists of triangular and spindle-shaped cells

On a functional basis, the neurons of the cerebral cortex can be divided into three main groups.

1. Sensory neurons cerebral cortex, the so-called stellate neurons, which in particular in large numbers are located in the III and IV layers of the sensory areas of the cortex. The axons of the third neurons of specific afferent pathways terminate on them. These cells provide the perception of afferent impulses coming to the cerebral cortex from the nuclei of the visual tubercles.
2. Motor (effector) neurons - cells that send impulses to the underlying parts of the brain- to the subcortical nuclei, the brain stem and spinal cord. These are large pyramidal neurons, which were first described by V. A. Betz in 1874. They are concentrated mainly in the V layer of the motor cortex. Some spindle-shaped cells also take part in the implementation of the effector function of the cortex.
3. Contact, or intermediate, neurons- cells that communicate between different neurons of the same or different zones bark. These include small and medium pyramidal and fusiform cells.

STRUCTURE OF MYELIN FIBERS (MYELOARCHITECTONICS)

Myeloarchitectonically, the human cerebral cortex is also divided mainly into six layers corresponding to the indicated cell layers. The myeloarchitectonic layers, to an even greater extent than the cytoarchitectonic layers, break up into sublayers and are extremely variable in different parts of the cortex.

In the complex structure of the nerve fibers of the cerebral cortex, there are

• horizontal fibers connecting different parts of the cortex, and
• radial fibers that connect gray and white matter.

The above description of the cellular structure of the cortex is to a certain extent schematic, since there are significant variations in the degree of development of these layers in different areas of the cortex.

Nervous tissue is a collection of interconnected nerve cells (neurons, neurocytes) and auxiliary elements (neuroglia), which regulates the activity of all organs and systems of living organisms. This is the main element of the nervous system, which is divided into central (includes the brain and spinal cord) and peripheral (consisting of ganglions, trunks, endings).

The main functions of the nervous tissue

1. Perception of irritation;
2. the formation of a nerve impulse;
3. rapid delivery of excitation to the central nervous system;
4. data storage;
5. production of mediators (biologically active substances);
6. adaptation of the organism to changes in the external environment.

properties of nervous tissue

• Regeneration- occurs very slowly and is possible only in the presence of an intact perikaryon. Restoration of the lost shoots goes by germination.
• Braking- prevents the occurrence of arousal or weakens it
• Irritability- response to the influence of the external environment due to the presence of receptors.
• Excitability- generation of an impulse when the threshold value of irritation is reached. There is a lower threshold of excitability, at which the smallest influence on the cell causes excitation. The upper threshold is the amount of external influence that causes pain.

The structure and morphological characteristics of nerve tissues

The main structural unit is neuron. It has a body - the perikaryon (in which the nucleus, organelles and cytoplasm are located) and several processes. It is the shoots that are hallmark cells of this tissue and serve to transfer excitation. Their length ranges from micrometers to 1.5 m. The bodies of neurons are also of different sizes: from 5 microns in the cerebellum to 120 microns in the cerebral cortex.

Until recently, it was believed that neurocytes are not capable of division. It is now known that the formation of new neurons is possible, although only in two places - this is the subventricular zone of the brain and the hippocampus. The lifespan of neurons is equal to the lifespan of an individual. Every person at birth has about trillion neurocytes and in the process of life loses 10 million cells every year.

offshoots There are two types - dendrites and axons.

The structure of the axon. It starts from the body of the neuron as an axon mound, does not branch out throughout, and only at the end is divided into branches. An axon is a long process of a neurocyte that carries out the transmission of excitation from the perikaryon.

The structure of the dendrite. At the base of the cell body, it has a cone-shaped extension, and then it is divided into many branches (this is the reason for its name, “dendron” from ancient Greek - a tree). The dendrite is a short process and is necessary for the translation of the impulse to the soma.

According to the number of processes, neurocytes are divided into:

• unipolar (there is only one process, the axon);
• bipolar (both axon and dendrite are present);
• pseudo-unipolar (one process departs from some cells at the beginning, but then it divides into two and is essentially bipolar);
• multipolar (have many dendrites, and among them there will be only one axon).

Multipolar neurons prevail in the human body, bipolar neurons are found only in the retina of the eye, in the spinal nodes - pseudo-unipolar. Monopolar neurons are not found at all in the human body; they are characteristic only of poorly differentiated nervous tissue.

neuroglia

Neuroglia is a collection of cells that surrounds neurons (macrogliocytes and microgliocytes). About 40% of the CNS is accounted for by glial cells, they create conditions for the production of excitation and its further transmission, perform supporting, trophic, and protective functions.

Macroglia:

Ependymocytes- are formed from glioblasts of the neural tube, line the canal of the spinal cord.

astrocytes- stellate, small in size with numerous processes that form the blood-brain barrier and are part of the gray matter of the GM.

Oligodendrocytes- the main representatives of neuroglia, surround the perikaryon along with its processes, performing the following functions: trophic, isolation, regeneration.

neurolemocytes- Schwann cells, their task is the formation of myelin, electrical insulation.

microglia - consists of cells with 2-3 branches that are capable of phagocytosis. Provides protection against foreign bodies, damage, as well as removal of products of apoptosis of nerve cells.

Nerve fibers- these are processes (axons or dendrites) covered with a sheath. They are divided into myelinated and unmyelinated. Myelinated in diameter from 1 to 20 microns. It is important that myelin is absent at the junction of the sheath from the perikaryon to the process and in the area of ​​axonal ramifications. Unmyelinated fibers are found in the autonomic nervous system, their diameter is 1-4 microns, the impulse travels at a speed of 1-2 m/s, which is much slower than myelinated ones, they have a transmission speed of 5-120 m/s.

Neurons are subdivided according to functionality:

• Afferent- that is, sensitive, accept irritation and are able to generate an impulse;
• associative- perform the function of impulse translation between neurocytes;
• efferent- complete the transfer of the impulse, performing a motor, motor, secretory function.

Together they form reflex arc, which ensures the movement of the impulse in only one direction: from sensory fibers to motor ones. One individual neuron is capable of multidirectional transmission of excitation, and only as part of a reflex arc does a unidirectional impulse flow occur. This is due to the presence of a synapse in the reflex arc - an interneuronal contact.

Synapse consists of two parts: presynaptic and postsynaptic, between them there is a gap. The presynaptic part is the end of the axon that brought the impulse from the cell, it contains mediators, it is they that contribute to the further transmission of excitation to the postsynaptic membrane. The most common neurotransmitters are: dopamine, norepinephrine, gamma-aminobutyric acid, glycine, for which there are specific receptors on the surface of the postsynaptic membrane.

Chemical composition of nervous tissue

Water is contained in a significant amount in the cerebral cortex, less in white matter and nerve fibers.

Protein substances represented by globulins, albumins, neuroglobulins. Neurokeratin is found in the white matter of the brain and axon processes. Many proteins in the nervous system belong to mediators: amylase, maltase, phosphatase, etc.

The chemical composition of the nervous tissue also includes carbohydrates are glucose, pentose, glycogen.

Among fat phospholipids, cholesterol, cerebrosides were found (it is known that newborns do not have cerebrosides, their number gradually increases during development).

trace elements in all structures of the nervous tissue are distributed evenly: Mg, K, Cu, Fe, Na. Their importance is very great for the normal functioning of a living organism. So magnesium is involved in the regulation of the nervous tissue, phosphorus is important for productive mental activity, potassium ensures the transmission of nerve impulses.

"Nerve cells are not restored," we are accustomed to hearing and repeating for a long time. And this expression could well be included in common truths. Nevertheless, at the first congress on the regeneration of the central nervous system held in the United States in 1970, reports were made that testified : nerve cells can be regenerated, and even to a wider extent than scientists previously thought.

Ten years have passed, and new facts have appeared. Thus, studies carried out in medical institute State of Maryland, made it possible to establish that the nerve cells of the brain and spinal cord after their damage are regenerated as a result of the massive growth of special cells that form a dense plexus at the site of damage. Encouraging results were obtained when parts of peripheral nerve cells were transplanted into damaged areas of the spinal cord, and then parts of the nervous tissue were transplanted into degenerated areas. True, research is still being carried out on laboratory animals, experiments on humans are considered risky. If cut optic nerve a frog or a fish, then, as you know, it often recovers, finding the "right path" for itself. The "ruling factor" is probably some chemical substance discovered by Rita Levi-Montalcini that stimulates nerve cells to grow in the ganglia of the sympathetic nervous system. However, something is produced by the neurons themselves. Many years ago, the neurobiologist Paul Weiss established that matter is constantly moving inside nerve cells, and the speed of its movement can be different - from a millimeter to several tens of centimeters per day. Is this related to the process of regeneration of nerve cells?

A neuron is a structural and functional unit of the nervous system. These nerve cells have a complex structure in structure, they contain a nucleus, a cell body and processes. There are over eighty-five billion neurons in the human body.

Nerve cells consist of protoplasm (cytoplasm and nucleus), externally limited by a membrane of a double layer of lipids (bilipid layer). There are proteins on the membrane: on the surface (in the form of globules), on which outgrowths of polysaccharides can be observed, due to which cells perceive external irritation, and integral proteins penetrating the membrane through, in which there are ion channels. The neuron consists of a body with a diameter of 3 to 130 microns, containing a nucleus and organelles, as well as processes. There are two types of processes: dendrites and axons. The neuron has a developed and complex cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell.

An axon is usually a long process of a nerve cell adapted to conduct excitation and information from the body of a neuron or from a neuron to an executive organ. Dendrites are short and highly branched processes of a neuron that serve as the main site for the formation of excitatory and inhibitory synapses that affect the neuron, and which transmit excitation to the body of the nerve cell.

nervous tissue- the main structural element of the nervous system. AT composition of nervous tissue contains highly specialized nerve cells neurons, and neuroglial cells performing supporting, secretory and protective functions.

Neuron is the main structural and functional unit of the nervous tissue. These cells are able to receive, process, encode, transmit and store information, establish contacts with other cells. The unique features of a neuron are the ability to generate bioelectric discharges (impulses) and transmit information along the processes from one cell to another using specialized endings -.

The performance of the functions of a neuron is facilitated by the synthesis in its axoplasm of substances-transmitters - neurotransmitters: acetylcholine, catecholamines, etc.

The number of brain neurons approaches 10 11 . One neuron can have up to 10,000 synapses. If these elements are considered information storage cells, then we can conclude that the nervous system can store 10 19 units. information, i.e. capable of containing almost all the knowledge accumulated by mankind. Therefore, the notion that the human brain remembers everything that happens in the body and when it communicates with the environment is quite reasonable. However, the brain cannot extract from all the information that is stored in it.

Certain types of neural organization are characteristic of various brain structures. Neurons that regulate a single function form the so-called groups, ensembles, columns, nuclei.

Neurons differ in structure and function.

By structure(depending on the number of processes extending from the cell body) distinguish unipolar(with one process), bipolar (with two processes) and multipolar(with many processes) neurons.

According to functional properties allocate afferent(or centripetal) neurons that carry excitation from receptors in, efferent, motor, motor neurons(or centrifugal), transmitting excitation from the central nervous system to the innervated organ, and intercalary, contact or intermediate neurons connecting afferent and efferent neurons.

Afferent neurons are unipolar, their bodies lie in the spinal ganglia. The process extending from the cell body is divided in a T-shape into two branches, one of which goes to the central nervous system and performs the function of an axon, and the other approaches the receptors and is a long dendrite.

Most efferent and intercalary neurons are multipolar (Fig. 1). Multipolar intercalary neurons are located in large numbers in the posterior horns of the spinal cord, and are also found in all other parts of the central nervous system. They can also be bipolar, such as retinal neurons that have a short branching dendrite and a long axon. Motor neurons are located mainly in the anterior horns of the spinal cord.

Rice. 1. The structure of the nerve cell:

1 - microtubules; 2 - a long process of a nerve cell (axon); 3 - endoplasmic reticulum; 4 - core; 5 - neuroplasm; 6 - dendrites; 7 - mitochondria; 8 - nucleolus; 9 - myelin sheath; 10 - interception of Ranvier; 11 - the end of the axon

neuroglia

neuroglia, or glia, - a set of cellular elements of the nervous tissue, formed by specialized cells of various shapes.

It was discovered by R. Virchow and named by him neuroglia, which means "nerve glue". Neuroglia cells fill the space between neurons, accounting for 40% of the brain volume. Glial cells are 3-4 times smaller than nerve cells; their number in the CNS of mammals reaches 140 billion. With age, the number of neurons in the human brain decreases, and the number of glial cells increases.

It has been established that neuroglia is related to the metabolism in the nervous tissue. Some neuroglia cells secrete substances that affect the state of excitability of neurons. It has been noted that for various mental states the secretion of these cells changes. Long-term trace processes in the CNS are associated with the functional state of neuroglia.

Types of glial cells

According to the nature of the structure of glial cells and their location in the CNS, they distinguish:

• astrocytes (astroglia);
• oligodendrocytes (oligodendroglia);
• microglial cells (microglia);
• Schwann cells.

Glial cells perform supporting and protective functions for neurons. They are included in the structure. astrocytes are the most numerous glial cells, filling the spaces between neurons and covering. They prevent the spread of neurotransmitters diffusing from the synaptic cleft into the CNS. Astrocytes have receptors for neurotransmitters, the activation of which can cause fluctuations in the membrane potential difference and changes in the metabolism of astrocytes.

Astrocytes tightly surround the capillaries of the blood vessels of the brain, located between them and neurons. On this basis, it is suggested that astrocytes play an important role in the metabolism of neurons, by regulating capillary permeability for certain substances.

One of the important functions of astrocytes is their ability to absorb excess K+ ions, which can accumulate in the intercellular space during high neuronal activity. Gap junction channels are formed in the areas of close adherence of astrocytes, through which astrocytes can exchange various small ions and, in particular, K+ ions. This increases the ability of them to absorb K+ ions. Uncontrolled accumulation of K+ ions in the interneuronal space would lead to an increase in the excitability of neurons. Thus, astrocytes, absorbing an excess of K+ ions from the interstitial fluid, prevent an increase in the excitability of neurons and the formation of foci of increased neuronal activity. The appearance of such foci in the human brain may be accompanied by the fact that their neurons generate a series of nerve impulses, which are called convulsive discharges.

Astrocytes are involved in the removal and destruction of neurotransmitters entering extrasynaptic spaces. Thus, they prevent the accumulation of neurotransmitters in the interneuronal spaces, which could lead to brain dysfunction.

Neurons and astrocytes are separated by intercellular gaps of 15–20 µm, called the interstitial space. Interstitial spaces occupy up to 12-14% of the brain volume. An important property of astrocytes is their ability to absorb CO2 from the extracellular fluid of these spaces, and thereby maintain a stable brain pH.

Astrocytes are involved in the formation of interfaces between the nervous tissue and brain vessels, nervous tissue and brain membranes in the process of growth and development of the nervous tissue.

Oligodendrocytes characterized by the presence of a small number of short processes. One of their main functions is myelin sheath formation of nerve fibers within the CNS. These cells are also located in close proximity to the bodies of neurons, but the functional significance of this fact is unknown.

microglial cells make up 5-20% of the total number of glial cells and are scattered throughout the CNS. It has been established that the antigens of their surface are identical to the antigens of blood monocytes. This indicates their origin from the mesoderm, penetration into the nervous tissue during embryonic development and subsequent transformation into morphologically recognizable microglial cells. In this regard, it is generally accepted that the most important function of microglia is to protect the brain. It has been shown that when the nervous tissue is damaged, the number of phagocytic cells increases due to blood macrophages and activation of the phagocytic properties of microglia. They remove dead neurons, glial cells and their structural elements, phagocytize foreign particles.

Schwann cells form the myelin sheath of peripheral nerve fibers outside the CNS. The membrane of this cell repeatedly wraps around, and the thickness of the resulting myelin sheath can exceed the diameter of the nerve fiber. The length of the myelinated sections of the nerve fiber is 1-3 mm. In the intervals between them (interceptions of Ranvier), the nerve fiber remains covered only by a surface membrane that has excitability.

One of the most important properties myelin is its high resistance electric current. It is due to the high content of sphingomyelin and other phospholipids in myelin, which give it current-insulating properties. In areas of the nerve fiber covered with myelin, the process of generating nerve impulses is impossible. Nerve impulses are generated only at the Ranvier interception membrane, which provides a higher speed of nerve impulse conduction in myelinated nerve fibers compared to unmyelinated ones.

It is known that the structure of myelin can be easily disturbed in infectious, ischemic, traumatic, toxic damage to the nervous system. At the same time, the process of demyelination of nerve fibers develops. Especially often demyelination develops with a disease multiple sclerosis. As a result of demyelination, the rate of conduction of nerve impulses along the nerve fibers decreases, the rate of delivery of information to the brain from receptors and from neurons to the executive organs decreases. This can lead to impaired sensory sensitivity, movement disorders, regulation of internal organs and other serious consequences.

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 during their propagation along the dendrite membrane attenuate, but they 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. In the dendritic membrane, no voltage-dependent sodium and potassium channels. 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 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 occurs with attenuation, 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 areas 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.