What a human nerve cell consists of. Nerve Cells (Neurons)

This cell has a complex structure, is highly specialized and in structure contains the nucleus, cell body and processes. In the human body, there are more than one hundred billion neurons.

Overview

The complexity and variety of functions of the nervous system are determined by the interaction between neurons, which, in turn, is a set of various signals transmitted in the framework of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions generating an electric charge that moves along a neuron.

Structure

A neuron consists of a body with a diameter of 3 to 130 μm, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough ESR with active ribosomes, the Golgi apparatus), as well as processes. There are two types of processes: dendrites and. A neuron has a developed and complex cytoskeleton that penetrates its processes. The cytoskeleton maintains the shape of the cell; its filaments serve as “rails” for transporting organelles and substances packed into membrane vesicles (for example, neurotransmitters). The cytoskeleton of a neuron consists of fibrils of different diameters: Microtubules (D \u003d 20-30 nm) - consist of tubulin protein and stretch from the neuron along the axon, up to the nerve endings. Neurofilaments (D \u003d 10 nm) - together with microtubules provide intracellular transport of substances. Microfilaments (D \u003d 5 nm) - consist of actin and myosin proteins, especially expressed in growing nerve processes and c. A developed synthetic apparatus is revealed in the body of the neuron, the granular EPS of the neuron is colored basophilically and is known as the “tigroid”. The tigroid penetrates the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon.

Anterograd (from the body) and retrograde (to the body) axon transport is distinguished.

Dendrites and axon

Axon is usually a long process adapted to guide a neuron from the body. Dendrites are usually short and highly branched processes that serve as the main formation site of excitatory and inhibitory synapses affecting a neuron (different neurons have a different ratio of axon and dendrite lengths). A neuron can have several dendrites and usually only one axon. One neuron can have connections with many (up to 20 thousand) other neurons.

Dendrites divide dichotomously, while axons produce collaterals. At the branch nodes, mitochondria are usually concentrated.

Dendrites do not have a myelin sheath, but axons can have it. The place of generation of excitation in most neurons is the axon knoll - formation at the site of axon discharge from the body. For all neurons, this zone is called the trigger.

Synapse   (Greek σύναψις, from συνάπτειν - hug, grab, shake hands) - the place of contact between two neurons or between a neuron and a receiving effector cell. Serves for transmission between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. Some synapses cause depolarization of a neuron, others - hyperpolarization; the former are exciting, the latter inhibitory. Usually, stimulation of a neuron requires irritation from several excitatory synapses.

The term was introduced in 1897 by the English physiologist Charles Sherrington.

Classification

Structural classification

Based on the number and location of dendrites and axon, neurons are divided into non-axon, unipolar neurons, pseudo-unipolar neurons, bipolar neurons and multipolar (many dendritic trunks, usually efferent) neurons.

Axonless Neurons   - small cells, grouped close in the intervertebral ganglia, which do not have anatomical signs of the division of processes into dendrites and axons. All processes in the cell are very similar. The functional purpose of axon-free neurons is poorly understood.

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

Bipolar neurons   - neurons having one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia.

Multipolar neurons   - neurons with one axon and several dendrites. This type of nerve cell prevails in.

Pseudo-Unipolar Neurons - are unique in their kind. One process leaves the body, which immediately T-divides. This whole single tract is covered with a myelin sheath and structurally represents an axon, although excitation along one of the branches does not come from, but to the body of the neuron. The dendrites are structurally branched at the end of this (peripheral) process. The trigger zone is the beginning of this branching (that is, located outside the cell body). Such neurons are found in the spinal ganglia.

Functional classification

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

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

Efferent neurons   (effector, motor or motor). The neurons of this type include terminal neurons - ultimatum and penultimate - not ultimatum.

Associative neurons   (insertion or interneurons) - a group of neurons communicates between efferent and afferent, they are divided into intrusive, commissural and projection.

Secretory neurons   - neurons secreting highly active substances (neurohormones). They have a well-developed Golgi complex, the axon ends with axovasal synapses.

Morphological classification

The morphological structure of neurons is diverse. In this regard, in the classification of neurons apply several principles:

• take into account the size and shape of the body of the neuron;
• the number and nature of branching of the processes;
• the length of the neuron and the presence of specialized membranes.

By the shape of the cell, neurons can be spherical, granular, star-shaped, pyramidal, pear-shaped, spindle-shaped, irregular, etc. The body size of a neuron varies from 5 μm in small granular cells to 120-150 μm in giant pyramidal neurons. The length of a neuron in humans is from 150 microns to 120 cm.

The following morphological types of neurons are distinguished by the number of processes:

• unipolar (with one process) neurocytes present, for example, in the sensory nucleus of the trigeminal nerve;
• pseudo-unipolar cells grouped nearby in the intervertebral ganglia;
• bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
• multipolar neurons (have one axon and several dendrites), prevailing in the central nervous system.

Neuron development and growth

A neuron develops from a small progenitor cell, which ceases to divide even before it releases its processes. (However, the question of neuron division currently remains controversial) As a rule, the axon begins to grow first, and dendrites form later. At the end of the developing process of the nerve cell, a thickening of an irregular shape appears, which, apparently, paves the way through the surrounding tissue. This thickening is called a nerve cell growth cone. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspikes have a thickness of 0.1 to 0.2 μm and can reach 50 μm in length, the wide and flat region of the growth cone has a width and length of about 5 μm, although its shape can vary. The gaps between the microspikes of the growth cone are covered with a folded membrane. Microspikes are in constant motion - some are drawn into the growth cone, others are elongated, deviate in different directions, touch the substrate and can stick to it.

The growth cone is filled with small, sometimes connected to each other, irregular membrane vesicles. A dense mass of entangled actin filaments is located directly under the folded sections of the membrane and in the spines. The growth cone also contains mitochondria, microtubules, and neurofilaments present in the body of the neuron.

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

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

The main function of the nervous system is the transmission of information through electrical stimuli. To do this, you must:

1. Exchange of chemicals with the environment - membrane   –Long information processes.

2. Fast signal exchange - special areas on the membrane - synapses

3. The mechanism for the rapid exchange of signals between cells - special chemicals - pickssecreted by some cells and perceived by others in synapses

4. The cell responds to changes in synapses located on short processes - dendrites   by slow changes in electrical potentials

5. A cell transmits signals over long distances using fast electrical signals along long processes - axons

Axon   - one in the neuron, has an extended structure, conducts fast electrical impulses from the cell body

Dendrites   - may be many, branch, short, conducts slow gradual electrical impulses to the cell body

Nerve cell,   or neuron,   consists of a body and processes of two types. Body   the neuron is represented by the nucleus and the surrounding area of \u200b\u200bthe cytoplasm. This is the metabolic center of a nerve cell; when it is destroyed, it dies. The bodies of neurons are located mainly in the brain and spinal cord, i.e., in the central nervous system (CNS), where their clusters form gray matter of the brain. Accumulations of nerve cell bodies outside the central nervous system form nerve nodes, or ganglia.

Short, tree-branching processes extending from the body of a neuron are called dendrites. They perform the functions of perceiving irritation and transmitting excitation to the body of a neuron.

The most powerful and longest (up to 1 m) non-branching process is called an axon, or nerve fiber. Its function is to conduct excitation from the body of the nerve cell to the end of the axon. It is covered with a special white lipid membrane (myelin), which plays the role of protecting, nourishing and isolating nerve fibers from each other. Accumulations of axons in the central nervous system form the white matter of the brain. Hundreds and thousands of nerve fibers extending beyond the central nervous system, with the help of connective tissue are combined into bundles - nerves that give numerous branches to all organs.

Side branches extend from the ends of the axons, ending with extensions - axoppy endings, or terminals. This is the area of \u200b\u200bcontact with other nerve, muscle or glandular marks. It is called a synapse, the function of which is to transmit excitation. One neuron through its synapses can connect to hundreds of other cells.

According to the functions performed, three types of neurons are distinguished. Sensitive (centripetal) neurons perceive irritation from receptors that are excited by stimuli from the external environment or from the human body itself, and in the form of a nerve impulse transmit excitation from the periphery to the central nervous system. Motor (centrifugal) neurons send a nerve signal from the central nervous system to muscles, glands, t i.e. to the periphery. Nerve cells that perceive excitation from other neurons and transmit it also to nerve cells are insertion neurons, or interneurons. They are located in the central nervous system. Nerves, which include both sensory and motor fibers, are called mixed.

Anya:Neurons, or nerve cells, are the building blocks of the brain. Although they have the same genes, the same general structure and the same biochemical apparatus as other cells, they also have unique features that make the brain function completely different from the functions of, say, the liver. It is believed that the human brain consists of 10 in the 10th neuron: about the same as the number of stars in our galaxy. There are no two neurons that are identical in appearance. Despite this, their forms usually fit into a small number of categories, and most neurons have certain structural features that make it possible to distinguish three areas of the cell: the cell body, dendrites and axon.

The cell body - the catfish, contains the nucleus and biochemical apparatus for the synthesis of enzymes and various molecules necessary for the life of the cell. Typically, the body has an approximately spherical or pyramidal shape, ranging in size from 5 to 150 microns in diameter. Dendrites and axon are processes that extend from the body of a neuron. Dendrites are thin tubular outgrowths that branch many times, forming a crown of a tree around the body of a neuron (dendron tree). Through dendrites, nerve impulses enter the body of a neuron. Unlike numerous dendrites, the axon is the only one and differs from dendrites both in structure and in the properties of its outer membrane. The length of the axon can reach one meter, it practically does not branch, forming processes only at the end of the fiber, its name comes from the word axis (ass-axis). Along the axon, a nerve impulse leaves the cell body and is transmitted to other nerve cells or to the executive organs - muscles and glands. All axons are enclosed in a shell of Schwann cells (a type of glial cells). In some cases, Schwann cells simply wrap the axon in a thin layer. In many cases, the Schwann cell rotates around the axon, forming several dense layers of insulation called myelin. The myelin sheath is interrupted approximately every millimeter along the length of the axon by narrow slots - the so-called Ranvier intercepts. In axons having a shell of this type, the propagation of a nerve impulse occurs by jumping from interception to interception, where the extracellular fluid is in direct contact with the cell membrane. This conduction of a nerve impulse is called saltotropic. The evolutionary meaning of the myelin sheath, apparently, is to save the metabolic energy of the neuron. As a rule, myelinated nerve fibers conduct nerve impulses faster than non-myelinated ones.

By the number of processes, neurons are divided into unipolar, bipolar and multipolar.

According to the structure of the cell body, neurons are divided into stellate, pyramidal, granular, oval, etc.

Neural tissue   - The main structural element of the nervous system. AT nerve tissue composition   highly specialized nerve cells come in - neurons, and neuroglia cellsperforming supporting, secretory and protective functions.

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

The neuron functions are facilitated by the synthesis in its axoplasm of the transmitter substances - neurotransmitters: acetylcholine, catecholamines, etc.

The number of brain neurons approaches 10 11. One neuron can have up to 10,000 synapses. If we consider these elements as information storage cells, we can conclude that the nervous system can store 10 19 units. information i.e. able to accommodate almost all the knowledge accumulated by mankind. Therefore, the idea that the human brain remembers everything that happens in the body and during its communication with the environment is quite justified. However, the brain cannot extract from all the information that is stored in it.

Various 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 vary in structure and function.

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

By functional properties   emit afferent   (or centripetala) neurons carrying excitation from receptors in, efferent, motor, motor neurons   (or centrifugal) transmitting excitation from the central nervous system to the innervated organ, and insertion, 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 body of the cell is T-shaped into two branches, one of which goes to the central nervous system and acts as 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, as well as in all other parts of the central nervous system. They can be bipolar, for example, retinal neurons with a short branching dendrite and a long axon. Motor neurons are located mainly in the anterior horns of the spinal cord.

Fig. 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, - the set of cellular elements of the nervous tissue formed by specialized cells of various shapes.

It was discovered by R. Virkhov 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 central nervous system of mammals reaches 140 billion. With age in humans, the number of neurons in the brain decreases, and the number of glial cells increases.

It has been established that neuroglia is related to metabolism in the nervous tissue. Some neuroglia cells secrete substances that affect the state of neuronal excitability. It was noted that under various mental conditions, the secretion of these cells changes. Long functional processes in the central nervous system are associated with the functional state of neuroglia.

Types of Glial Cells

By the nature of the structure of glial cells and their location in the central nervous system, there are:

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

Glial cells perform supporting and protective functions for neurons. They are part of the structure. Astrocytes   are the most numerous glial cells filling the spaces between neurons and covering. They prevent the spread in the central nervous system of neurotransmitters that diffuse from the synaptic cleft. In astrocytes there are 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 believed that astrocytes play an important role in the metabolism of neurons, 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 at high neural activity. Channels of gap junctions are formed in the areas of tight adherence of astrocytes, through which astrocytes can exchange various small ions and, in particular, K + ions.This increases the possibility of their absorption of K + ions. Uncontrolled accumulation of K + ions in the interneuron space would increase the excitability of neurons. Thus, astrocytes, absorbing excess K + ions from the interstitial fluid, prevent an increase in the excitability of neurons and the formation of foci of increased neural activity. The appearance of such foci in the human brain can 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 impaired brain function.

Neurons and astrocytes are separated by intercellular clefts of 15-20 microns, 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 these extracellular fluids, and thereby maintain a stable brain pH.

Astrocytes are involved in the formation of interfaces between nerve tissue and blood vessels of the brain, nerve tissue and brain membranes during the growth and development of nerve tissue.

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

Microglia cells make up 5-20% of the total number of glial cells and are scattered throughout the central nervous system. It was 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 microglia cells. In this regard, it is believed that the most important function of microglia is to protect the brain. It was shown that in case of damage to the nervous tissue, 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, phagocytose foreign particles.

Schwann cells   form the myelin sheath of peripheral nerve fibers outside the central nervous system. The membrane of this cell is repeatedly wrapped around, and the thickness of the resulting myelin sheath may exceed the diameter of the nerve fiber. The length of myelinated sections of the nerve fiber is 1-3 mm. In the spaces between them (Ranvier intercepts), the nerve fiber remains covered only by a surface membrane with excitability.

One of the most important properties of myelin is its high resistance to 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 nerve fiber coated with myelin, the process of generating nerve impulses is impossible. Nerve impulses are generated only on the Ranvier intercept membrane, which provides a higher speed of nerve impulses for myelinated nerve fibers compared to non-myelinated ones.

It is known that the structure of myelin can easily be violated with 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 of multiple sclerosis. As a result of demyelination, the speed of conduction of nerve impulses along nerve fibers decreases, the speed of delivery to the brain of information from receptors and from neurons to the executive organs decreases. This can lead to impaired sensory sensitivity, impaired movement, regulation of internal organs, and other serious consequences.

The structure and function of neurons

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

The anatomical structure and properties of the neuron ensure its implementation main functions: the implementation of metabolism, energy, the perception of various signals and their processing, the formation or participation in responses, the generation and conduct of nerve impulses, the union of neurons in neural circuits, providing both the simplest reflex reactions and the highest integrative functions of the brain.

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

Fig. 2. The structure of the neuron

Nerve cell body

Body (pericarion, catfish)   the neuron and its processes throughout are covered with 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.

The neuroplasma is located in the body of the neuron and the core is delimited by membranes, the rough and smooth endoplasmic reticulum, Golgi apparatus, and mitochondria are located. 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 a neuron, its processes and synapses. These are proteins that perform the functions of enzymes, carriers, ion channels, receptors, and others. Some proteins perform functions while in the neuroplasm, others integrate into 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 via axonal transport. In the body of the cell, peptides are synthesized, which are necessary for the functioning of axons and dendrites (for example, growth factors). Therefore, with damage to the body of a neuron, its processes degenerate and are destroyed. If the body of the neuron is preserved, and the process is damaged, then it slowly regenerates (regenerates) and restores the innervation of denervated muscles or organs.

The site of protein synthesis in the bodies of neurons is a 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 a smooth endoplasmic reticulum and Golgi apparatus, proteins acquire their spatial conformation, sorted and sent to transport flows to the structures of the cell body, dendrites or axon.

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

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

Nerve cell dendrites

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). There are thousands of synapses on neuron dendrites formed by axons or dendrites of other neurons.

Fig. 3. Synaptic contacts of the interneuron. The arrows on the left indicate the arrival 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) to 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 spikes. There are channels in the spine membrane, the permeability of which depends on the transmembrane potential difference. Secondary mediators of intracellular signal transmission, as well as ribosomes, on which protein is synthesized in response to the arrival of synaptic signals, were found in the cytoplasm of dendrites in the area of \u200b\u200bspines. The exact role of the spines remains unknown, but it is obvious that they increase the surface area of \u200b\u200bthe dendritic tree for the formation of synapses. Spikes are also structures of a neuron for receiving input signals and processing them. Dendrites and spines provide the transfer of information from the periphery to the body of the neuron. The mantle dendrite membrane is polarized 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 transmission of information along the membrane in the form of local circular currents (electrotonically) that arise between postsynaptic membranes and the dendrite membrane adjacent to them.

Local currents, when they propagate along the dendritic membrane, decay, but turn out to be large enough to transmit signals to the membrane of the neuron body through the synaptic inputs to the dendrites. No potential-dependent sodium and potassium channels have yet been detected in the dendrite membrane. It does not have excitability and the ability to generate action potentials. However, it is known that the action potential arising on the membrane of the axon knoll can spread 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 large in the dendrites of neurons of the cerebellar cortex, basal ganglia, and cerebral cortex. The area of \u200b\u200bthe dendritic tree and the number of synapses decrease in some fields of the cerebral cortex of the elderly.

Axon neuron

Axon -   a process of a nerve cell 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 site of the exit of the axon from the body of the neuron there is a thickening - the axon knoll covered with a plasma membrane, which is soon covered with myelin. The site of the axon knoll uncovered by myelin is called the initial segment. The axons of neurons up to their final branches are covered with a myelin sheath, interrupted by Ranvier intercepts - microscopic myelin-free sections (about 1 μm).

Throughout the axon (myelinated and non-myelinated fiber) is covered with a bilayer phospholipid membrane with protein molecules embedded in it, which perform the functions of ion transport, voltage-dependent ion channels, etc. Proteins are distributed evenly in the membrane of the non-myelinated nerve fiber, and they are located in the membrane of the myelinated nerve fiber mainly in the field of intercepts Ranvier. Since there are 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 via axonal transport.

Properties of the membrane covering the body and axon of a neuronare different. This difference applies primarily to membrane permeability for mineral ions and is due to the content of various types. While 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 Ranvier intercepts, there is a high density of voltage-dependent sodium and potassium channels.

The lowest polarization value (about 30 mV) is possessed by the membrane of the initial axon segment. In axon regions farther from the cell body, the transmembrane potential is about 70 mV. The low polarization of the membrane of the initial segment of the axon makes it possible for the neuron membrane to have the highest excitability in this region. The postsynaptic potentials that arose on the membrane of the dendrites and body of the cell as a result of the transformation of the information signals received by the neuron at the synapses spread through the membrane of the neuron’s body using local circular electric currents. If these currents cause depolarization of the axon knoll membrane to a critical level (E k), the neuron will respond to signals from other nerve cells receiving it by generating its action potential (nerve impulse). The resulting nerve impulse is then carried out 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 GABA-ergic inhibitory synapses are formed. The arrival of signals from these from other neurons can prevent the generation of a nerve impulse.

Classification and types of neurons

Classification of neurons is carried out both by morphological and functional characteristics.

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

By the nature of connections with other cells and the function performed, they are distinguished   sensory, insertion   and   motor   neurons.   Sensory   neurons are also called afferent neurons, and their processes are centripetal. Neurons that perform the function of transmitting signals between nerve cells are called   insertion, or   associative.Neurons whose axons form synapses on effector cells (muscle, glandular) are classified as   motor,or   efferent, their axons are called centrifugal.

Afferent (Sensitive) Neurons   perceive information by sensory receptors, convert it into nerve impulses and conduct to the brain and spinal cord. The bodies of sensitive neurons are located in the spinal and cranial brain. These are pseudo-unipolar neurons, the axon and dendrite of which depart from the body of the neuron together and then separate. The dendrite follows to the periphery to organs and tissues as part of sensitive 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 in the brain.

Insert, or   associative neurons   perform the functions of processing incoming information and, in particular, provide 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 received information 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 of plasma membranes, cytoplasm and nucleus. Signal transmission uses many different types of neurotransmitters, neuromodulators, and other signaling molecules. Obviously, for the formation of a response to the simultaneous receipt of multiple signals, the neuron must have the ability to integrate them.

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

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

Fig. 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. Perceived signals are converted at synapses to the oscillation of 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 - the synapses in the diagram are shown as light circles) or hyperpolarizing (TPPS - synapses in the diagram are shown in black circles). At different points of the neuron, many signals can simultaneously arrive, some of which are transformed into EPSP, and others into TPPS.

These potential difference oscillations propagate using local circular currents along the neuron membrane in the direction of the axon knoll in the form of depolarization waves (in the white color scheme) and hyperpolarization (in the black color scheme) superimposed on each other (gray sections in the scheme). In this overlap, the amplitudes of the waves of one direction are summed, while the opposite - decrease (smooth). Such an algebraic summation of the potential difference on the membrane is called   spatial summation   (Fig. 4 and 5). The result of this summation can be either depolarization of the axon knoll 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 knoll membrane (about 30 mV) to E k, it must be depolarized by 10-20 mV. This will lead to the discovery of potential-dependent sodium channels in it and the generation of a nerve impulse. Since upon receipt of one PD and its conversion into EPSP, membrane depolarization can reach up to 1 mV, and all propagation to the axon knoll proceeds with attenuation, for the generation of a nerve impulse, 40-80 nerve impulses from other neurons and summation to the neuron at the same time are required and summation same amount of EPSP.

Fig. 5. Spatial and temporal summation of EPSP by a neuron; a - BPSP for a single stimulus; and - EPSP for 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 while increasing the flow of signals through excitatory synapses. Under conditions when signals arriving through inhibitory synapses cause hyperpolarization of the neuron membrane equal to or greater than the depolarization caused by signals arriving through excitatory synapses, depolarization of the axon knoll membrane will be impossible, the neuron will not generate nerve impulses and become inactive.

Neuron also carries out   temporary summation   EPSP and TPSC signals arriving at it almost simultaneously (see Fig. 5). The changes in the potential difference caused by them in the near-synaptic regions can also be algebraically summed, which is called the temporary summation.

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

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

Molecular receptors are involved in the conversion and integration of signals arriving at the neuron. At the same time, their stimulation by signaling molecules can lead through changes in the state of ion channels initiated by (G-proteins, second mediators), transformation of the perceived signals into fluctuation of the potential difference of the neuron membrane, summing up and forming a neuron response in the form of generation of a nerve impulse or its inhibition.

Signal conversion by metabotropic molecular receptors of a neuron is accompanied by its response in the form of triggering a cascade of intracellular transformations. The response of a neuron in this case may be an acceleration of general 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 activities.

Intracellular transformations in a neuron, initiated by the received signals, often lead to increased 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.

Receiving a number of signals by a neuron may be accompanied by the expression or repression of certain genes, for example, 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 may have a stronger or weaker effect on other nerve cells controlled by it. Considering that the modulating effect of neuropeptides can last for a long time, the effect of a neuron on other nerve cells can also last 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, allowing it to effectively adapt to the nature of the incoming signals and use them to regulate the functions of other cells.

Neural circuits

CNS neurons interact with each other, forming a variety of synapses at the site 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 axon collateral to the neuron (2), forming an axosomatic synapse on its body, and the second one will form an axon synapse on the body of the first neuron. Local neural networks can serve as traps in which nerve impulses are able to circulate for a long time in a circle formed by several neurons.

The possibility of prolonged circulation of the once generated wave of excitation (nerve impulse) due to the transmission of the ring structure, experimentally shown by Professor I.A. Vetokhin in experiments on the jellyfish nerve ring.

The circular circulation of nerve impulses along local neural circuits performs the function of transforming the rhythm of excitations, provides the possibility of long-term excitation after the cessation of the arrival of signals to them, and participates in the mechanisms of memorizing incoming information.

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

Fig. 6. The simplest neural circuits of the central nervous system. Description in text

In this case, the excitation that arose in the motor neuron propagates along the axon branch, activates the Renshaw cell, which inhibits the a-motor neuron.

Convergent chains   formed by several neurons, on one of which (usually efferent) axons of a number of other cells converge or converge. Such chains are widespread in the central nervous system. For example, axons of many neurons of sensitive fields of the cortex converge onto the pyramidal neurons of the primary motor cortex. Axons of thousands of sensitive and intercalated neurons of various levels of the central nervous system converge on motor neurons of the ventral horns of the spinal cord. Convergent chains play an important role in integrating signals with efferent neurons and coordinating physiological processes.

Divergent Single Entry Circuits   formed by a neuron with a branching axon, each of the branches of which 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 brain stem. They provide a rapid increase in the excitability of numerous parts of the brain and the mobilization of its functional reserves.

NEURON - it is a separate nerve cell, the building block of the brain. It transmits nerve impulses along a single long fiber (axon) and receives them through numerous short fibers (dendrites)   (C. Stevens).

Although neurons, or nerve cells, have the same genes, the same general structure and the same biochemical apparatus as other cells, they also have unique features that make brain function completely different from, say, the liver. Important features of neurons are the characteristic shape, the ability of the outer membrane to generate nerve impulses and the presence of a unique structure - a synapse, which serves to transfer information from one neuron to another.

It is believed that the human brain consists of 10 11 neurons: this is approximately the same as the stars in our galaxy. There are no two neurons that are identical in appearance. Despite this, their forms usually fit into a small number of broad categories, and most neurons have certain structural features that make it possible to distinguish three areas of the cell: the cell body, dendrites and axon. The body contains the nucleus and biochemical apparatus for the synthesis of enzymes and other molecules necessary for the life of the cell. Typically, the body of a neuron has an approximately spherical or pyramidal shape. Dendrites are thin tubular outgrowths that repeatedly divide and form a branching tree around the cell body. They create that basic physical surface onto which signals coming to a given neuron arrive. The axon stretches far from the body of the cell and serves as the communication line through which the signals generated in the body of this cell can be transmitted over long distances to other parts of the brain and the rest of the nervous system. Axon differs from dendrites both in structure and in the properties of its outer membrane. Most axons are longer and thinner than dendrites and have a different branching character: if the processes of dendrites are mainly grouped around the cell body, then the processes of axons are located at the end of the fiber, in the place where the axon interacts with other neurons.

The functioning of the brain is associated with the movement of information flows along complex circuits consisting of neural networks. Information is transmitted from one cell to another in specialized places of contact - synapses. A typical neuron can have from 1000 to 10000 synapses and receive information from 1000 other neurons. Although most synapses form between the axons of one cell and the dendrites of another, there are other types of synaptic contacts: between axon and axon, between the dendrite and dendrite and between the axon and the body of the cell. In the synapse region, the axon usually expands, forming at the end a presynaptic plaque, which is an information transferring part of the contact. The terminal plaque contains small spherical formations called synaptic vesicles, each of which contains several thousand molecules of a chemical mediator. Upon arrival at the presynaptic end of the nerve impulse, some of the vesicles eject their contents into a narrow gap separating the plaque from the dendritic membrane of another cell designed to receive such chemical signals. Thus, information is transmitted from one neuron to another using some mediator or mediator. Impulse of a neuron reflects the activation of hundreds of synapses by acting neurons. Some synapses are excitatory, i.e. they contribute to the generation of impulses, while others - inhibitory - can cancel the action of signals that, in their absence, could excite a postsynaptic neuron.

Although neurons are the building blocks of the brain, they are not the only cells in it. So, oxygen and nutrients are supplied by a dense network of blood vessels. There is a need for connective tissue, especially on the surface of the brain. One of the important classes of cells of the central nervous system, as previously noted, are glial cells, or glia. Glia occupies almost the entire space in the nervous system that is not occupied by the neurons themselves. Although the function of glia has not yet been fully studied, it appears to provide structural and metabolic support for a network of neurons.

In axons with a myelin sheath, the propagation of a nerve impulse occurs by jumping from interception to interception, where the extracellular fluid is in direct contact with the cell membrane. The evolutionary meaning of the myelin sheath, apparently, is to save the metabolic energy of the neuron. As a rule, myelinated nerve fibers conduct nerve impulses faster than non-myelinated ones.

Neurons are able to fulfill their function only due to the fact that their outer membrane has special properties. The axon membrane along its entire length is specialized for conducting an electrical impulse. A membrane of axon endings is able to secrete a mediator, and a membrane of dendrites responds to a mediator. In addition, the membrane provides recognition of other cells in the process of embryonic development, so that each cell finds its intended place in a network of 10 11 cells. In this regard, many modern studies are focused on the study of all those membrane properties that are responsible for nerve impulse, synaptic transmission, cell recognition and the establishment of contacts between cells.

The neuron’s membrane, like the outer membrane of any cell, has a thickness of about 5 nm and consists of two layers of lipid molecules arranged in such a way that their hydrophilic ends are turned towards the aqueous phase inside and outside the cell, and the hydrophobic ends are turned towards from the aqueous phase and form the inner part of the membrane. The lipid part of the membrane is approximately the same in all types of cells. What makes one membrane different from another is the specific proteins that are bound to the membrane in one way or another. Proteins that are actually embedded in the double lipid layer are called internal proteins. Other proteins, peripheral membrane proteins, are attached to the membrane surface, but are not an integral part of its structure. Due to the fact that membrane lipids are liquids, even internal proteins can often move freely from place to place by diffusion. However, in some cases, proteins are rigidly fixed using auxiliary structures.

Membrane proteins of all cells break down into five classes: pumps, channels, receptors, enzymes, and structural proteins. Pumps consume metabolic energy to move ions and molecules against concentration gradients and maintain the necessary concentrations of these molecules in the cell. Since charged molecules cannot pass through the double lipid layer itself, the cells acquired protein channels during evolution, which provide selective pathways for the diffusion of specific ions. Cell membranes must recognize and attach many types of molecules. These functions are performed by receptor proteins, which are binding sites with high specificity and affinity. Enzymes are placed inside the membrane or on it, which facilitates the flow of chemical reactions at the membrane surface. Finally, structural proteins provide the connection of cells into organs and maintain the subcellular structure. These five classes of membrane proteins are not necessarily mutually exclusive. So, for example, one or another protein can be both a receptor, an enzyme, and a pump.

Membrane proteins are the key to understanding the functions of a neuron, and therefore the functions of the brain. Since they occupy such a central place in modern concepts of a neuron, attention should be paid to the description of the ion pump, various types of channels and a number of other proteins, which together give neurons their unique properties. The general idea is to summarize the important characteristics of membrane proteins and to show how these characteristics determine the nerve impulse and other complex features of neuron functions.

Like all other cells, a neuron is able to maintain the constancy of its internal environment, noticeably different in composition from the surrounding fluid. Particularly striking are the differences in the concentrations of sodium and potassium ions. The external environment is about 10 times richer than sodium than the internal, and the internal environment is about 10 times richer than potassium than the external. Both potassium and sodium are able to penetrate through the pores in the cell membrane; therefore, some pump must continuously exchange sodium ions that enter the cell for potassium ions from the external environment. Such pumping out of sodium is carried out by an internal membrane protein called the Na-K-adenosine triphosphatase pump, or, as it is often called, the sodium pump.

The protein molecule of the sodium pump (or a complex of protein subunits) has a molecular weight of about 275,000 atomic units and sizes of the order of 6x8 nm 2, which is slightly larger than the thickness of the cell membrane. Each sodium pump can use the energy stored in the form of a phosphate bond in adenosine triphosphate (ATP) in order to exchange three sodium ions of the internal environment of the cell for two potassium ions of the external environment. Operating at maximum speed, each pump is able to transport about 200 sodium ions and 130 potassium ions per second through the membrane. However, the actual speed is adjusted according to the needs of the cell. Most neurons have from 100 to 200 sodium pumps per square micron of the membrane surface, but in some parts of this surface their density is almost 10 times higher. A typical small neuron apparently has about a million sodium pumps capable of transporting about 200 million sodium ions per second. It is the transmembrane gradients of sodium and potassium that make it possible to conduct a nerve impulse along a neuron.

Membrane proteins, which serve as channels, are essential for many aspects of neuron activity, and in particular for generating nerve impulses and synaptic transmission. To represent the significance of the channels for the electrical activity of the brain, one should describe the formation and consider the properties of these channels.

Since the concentrations of sodium and potassium ions on either side of the membrane are different, the inside of the axon has a negative potential of about 70 mV with respect to the external environment. In the middle of the XX century. British researchers A. Hodgkin, A. Huxley and B. Katz in their classic work on the transmission of a nerve impulse along a giant squid axon showed that the propagation of a nerve impulse is accompanied by sharp changes in the permeability of the axon membrane for sodium and potassium ions. When a nerve impulse arises at the base of the axon (in most cases it is generated by the cell body in response to activation of dendritic synapses), the transmembrane potential difference at this point locally decreases. Immediately in front of the area with a changed potential (in the direction of propagation of the nerve impulse), membrane channels open that allow sodium ions to enter the cell.

This process is self-enhancing: the flow of sodium ions through the membrane helps to open more channels, makes it easier for other ions to follow them. Penetrated into the cell sodium ions change the negative internal potential of the membrane to positive. Shortly after opening, the sodium channels close, but now another group of channels opens, which allows potassium ions to go out. This flow restores the potential inside the axon to the value of its resting potential, i.e. up to 70 mV. A sharp jump in potential, first in the positive and then in the negative direction, which looks like a peak (“spike”) on the oscilloscope’s screen, is known as action potentialand is an electrical expression of a nerve impulse. A wave of potential change sweeps along the axon to its very end in much the same way as the flame runs along the Bikford cord.

This brief description of the nerve impulse illustrates the importance of channels for the electrical activity of neurons and emphasizes two fundamental properties of channels: selectivity and the presence of gate mechanisms. Channels are selectively permeable, and the degree of selectivity varies widely. So, channels of one type allow sodium ions to pass, but strongly interfere with the passage of potassium ions, while channels of another type do the opposite. However, selectivity is rarely absolute. A channel of the same type, which has practically no selectivity, allows about 85 sodium ions to pass for every 100 potassium ions; the other channel, with greater selectivity, transmits only about 7 sodium ions for every 100 potassium ions. A channel of the first type, known as activated by acetylcholine, has a pore with a diameter of about 0.8 nm, which is filled with water. The second type of channel, known as the potassium channel, has significantly less pore and contains less water.

Sodium ion is approximately 30% less than potassium ion. The exact molecular structure that allows larger ions to pass through the cell membrane more easily than smaller ones is unknown. However, the general principles underlying such discrimination are understandable. They include interactions between ions and sections of the channel structure, combined with the specific ordering of water molecules inside the pore.

Gate mechanisms regulating the opening and closing of membrane channels are represented by two main types. A channel of the same type, mentioned above when describing a nerve impulse, opens and closes in response to changes in the potential of the cell membrane, so they say that it is controlled electrically. The second type of channels is chemically controlled. Such channels react only weakly, if at all, to changes in potential, but open when a special molecule - the mediator - binds to a certain receptor region on the channel protein. Chemically controlled channels are found in the receptive membrane of synapses: they are responsible for translating the chemical signals sent by the ends of the axon during synaptic transmission into changes in ion permeability. Chemically controlled channels are usually named according to their specific mediator. So, for example, they talk about AH-activated channels or GABA-activated channels (AH - acetylcholine, GABA - gamma-aminobutyric acid). Electrically controlled channels are usually called the ion that most easily passes through this channel.

When functioning, proteins usually change shape. Such changes in shape, called conformational, are especially pronounced in contractile proteins responsible for cell movement, but they are no less important for many enzymes and other proteins. Conformational changes in channel proteins form the basis of the gate mechanisms, since they provide opening and closing of the channel due to small movements of parts of the molecule located in a critical place and allowing to block or release the pore.

When electrically or chemically controlled channels open and pass ions, an electric current arises that can be measured. In several cases, it was possible to detect the current passing through a single channel, so that its opening and closing could be investigated directly. It was found that the time during which the channel remains open varies randomly, since the opening and closing of the channel is the result of some conformational changes in the protein molecule embedded in the membrane. The presence of randomness in the gate processes stems from random collisions of water molecules and other molecules with the structural elements of the channel.

Back in the 50-60s. The 20th century neuron, as it was usually described in textbooks, seemed to be a very simple structure. Now, thanks to such effective research methods as electron microscopy and intracellular registration using microelectrodes, it is known that neurons have an extremely complex morphological and functional organization and are very diverse.

The ultimate goal of a complex of sciences (anatomy and physiology of the central nervous system, physiology of GNI and neuropsychology) is to explain how neurons, acting together, can lead to the implementation of the behavior observed in the whole organism. Therefore, it is extremely important first of all to establish what they consist of, how they are constructed, what individual neurons can and cannot do. This need requires the study of anatomy and physiology. If the object of research is “at the junction of sciences”, then research is inevitably fraught with difficulties. A competent psychologist must know the anatomy and physiology and at the same time have a solid knowledge of psychology.

Until the middle of the XIX century. The view of the nervous system as a continuous plexus of tubules (like the vascular system) through which fluid or electricity flows was widespread. The work of anatomists - His, Kölliker, Ramon-i-Cajal - allowed Valdeyer to put forward a "neural theory." Valdeyer was convinced that the nervous system consists of many separate cells called “neurons” and that “nervous energy” is carried from one cell to another. As early as 1935, there were such scientists who did not share this belief, but with the invention of the electron microscope, it became possible to demonstrate the presence of gaps between individual cells. In the course of these and many other studies, it was unequivocally found that nerve cell, or neuron,   is the main structural and functional unit of the nervous system.

The first studies on the physiology of neurons were carried out to a large extent on isolated sections of peripheral nerves, which retain normal functions for some time if placed in appropriate conditions. As a result, many of the properties that were identified and attributed to neurons in general, in reality, applied only to certain parts of some rather atypical neurons. Over the years, the most widely distributed theory of neural conduction,arguing that an electric current called a pulse in one neuron is responsible for the discharge of other neurons with which it is in contact.

This theory, although it was incorrect, has brought to life many valuable studies on such simple nerve chains as the neuromuscular junction and spinal connections responsible for reflex reactions. But gradually the data, which contradicted the electrical theory of neural conduction, became more and more, and they could not be ignored. Finally, over the past 20–25 years, a more complex and close to truth neuron model has been created.

CLASSIFICATION OF NEURONS:

Classification of neurons by the number of processes

1. Unipolar neurons have 1 process. According to most researchers, such neurons are not found in the nervous system of mammals and humans.

2. Bipolar neurons - have 2 processes: axon and dendrite. A variety of bipolar neurons are pseudo-unipolar neurons of the spinal ganglion, where both processes (axon and dendrite) depart from a single outgrowth of the cell body.

3. Multipolar neurons - have one axon and several dendrites. They can be distinguished in any part of the nervous system.

Classification of neurons by shape

Spindle-shaped, pear-shaped, pyramidal, polygonal. This approach underlies the study of brain cytoarchitectonics.

Functional classification

Sensitive (afferent) - helping to perceive external stimuli (stimuli).

Associative (insertion interneuron).

Motor (efferent) - causing contractions and movements. It is these neurons that received the name "motor neurons", i.e. motor neurons concentrated in the motor nuclei of the anterior horns of the spinal cord and brain stem.

Biochemical classification

1. Cholinergic (mediator - AH - acetylcholine).

2. Catecholaminergic (A, HA, dopamine).

3. Amino acids (glycine, taurine).

According to the principle of their position in the network of neurons

Primary, secondary, tertiary, etc.

Based on this classification, types of nerve networks are also distinguished:

hierarchical (ascending and descending);

local - transmitting excitement at any one level;

divergent with one input (located mainly only in the midbrain and in the brain stem) - communicating immediately with all levels of the hierarchical network. Neurons of such networks are called "non-specific."

It refers to non-specific networks reticular neurons - polygonal neurons that form the intermediate zone of the gray matter of the spinal cord (including the lateral horns), the nuclei of the reticular formation of the medulla oblongata and midbrain (including the vegetative nuclei of the corresponding cranial nerves), the formation of the subthalamic and hypothalamic regions of the diencephalon.

Neurons can be distinguished depending on whether they have long (Golgi cell, type 1) or short axons (Golgi cell, type 2). In the framework of this classification, those axons are considered short whose branches remain in close proximity to the cell body. So, golgi type 1 cells (efferent) - neurons with a long axon, continuing in the white matter of the brain. BUT type 2 cells Golgi (insert) Are neurons with a short axon, the branches of which extend beyond the gray matter of the brain.

Gasser cells A, B and C-types

Neurons also differ in the speed of impulses along axons. Gasser divided the fibers into three main groups: A, B and C. The fibers of groups A and B are myelinated. The differences between groups A and B are not significant; type B neurons are found only in the preganglionic part of the autonomic nervous system. The diameter of type A fibers varies from 4 to 20 μm, and the speed with which the pulses pass through them, determined in m / s, is approximately equal to their diameter in microns, multiplied by 6. C-fibers are much smaller in diameter (0.3 up to 1.3 μm), and the speed of the pulses in them is slightly less than the diameter multiplied by 2.

Gasser subdivided A-fibers according to their speed. The fibers with the highest speed were called “A-alpha”, medium - “A-beta” and the lowest - “A-gamma". Since the conduction speed is directly proportional to the diameter, these designations are sometimes used to classify the types of myelinated fibers. In this regard, Lloyd proposed a classification based directly on the diameter of the fibers. Group 1 includes myelinated fibers with a diameter of 12-21 microns, group 2 - 6-12 microns, group 3 - 1-6 microns. Gasser C-fibers make up group 4.

Forms of nerve cells. Betz's pyramidal neurons

There is a classification of nerve cells, according to which in the cerebral cortex, neurons are divided into three main types (in shape): pyramidal, star-shaped and spindle-shaped; there are also transitional forms. These types of nerve cells of the cortex can be determined on preparations stained with the Nissl method, which does not, however, permit the detection of dendrites, axons, and their branches. To identify these details, it is necessary to apply the Golgi method.

Pyramidal neuronsin the cortex have a different size. They are found in all layers of the cortex. The largest pyramidal neurons are located in layer IV of the visual region of the cortex and in layers III and V of other cortical zones. Particularly large pyramidal neurons - Betz neurons (named after V.A. Betz, who first described them) were found in the region of the cortical end of the motor analyzer. In certain regions of the cortex, pyramidal neurons are especially richly represented in layer III; in the places of dividing this layer into three sublayers, the largest pyramidal neurons are found in the third sublayer. They, as a rule, have an apical (alikal) dendrite with significant branching, directed to the surface of the cortex. In most cases, apical dendrites reach the layer I of the cortex, where they branch in the horizontal direction. From the base of the pyramidal neuron in the horizontal direction, basal and lateral dendrites depart, which also gradually give rise to branches of various lengths. The only long axon extending from the pyramidal neuron is directed down into the white matter and gives collateral branches branched in different directions. Sometimes its branches form an arc and are directed to the surface of the cortex, giving along the path processes that form interneuron connections.

Stellate and fusiform neurons

Very diverse stellate cells cerebral cortex, especially in humans. The system of stellate neurons with the richest branching of dendrites in phylogenesis and ontogenesis progressively grows and becomes more complicated in the cortical ends of the analyzers. Neurons of this type make up a significant part of all cellular elements of the cerebral cortex of the human brain. Their dendritic and axonal endings are very diverse and rich in branches, especially in the upper layers of the cortex, i.e. in phylogenetically the newest formations. Axons of stellate neurons, in contrast to axons of pyramidal and spindle-shaped cells, as a rule, do not go beyond the cortex of the cerebral hemispheres, and often beyond the limits of one layer. In the cerebral cortex, significant differences are observed in the complexity of the forms and variety of dendritic and axonal branches of stellate neurons: interneuronal connections are especially diverse.

If pyramidal and stellate cells are found in almost all layers of the cerebral cortex, then the so-called fusiform neuronscharacteristic mainly for the VI-VII layers of the cortex. However, fusiform neurons are often found in the V layer. The most characteristic feature of spindle-shaped neurons is the presence of two dendrites in opposite directions. Often, along with these basic dendrites and their branches, a lateral dendrite extending in the horizontal direction departs from the body of the spindle-shaped cells. Fusiform dendrites usually form a few branches. Branching of the axons of the spindle cells is also very insignificant in comparison with the branches of stellate and pyramidal neurons. The apical dendrite of the spindle cell, rising upward, can reach the I layer, however, for the most part, these dendrites end in layers V, IV and III.

Consists of highly specialized cells. They have the ability to perceive all kinds of stimuli. In response, human nerve cells can form an impulse, as well as transmit it to each other and other working elements of the system. As a result, a reaction is formed that is adequate to the effect of the stimulus. The conditions under which certain functions of the nerve cell are manifested form glial elements.

Development

The laying of nerve tissue occurs in the third week of the embryonic period. At this time, a plate is formed. From it develop:

• Oligodendrocytes.
• Astrocytes.
• Ependymocytes.
• Macroglia.

During further embryogenesis, the neural plate turns into a tube. In the inner layer of its wall there are stem ventricular elements. They proliferate and go out. In this area, part of the cells continues to divide. As a result, they are divided into spongioblasts (components of microglia), glioblasts and neuroblasts. Nerve cells form from the latter. 3 layers stand out in the tube wall:

At 20-24 weeks in the cranial segment of the tube begins the formation of blisters, which are the source of brain formation. The remaining sections serve for the development of the spinal cord. From the edges of the neural gutter, the cells participating in the formation of the crest depart. It is located between the ectoderm and the tube. Ganglion plates are formed from these cells, which serve as the basis for myelocytes (pigmented skin elements), peripheral nerve nodes, melanocytes of the integument, and components of the APUD system.

Components

Glyocytes in the system are 5-10 times more than nerve cells. They perform various functions: supporting, protective, trophic, stromal, excretory, suction. In addition, gliocytes have the ability to proliferate. Ependymocytes are prismatic. They make up the first layer, line the brain cavities and the central spinal section. Cells are involved in the production of cerebrospinal fluid and have the ability to absorb it. The basal part of ependymocytes has a conical truncated shape. It passes into a long thin process penetrating the medulla. On its surface, it forms a glial delimiting membrane. Astrocytes are represented by multi-process cells. They are:

Oliodendrocytes are small elements with outgoing short tails located around neurons and their endings. They form the glial membrane. Through it, impulses are transmitted. At the periphery, these cells are called mantle (lemmocytes). Microglia is part of the macrophage system. It is presented in the form of small mobile cells with sparsely branched short processes. The elements contain a bright core. They can form from blood monocytes. Microglia restores the structure of the damaged nerve cell.

The main component of the central nervous system

It is represented by a nerve cell - a neuron. In total, there are about 50 billion. Depending on the size, giant, large, medium, small nerve cells are secreted. In their form, they can be:

There is also a classification by the number of endings. So, only one process of a nerve cell can be present. This phenomenon is characteristic of the embryonic period. In this case, nerve cells are called unipolar. Bipolar elements are found in the retina. They are extremely rare. Such nerve cells have 2 endings. Pseudo-unipolar are also distinguished. A long cytoplasmic outgrowth, which is divided into two processes, departs from the body of these elements. Multipolar structures are found primarily directly in the central nervous system.

Nerve cell structure

The element distinguishes the body. It contains a large bright nucleus with one or two nucleoli. The cytoplasm contains all organelles, especially tubules from granular EPS. Accumulations of basophilic substance are widespread throughout the cytoplasmic surface. They are formed by ribosomes. In these clusters, the process of synthesis of all the necessary substances that are transported from the body to the processes takes place. Due to stress, these blocks are destroyed. Thanks to intracellular regeneration, a recovery-destruction process is constantly taking place.

Impulse formation and reflex activity

Among the processes dendrites are common. Branching, they form a dendritic tree. Due to them, synapses with other nerve cells are formed and information is transmitted. The more dendrites there are, the more powerful and vast the receptor field and, accordingly, more information. Along them, impulses propagate to the body of the element. Nerve cells contain only one axon. At the base of it a new impulse is formed. He moves away from the body along the axon. The process of a nerve cell can have a length of several microns to one and a half meters.

There is another category of elements. They are called neurosecretory cells. They can produce and release hormones in the blood. Nerve tissue cells are arranged in chains. They, in turn, form the so-called arcs. They determine the reflex activity of man.

Tasks

The following types of elements are distinguished by the function of the nerve cell:

• Afferent (sensitive).   They form 1 link in the reflex arch (spinal nodes). A long dendrite passes to the periphery. There it ends with an ending. In this case, the short axon enters the spinal cord in a reflex somatic arch. He is the first to respond to the stimulus, as a result of which a nerve impulse is formed.
• Conductor (insertion). These are nerve cells in the brain. They form a 2 link arc. These elements are also present in the spinal cord. Information from them is obtained by motor effector cells of nerve tissue, branched short dendrites and a long axon reaching the skeletal muscle fiber. Through the neuromuscular synapse, an impulse is transmitted. Also isolated and effector (efferent) elements.

Reflex arcs

In humans, they are mostly complex. In a simple reflex arc there are three neurons and three links. Their complication occurs due to an increase in the number of insertion elements. The leading role in the formation and subsequent conduct of the impulse belongs to the cytolemma. Under the influence of the stimulus, depolarization is performed in the area of \u200b\u200binfluence — charge inversion. In this form, the impulse spreads further along the cytolemma.

Fibers

Glial membranes are independently located around the nerve processes. In the complex, they form nerve fibers. Branches in them are called axial cylinders. There are non-myelin and myelin fibers. They differ in the structure of the glial membrane. Myelin-free fibers have a fairly simple device. The axial cylinder approaching the glial cell bends its cytolemma. The cytoplasm closes above it and forms a mesaxon - a double fold. One glial cell may contain several axial cylinders. These are "cable" fibers. Their branches can pass into the neighboring glial cells. The pulse passes at a speed of 1-5 m / s. Fibers of this type are found during embryogenesis and in the postganglionic areas of the autonomic system. Myelin segments are thick. They are located in the somatic system innervating the musculature of the skeleton. Lemmocytes (glial cells) pass sequentially, in a chain. They form a cord. In the center passes an axial cylinder. In the glial membrane are present:

• The inner layer of nerve cells (myelin).   It is considered the main one. In some areas between the layers of cytolemma there are extensions that form myelin notches.
• P peripheral layer.   It contains organelles and the nucleus - the neurilema.
• Thick basement membrane.

Hypersensitivity Places

In areas where adjacent lemocytes border, thinning of the nerve fiber occurs and the myelin layer is absent. These are places of increased sensitivity. They are considered the most vulnerable. The part of the fiber located between adjacent nodal intercepts is called the inter-nodal segment. Here, the pulse travels at a speed of 5-120 m / s.

Synapses

With their help, the cells of the nervous system are interconnected. There are different synapses: axo-somatic, -dendritic, -axonal (mainly inhibitory type). Electrical and chemical are also isolated (the former are rarely detected in the body). In the synapses, the post- and presynaptic parts are distinguished. The first contains a membrane in which highly specific protein (protein) receptors are present. They respond only to certain mediators. There is a gap between the pre- and postsynaptic parts. A nerve impulse reaches the first and activates special vesicles. They pass to the presynaptic membrane and fall into the gap. From there, they affect the postsynaptic film receptor. This provokes its depolarization, transmitted, in turn, through the central process of the next nerve cell. In the chemical synapse, information is transmitted in only one direction.

Varieties

Synapses are divided into:

• Brake, containing inhibitory neurotransmitters (gamma-aminobutyric acid, glycine).
• Excitants in which the corresponding components are present (adrenaline, acetylcholine, glutamine to-that, norepinephrine).
• Effector, ending on working cells.

Neuromuscular synapses are formed in the fiber of skeletal muscle. They have a presynaptic part formed by the terminal end section of the axon from the motor neuron. It is embedded in the fiber. The adjacent site forms the postsynaptic part. There are no myofibrils in it, but mitochondria and the nucleus are present in large numbers. The postsynaptic membrane is formed by a sarcolemma.

Sensitive endings

They are very diverse:

• Free are found exclusively in the epidermis. The fiber, passing through the basement membrane and discarding the myelin sheath, freely interacts with epithelial cells. These are pain and temperature receptors.
• Unencapsulated non-free endings are present in connective tissue. Glia accompanies branching in the axial cylinder. These are tactile receptors.
• Encapsulated endings are branches from the axial cylinder, accompanied by a glial inner bulb and outer connective tissue membrane. These are also tactile receptors.