Types of nerve cells. Neurons and nerve tissue

Date: 22.09.2019

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

Anaxon neurons- small cells, grouped near the spinal cord in the intervertebral ganglia, without anatomical signs of separation of processes into dendrites and axons. All processes of the cell are very similar. The functional purpose of non-axon neurons is poorly understood.

Unipolar neurons- neurons with one process, are present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain. Many morphologists believe that unipolar neurons do not occur in the human body and higher vertebrates.

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

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

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

Functional classification

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

Afferent neurons(sensitive, sensory, receptor or centripetal). This type of neurons includes primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free ends.

Efferent neurons(effector, motor, motor or centrifugal). Neurons of this type include end neurons - ultimatum and penultimate - not ultimatum.

Associative neurons(interneurons or interneurons) - a group of neurons makes a connection between efferent and afferent, they are divided into intrisit, 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, several principles are used when classifying neurons:

take into account the size and shape of the neuron body;

the number and nature of branching of the processes;

the length of the axon and the presence of specialized membranes.

According to the number of processes, the following morphological types of neurons are distinguished:

unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain;

pseudo-unipolar cells grouped near the spinal cord 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), predominant in the central nervous system

General structure of the human nervous system

The human nervous system can be divided into sections based on the characteristics of their structure, location, or functional properties.

The first classification is based on morphological characteristics (structure):

Functionally (based on the tasks performed), the human nervous system can be divided into a number of departments:

The somatic nervous system regulates the skeletal muscles and sensory organs. It provides the body's connection with the external environment and an adequate response to its change.

The vegetative (autonomic) nervous system regulates the activity of internal organs and ensures the maintenance of homeostasis. As a rule, the activity of the autonomous NS does not obey the human consciousness (the exception is the phenomena of yoga, hypnosis).

The nervous system is composed of neurons, or nerve cells, and neuroglia, or neuroglial cells. Neurons are the main structural and functional elements in both the central and peripheral nervous systems. Neurons are excitable cells, meaning they are capable of generating and transmitting electrical impulses (action potentials). Neurons have different shapes and sizes and form processes of two types: axons and dendrites. A neuron usually has several short branched dendrites, along which impulses follow to the body of the neuron, and one long axon, along which impulses go from the body of the neuron to other cells (neurons, muscle or glandular cells). The transfer of excitation from one neuron to other cells occurs through specialized contacts - synapses.

Neuroglia

Glial cells are more numerous than neurons and account for at least half the volume of the central nervous system, but unlike neurons, they cannot generate action potentials. Neuroglial cells are different in structure and origin; they perform auxiliary functions in the nervous system, providing support, trophic, secretory, demarcation and protective functions.

The first generalizations concerning the essence of the psyche can be found in the works of ancient Greek and Roman scientists (Thales, Anaximenes, Heraclitus, Democritus, Plato, Aristotle, Epicurus, Lucretius, Galen). Already among them were materialists, who believed that the psyche arose from natural principles (water, fire, earth, air), and idealists, who derived mental phenomena from an immaterial substance (soul).

Representatives of the materialistic direction (Heraclitus, Democritus) believed that the soul and body are one, and did not see any special differences between the human soul and the souls of animals. On the contrary, representatives of the idealistic worldview, Socrates and Plato, viewed the soul as a phenomenon not associated with the body and having a divine origin. Plato believed that the soul is older than the body, that the souls of humans and animals are sharply different, that the human soul is dual: a higher and a lower order. The first is immortal, it has purely mental power and can pass from one organism to another and even exist independently, independently of the body. The second (lower order) soul is mortal. For animals, only the lowest form of the soul is characteristic - motivation, instinct (from Lat. Instinctus - motivation).

The philosophical currents of ancient Greece - materialism and idealism - reflected an acute class struggle. The struggle of the materialistic "line of Democritus" with the idealistic "line of Plato" in ancient Greece was the struggle of the progressive slave-owning democracy against the reactionary land-based slave-owning aristocracy.

The participation of the Greeks in international trade, their communication with various peoples, acquaintance with various cultures and religious ideas contributed to the development of that extremely peculiar worldview among the Greeks, which went down in the history of philosophy under the name of the so-called Greek natural philosophy.

Democritus (about 460-360 BC) was a major representative of materialism in Ancient Greece. Democritus taught that the foundation of the world is not God, not any spirit, but matter. Everything that exists has arisen from primal matter. Matter is made up of tiny particles (atoms). These particles are in constant motion - they join, then they separate. Democritus explained all the variety of natural phenomena by various combinations of atoms. Nature is one and in perpetual motion. Thus, Democritus struck a blow to religion, which explained everything by the activity of the gods. Atomistic materialism opposed the idea of \u200b\u200bthe intervention of gods in the fate of the world and individuals, against superstitions.

Another position of Greek philosophy was the view of nature as something that is in perpetual motion, in a continuous flow, in relentless change. There is no peace in the world, but there is a constant process of becoming, one state is constantly being replaced by another. Heraclitus taught: "Everything flows, everything changes, there is nothing motionless, everything in the universe is swept by the flow of motion, everything is in the process of eternal change, eternal motion." He paid considerable attention to Democritus and medicine; he wrote about pulse, inflammation, rabies. “People ask the gods for health in their prayers, but they don’t know that they themselves have the means at their disposal,” Democritus wrote to his contemporary physician Hippocrates. In these statements the general materialistic views of Democritus found expression. Epicurus was the successor of Democritus.

Greek natural philosophy has had a significant impact on the development of materialistic concepts of disease.

Idealistic trends were represented by the school of Pythagoras (end of the 6th century BC), and later, from the 4th century, by the philosophy of Plato. These idealist philosophers were representatives of the slave-owning aristocracy. They ignored the study of concrete nature, explained everything that was happening by the influence of a force standing over the world in the form of either mystical "numbers" (Pythagoras), or eternal ideas (Plato).

The first draft of a mechanistic theory was developed by the naturalist Rene Descartes. Man and any living organism Descartes saw a simple mechanism, and not a body that possesses a soul and is controlled by it. Such thoughts became widespread due to the technological progress that took place in those years in Europe. The popularity of technology forced scientists to consider living organisms from the point of view of mechanics. The mechanistic theory was first confirmed by William Harvey, who discovered the circulatory system: from the point of view of mechanics, the heart acted as a pump that pumped blood without, by the way, requiring any participation of the soul. Descartes followed the mechanistic theory, introducing the concept of a reflex, thereby refuting the existence of the soul not only in the internal organs of a person, but also in the entire external work of the body. The concept of a reflex was introduced much later than Descartes's idea. Since at that time knowledge about the nervous system was insufficient, Descartes explained it as a system of pipes through which certain "animal spirits" move. These particles move to the brain, and from the brain to the muscles under the influence of an external impulse. That is, Descartes saw the reflex as a semblance of reflection of sunlight from a surface. Despite the fact that Descartes's hypothesis was not based on experience, it played an important role in psychology, for the first time, at that time, giving an explanation of human behavior without resorting to the theory of the soul. Another issue that interested Descartes was the ability to restructure behavior. Descartes confirmed this theory with the example of hunting dogs, which can be tamed to stop at the sight of game and run towards it when he heard a shot, instead of running away from the shot and immediately rushing at the game, which is the normal behavior of a dog. Descartes concluded that if it is possible to change behavior in animals, whose development is, of course, lower than that of humans, then humans can control their behavior even more successfully. Such a teaching system Descartes worked on the principle of restructuring the body, not strengthening the spirit, and gave a person absolute power over his own behavior and emotions. In his work "The Passion of the Soul" Descartes attributed not only reflexes to bodily functions, but also emotions, various mental states, the perception of ideas, memorization and internal aspirations. Under the passions, Descartes explained all the reactions of the body that reflect the "animal spirits". Denying the dominant role of the soul in human behavior, Descartes separates it from the body, turning it into an absolutely independent substance that has the ability to be aware of its own state and manifestations. That is - the only attribute of the soul is thinking and it always thinks (later this thinking of the soul acquired the name "introspection"). The most famous ahoism of Descartes was the words "I think - therefore I am". In the content of consciousness, Descartes identified three types of ideas: Ideas generated by a person - his sensory experience. These ideas do not provide knowledge of the surrounding world, giving only separate knowledge about objects or phenomena. Acquired phenomena are also separate knowledge that is transmitted through social experience. Only innate ideas, according to Descartes, give a person knowledge about the essence of the whole world. These laws are available only to the mind, without requiring information from the senses. This approach to knowledge is called "rationalism", and the disclosure and assimilation of innate ideas was called rational intuition. Also, Descartes was faced with the question of contacting two independent substances - how are the soul and body connected to each other? Descartes proposed to consider the pineal gland as a place of contact between soul and body. Through this gland, the body transfers passions to the soul, transforming them into emotions, and the soul regulates the work of the body, forcing changes in behavior. Thus, the perception of the body as a complex mechanism led to the emergence of the concept of mechanodetermenism. Thanks to the work of Descartes, the body was freed from the soul, and performed only motor functions through reflexes. The soul was freed from the body and performed only the functions of thinking, using reflection.

More details: http://www.anypsy.ru/content/mekhanisticheskie-vzglyady-dekarta.

Descartes proceeded from the fact that the interaction of organisms with surrounding bodies is mediated by a nervous machine, consisting of the brain as a center and neural "tubes" radiating from it at radii. The lack of any reliable data on the nature of the nervous process forced Descartes to present him on the model of the blood circulation process, knowledge of which acquired reliable reference points in experimental research. Descartes believed that by the movement of the heart of the blood "as the first and most general thing that is observed in animals, one can easily judge everything else" (5).

The nervous impulse was thought of as something akin - in composition and mode of action - to the process of moving blood through the vessels. It was assumed that the lightest and most mobile blood particles, being filtered from the rest, rise according to the general rules of mechanics to the brain. The streams of these particles Descartes designated the old term "animal spirits", having put into it a content that fully corresponded to the mechanistic interpretation of the body's functions. "What I here call" spirits "are nothing more than bodies that have no other property, except that they are very small and move very quickly" (5). Although Descartes does not have the term "reflex", the main contours of this concept are outlined quite clearly. "Considering the activity of animals, as opposed to human, machine-like, - notes IP Pavlov, - Descartes established the concept of a reflex as the main act of the nervous system."

Reflex means movement. Descartes understood it as the reflection of "animal spirits" from the brain to the muscles by the type of reflection of a light beam. In this regard, let us recall that the understanding of the nervous process as akin to heat and light phenomena has an ancient and ramified genealogy (cf. notions of stump). While the physical laws concerning the phenomena of heat and light, verified by experience and having a mathematical expression, remained unknown, the doctrine of the organic substrate of psychic manifestations depended on the doctrine of the soul as an expediently acting force. The picture began to change with the advances in physics, primarily optics. The achievements of Ibn al-Haytham and R. Bacon already in the Middle Ages prepared the conclusion that the sphere of sensations depends not only on the potencies of the soul, but also on the physical laws of motion and refraction of light rays.

Thus, the emergence of the concept of a reflex is the result of the introduction into psychophysiology of models that have developed under the influence of the principles of optics and mechanics. The extension of physical categories to the activity of the organism made it possible to understand its determinists, to bring it out from under the causal influence of the soul as a special essence.

According to the Cartesian scheme, external objects act on the peripheral end of the nerve "filaments" located inside the neural "tubes", the latter, by stretching, open the valves of the openings leading from the brain to the nerves, through the channels of which the "animal spirits" rush into the corresponding muscles, which, as a result "inflate". Thus, it was asserted that the first cause of the motor act lies outside it: what happens "at the exit" of this act is determined by material changes "at the entrance".

Descartes considered the "disposition of organs" to be the basis for the variety of pictures of behavior, meaning by this not only an anatomically fixed neuromuscular structure, but also its change. It occurs, according to Descartes, due to the fact that the pores of the brain, changing their configuration under the action of centripetal nerve "threads", do not return (due to insufficient elasticity) to their previous position, but become more extensible, giving the current of "animal spirits" a new direction.

After Descartes, the belief that explaining nervous activity by the forces of the soul is tantamount to turning to these forces to explain the operation of an automaton, for example, a clock, has become more and more firmly among naturalists.

The original methodological rule of Descartes was as follows: "What we experience in ourselves in such a way that we can admit it in the bodies of the inanimate should be attributed only to our body" (5). In this context, the inanimate body did not mean objects of inorganic nature, but mechanical structures, automata built by human hands. Having raised the question of how widely the possibility of modeling the processes of feeling, memory, etc. by purely mechanical means extends, Descartes comes to the conclusion that only two signs of human behavior do not lend themselves to modeling: speech and intelligence.

Descartes makes an attempt, proceeding from the reflex principle, to explain such a fundamental feature of the behavior of living bodies as their learning. From this attempt, ideas grew that give the right to consider Descartes as one of the forerunners of associationism. "When a dog sees a partridge, it naturally rushes to it, and when it hears a rifle shot, its sound naturally prompts it to run away. Nevertheless, cop dogs are usually taught that the sight of a partridge makes them stop, and the sound of a shot which they hear when shooting at a partridge made them run up to it. This is useful to know in order to learn how to control their passions. But since with some effort you can change the brain movements of animals devoid of intelligence, it is obvious that this is even better can be done with people and that people, even with a weak soul, could acquire extremely unlimited power over all their passions, if they made enough effort to discipline and guide them "(5).

A century later, the assumption that the connections of muscle reactions with the sensations that cause them can be changed, altered, and thereby give behavior the desired direction, will form the basis of Gartley's materialistic associative psychology. "It seems to me - he wrote, Gartley, defining the place of his concept among other systems - that Descartes would have been successful in implementing his plan in the form as it was proposed at the beginning of his treatise" On Man ", if he had at all a sufficient number of facts from the field of anatomy, physiology, pathology and philosophy "(3).

It seemed to Hartley that Descartes was unable to consistently carry out his plan due to a lack of facts. The real reasons for Descartes' inconsistency, his dualism (clearly manifested in the idea of \u200b\u200ba double determination of behavior: on the part of the soul and on the part of external stimuli) were of a methodological nature. The doctrine of the mechanistic basis of the behavior of living bodies, developed by Descartes, revolutionizing the minds of naturalists, freed the study of the neuromuscular system and its functions from idealistic delusions.

in contrast to Descartes and his followers, I.M.Sechenov was the first to put forward the concept of a reflex as a complex purposeful nervous activity of an animal, which underlies not only unconditioned instincts, but all, even the most complex forms of behavior, including conscious human activity.

The experimental studies of I.P. Pavlov and his school convincingly showed the complete scientific inconsistency of the Cartesian doctrine of the reflex and the mechanistic concept of the reflex arc that follows from it, as consisting of strictly fixed nervous processes. These studies revealed complex patterns and a variety of reflexes, the participation in their implementation not of any separate, precisely fixed neurons, but in general of the entire higher part of the animal's nervous system.

In this regard, the concept of a reflex arc has lost its former mechanistic character. This concept still retains its fundamental importance for explaining the essence of the reflex as a complex nervous process caused by external irritation and ending with a purposeful reaction of the organism. However, this reaction itself is understood by I.P. Pavlov not as a mechanical switching of nervous excitation caused by external stimulation to a strictly corresponding motor or secretory reaction, but as a reaction largely due to the past experience of the animal and the resulting complication of nervous activity.

In this regard, the structure and nature of the main links of the reflex arc is understood in a new way, dialectically: its afferent section does not mechanically receive external stimulation, but selectively, in accordance with the needs of the body and the information accumulated in its nervous system: the central section of the reflex arc becomes unusually complicated, including not one strictly fixed, but many combination neurons and, in this regard, involving various parts of the animal's brain in the reflex process each time in connection with a changing situation; finally, its effector department is understood not as unambiguous, stereotyped, precisely and forever determined by the nature and strength of the stimulus, but as carrying out an expedient reaction, the changing means of which are each time determined by the complex work of the central parts of the brain. For example, even such a relatively simple reflex as a defense reaction of the body in response to pain stimulation is performed in different ways, with the involvement of various muscle groups, depending on the position of the defending animal (standing, lying, sitting, etc. .).

Brain reflex- This, according to Sechenov, is a learned reflex, that is, not innate, but acquired in the course of individual development and depending on the conditions in which it is formed. Expressing the same idea in terms of his doctrine of higher nervous activity, I. P. Pavlov will say that this is a conditioned reflex, that this is a temporary connection. Reflex activity is an activity through which an organism with a nervous system realizes its connection with the conditions of life, all its variable relations with the outside world. Conditioned reflex activity as a signal is directed, according to Pavlov, to seek, in a constantly changing environment, the basic conditions of existence necessary for an animal, serving as unconditioned stimuli.

The third is inextricably linked with the first two features of the brain reflex. Being "learned", temporary, changing with changing conditions, the reflex of the brain cannot be determined morphologically once and for all by fixed pathways.

"Anatomical" physiology, which has dominated until now and in which everything is reduced to the topographic isolation of organs, is opposed physiological system, in which activity, a combination of central processes comes to the fore. Pavlov's reflex theory overcame the notion that the reflex is allegedly entirely determined by morphologically fixed pathways in the structure of the nervous system, on which the stimulus falls. She showed that the reflex activity of the brain (always including both unconditioned and conditioned reflexes) is a product of the dynamics of nervous processes confined to the brain structures, expressing the variable relations of the individual with the outside world.

Finally, and most importantly, the brain reflex is a reflex with a "mental complication." The advancement of the reflex principle to the brain led to the inclusion of mental activity in the reflex activity of the brain.

The core of the reflex understanding of mental activity is the provision according to which mental phenomena arise in the process of the brain's interaction with the world; therefore, mental processes that are inseparable from the dynamics of nervous processes can not be isolated either from the influences of the external world on a person, or from his actions, deeds, practical activities, for the regulation of which they serve.

Mental activity is not only a reflection of reality, but also a determinant of the meaning of the reflected phenomena for the individual, their relationship to his needs; therefore it regulates behavior. "Evaluation" of phenomena, attitude towards them are associated with the psychic from its very inception, as well as their reflection.

Reflex understanding of mental activity can be expressed in two positions:

1. Mental activity cannot be separated from a single reflex activity of the brain; she is the "integral part" of the latter.

2. The general scheme of the mental process is the same as for any reflex act: the mental process, like any reflex act, originates in external influence, continues in the central nervous system and ends with the individual's response activity (movement, deed, speech). Mental phenomena arise as a result of the "meeting" of the individual with the outside world.

The cardinal position of Sechenov's reflex understanding of the psychic concludes with the recognition that the content of mental activity as a reflex activity is not deduced from the “nature of nerve centers”, that it is determined by objective being and is its image. The assertion of the reflex nature of the mental is associated with the recognition of the mental as a reflection of being.

IM Sechenov always emphasized the real vital significance of the psychic. Analyzing the reflex act, he characterized its first part, beginning with the perception of sensory arousal, as a signal one. At the same time, sensory signals "inform" about what is happening in the environment. In accordance with the signals entering the central nervous system, the second part of the reflex act carries out movement. Sechenov emphasized the role of "feeling" in the regulation of movement. The working organ that carries out the movement participates in the emergence of the psychic as not an effector, but a receptor that gives sensory signals about the movement produced. The same sensory signals form "touching" with the beginning of the next reflex. At the same time, Sechenov clearly shows that mental activity can regulate actions, projecting them in accordance with the conditions in which they are performed, only because it analyzes and synthesizes these conditions.

Neurons

The neuron is the main element of the "biological processor" that allows animals to adapt to the environment, and to humans - to think and feel. By its structure, a neuron is a highly specialized cell of the nervous system,capable of generating and conducting electrical impulses. During ontogenesis, neurons lost their ability to reproduce.

As a rule, the neuron has a stellate shape, due to which the body is distinguished in it ( soma) and processes ( axon and dendrites). The axon of a neuron is always one, although it can branch, forming two or more nerve endings, and there can be a lot of dendrites. By the shape of the body, one can distinguish star-shaped, spherical, spindle-shaped, pyramidal, pear-shaped, etc. Some types of neurons, differing in body shape, are shown in Fig. 4.5.

Another, more common classification of neurons is their division into groups according to the number and structure of processes. Depending on their number, neurons are divided into unipolar (one shoots), bipolar (two branches) and multipolar (many processes) (Fig. 4.4). Unipolar cells (without dendrites) are not typical for adults and are observed only during embryogenesis. Instead, in the human body there are so-called pseudo-unipolar cells in which a single axon splits into two branches immediately after leaving the cell body. Bipolar neurons have one dendrite and one axon. They are found in the retina of the eye, and transmit excitation from photoreceptors to the ganglion cells that form the optic nerve. Multipolar neurons (having a large number of dendrites) make up the majority of the cells in the nervous system.

The sizes of neurons range from 5 to 120 microns and average 10-30 microns. The largest nerve cells in the human body are the spinal cord motor neurons and the giant Betz pyramids of the cerebral hemispheres. Both cells are motor cells by their nature, and their size is due to the need to take on a huge number of axons from other neurons. It is estimated that some motor neurons in the spinal cord have up to ten thousand synapses.

The third classification of neurons is according to their functions. According to this classification, all nerve cells can be divided into sensitive, intercalary and motor (Fig.6.5). Since "motor" cells can send orders not only to muscles, but also to glands, the term is often applied to their axons efferent, that is, directing impulses from the center to the periphery. Then the sensitive cells will be called afferent (along which nerve impulses move from the periphery to the center).

Thus, all classifications of neurons can be reduced to the three most commonly used (see Fig. 4.7):

This cell has a complex structure, is highly specialized and contains the nucleus, cell body and processes in structure. The human body contains over 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 different signals transmitted as part of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions that generate an electrical charge that travels along the 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 EPR with active ribosomes, the Golgi apparatus), as well as of processes. There are two types of processes: dendrites and. The neuron has a developed and complex cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its filaments serve as "rails" for the transport of organelles and substances packed in 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, are especially pronounced in the growing nerve processes and c. A developed synthetic apparatus is revealed in the body of the neuron, the granular EPS of the neuron is stained basophilically and is known as "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the origin of the axon, which serves as a histological sign of the axon.

There is a distinction between anterograde (from the body) and retrograde (to the body) axonal transport.

Dendrites and axon

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

Dendrites divide dichotomously, while axons give collaterals. Mitochondria are usually concentrated in the branch nodes.

Dendrites do not have a myelin sheath, but axons may have one. The place of generation of excitation in most neurons is the axonal mound - the formation at the site of the origin of the axon from the body. In all neurons, this zone is called the trigger zone.

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

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

Classification

Structural classification

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

Anaxon neurons - small cells, grouped close in the intervertebral ganglia, without anatomical signs of separation of processes into dendrites and axons. All processes of the cell are very similar. The functional purpose of non-axon neurons is poorly understood.

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

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

Multipolar neurons - neurons with one axon and several dendrites. This type of nerve cells predominates in.

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

Functional classification

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

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

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

Associative neurons (interneurons or interneurons) - a group of neurons makes a connection between efferent and afferent, they are divided into intrisit, 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, several principles are used when classifying neurons:

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

By the number of processes, the following morphological types of neurons are distinguished:

unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve;
pseudo-unipolar cells clustered 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), predominant in the central nervous system.

Neuron development and growth

A neuron develops from a small precursor cell that stops dividing even before it releases its processes. (However, the issue of neuronal division is currently 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, an irregular thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the nerve cell growth cone. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspines are 0.1 to 0.2 microns thick and can reach 50 microns in length, the wide and flat area of \u200b\u200bthe growth cone is about 5 microns wide and long, although its shape can vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others lengthen, deflect in different directions, touch the substrate and can stick to it.

The growth cone is filled with small, sometimes connected to each other, membrane vesicles of irregular shape. Immediately beneath the folded regions of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments found in the body of the neuron.

Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron. Since the average rate of advancement of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs during the growth of a neuron process at its far end. New membrane material is added, apparently at the end. The growth cone is an area of \u200b\u200brapid 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 the flow of fast axonal transport. The membrane material, apparently, is synthesized in the body of the neuron, is transferred to the growth cone in the form of bubbles and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell.

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

Neuron Mouse cortex pyramidal neuron, expressive green fluorescent protein (GFP)

Classification

Structural classification

Anaxon neurons - small cells, grouped near the spinal cord in the intervertebral ganglia, without anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of non-axon neurons is poorly understood.

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

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

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

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

Functional classification

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

Efferent neurons (effector, motor, motor or centrifugal). The neurons of this type are end neurons - ultimatum and penultimate - not ultimatum.

Associative neurons (interneurons or interneurons) - a group of neurons makes a connection between efferent and afferent, they are divided into intrisit, 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, several principles are used when classifying neurons:

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

According to the number of processes, the following morphological types of neurons are distinguished:

unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain;
pseudo-unipolar cells grouped near the spinal cord 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), predominant in the central nervous system.

Neuron development and growth

A neuron develops from a small precursor cell that stops dividing even before it releases its processes. (However, the question of neuronal division is currently controversial.) As a rule, the axon begins to grow first, and dendrites are formed later. At the end of the developing process of the nerve cell, an irregular thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the nerve cell growth cone. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspines are 0.1 to 0.2 microns thick and can reach 50 microns in length, the wide and flat area of \u200b\u200bthe growth cone is about 5 microns wide and long, although its shape can vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others lengthen, deflect in different directions, touch the substrate and can stick to it.

Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron. Since the average rate of advancement of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs during the growth of a neuron process at its distal end. New membrane material is added, apparently at the end. The growth cone is an area of \u200b\u200brapid 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 the flow of fast axonal transport. The membrane material, apparently, is synthesized in the body of the neuron, is transferred to the growth cone in the form of bubbles and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell.

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

Literature

Polyakov G.I., On the principles of the neural organization of the brain, M: MGU, 1965
Kositsyn NS Microstructure of dendrites and axodendritic connections in the central nervous system. Moscow: Nauka, 1976, 197 p.
Nemechek S. et al. Introduction to neurobiology, Avicennum: Prague, 1978, 400 pp.
Bloom F., Leiserson A., Hofstedter L. Brain, Mind and Behavior
Brain (collection of articles: D. Hubel, C. Stevens, E. Kandel, et al. - Scientific American issue (September 1979)). M.: Mir, 1980
Savelyeva-Novoselova N.A., Savelyev A.V. A device for modeling a neuron. A. s. No. 1436720, 1988
Savelyev A.V. Sources of variations in the dynamic properties of the nervous system at the synaptic level // magazine "Artificial Intelligence", NAS of Ukraine... - Donetsk, Ukraine, 2006. - No. 4. - S. 323-338.

Histology: Nerve tissue

Neurons
(Gray matter)

Soma Axon (Axon hillock, Axon terminal, Axoplasm, Axolemma, Neurofilaments)

PNS: Schwann cells Neurolemma Intercept Ranvier / Internodal segment Myelin notch

· Anaxon neurons - small cells, grouped near the spinal cord in the intervertebral ganglia, without anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of nonaxon neurons is poorly understood.

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

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

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

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

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

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

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

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

Morphological classification of neurons The morphological structure of neurons is diverse. In this regard, several principles are used when classifying neurons:

1.take into account the size and shape of the body of the neuron,

2.number and nature of branching of processes,

3. the length of the neuron and the presence of a specialized sheath.

By cell shape, neurons can be spherical, granular, stellate, pyramidal, pear-shaped, fusiform, irregular, etc. The size of the neuron body varies from 5 microns in small granular cells to 120-150 microns in giant pyramidal neurons. The length of a neuron in humans ranges 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 in the midbrain; - pseudo-unipolar cells grouped near the spinal cord in the intervertebral ganglia; - bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina of the eye, the olfactory epithelium and the bulb, the 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 precursor cell that stops dividing even before it releases its processes. (However, the question of neuronal division is currently controversial.) As a rule, the axon begins to grow first, and dendrites are formed later. At the end of the developing process of the nerve cell, an irregular thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the nerve cell growth cone. It consists of a flattened part of the process of a nerve cell with many thin spines. Microspines are 0.1 to 0.2 microns thick and can reach 50 microns in length, the wide and flat area of \u200b\u200bthe growth cone is about 5 microns wide and long, although its shape can vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others lengthen, deflect in different directions, touch the substrate and can stick to it. The growth cone is filled with small, sometimes connected to each other, membrane vesicles of irregular shape. Immediately beneath the folded regions of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments found in the body of the neuron. Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron.

Since the average rate of advancement of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs during the growth of a neuron process at its distal end. New membrane material is added, apparently at the end. The growth cone is an area of \u200b\u200brapid 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 the flow of fast axonal transport. The membrane material, apparently, is synthesized in the body of the neuron, is transferred to the growth cone in the form of bubbles and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell. The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons disperse and find a permanent place for themselves.

Neuroglia. Unlike nerve cells, glial cells are very diverse. Their number is tens of times greater than the number of nerve cells. Unlike nerve cells, glial cells are capable of dividing, their diameter is much smaller than the diameter of a nerve cell and is 1.5-4 microns.

For a long time, it was believed that the function of gliocytes is insignificant, and they perform only a supporting function in the nervous system. Thanks to modern research methods, it has been established that gliocytes perform a number of functions important for the nervous system: support, demarcation, trophic, secretory, protective.

Among gliocytes, according to the morphological organization, a number of types are distinguished: ependymocytes, astrocytes.

Ependymocytes form a dense layer of cells, elements lining the spinal canal and ventricles of the brain. During ontogeny, ependymocytes were formed from spongioblasts. Ependymocytes are slightly elongated cells with branching processes. Some ependymocytes perform a secretory function, releasing biologically active substances into the blood and brain ventricles. Ependymocytes form clusters on the capillary chain of the ventricles of the brain; when a dye is introduced into the blood, it accumulates in ependymocytes, which indicates that the latter perform the function of the blood-brain barrier.

Astrocytes perform a supporting function. This is a huge number of glial cells with many short processes. Among astrocytes there are 2 groups:

o plasma cells

o fibrous astrocytes

Oligodendrocytes - large glial cells, often concentrated around the nerve cell and are therefore called satillic gliacytes. Their function is very important for the trophism of the nerve cell. With functional overvoltage of the nerve cell, gliocytes are able to referee substances that enter the nerve cell by pinocytosis. Under functional loads, the depletion of the synthetic apparatus of glial cells occurs first, and then of the nerve cells. When restoring (repair), first, the functions of neurons are restored, and then - glial cells. Thus, gliocytes are involved in ensuring the functions of neurons. Glial cells are able to significantly influence the trophism of the brain, as well as the functional status of the nerve cell.
Nerves (nervi) - anatomical formations in the form of strands, built mainly of nerve fibers and providing a connection between the central nervous system and innervated organs, blood vessels and the skin of the body.

Nerves leave in pairs (left and right) from the brain and spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal N .; the set of N. and their derivatives constitutes the peripheral nervous system ( fig. ), in which, depending on the characteristics of the structure, functioning and origin, two parts are distinguished: the somatic nervous system, which innervates the skeletal muscles and the skin of the body, and autonomic nervous system, innervating internal organs, glands, circulatory system, etc.

Nerve fibers - processes of nerve cells (neurons) that have a shell and are capable of conducting a nerve impulse.
The main component of the nerve fiber is the process of the neuron, which forms, as it were, the axis of the fiber. For the most part, this is an axon. The neural process is surrounded by a complex shell, with which it forms a fiber. The thickness of a nerve fiber in the human body, as a rule, does not exceed 30 micrometers.
Nerve fibers are divided into pulpy (myelin) and non-pulp (myelin-free). The former have a myelin sheath covering the axon, the latter are devoid of the myelin sheath.

Both in the peripheral and in the central nervous system, myelin fibers predominate. Nerve fibers devoid of myelin are located mainly in the sympathetic division of the autonomic nervous system. At the point where the nerve fiber leaves the cell and in the area of \u200b\u200bits transition into the terminal ramifications, the nerve fibers can be devoid of any sheaths, and then they are called naked axial cylinders.

Depending on the nature of the signal conducted through them, nerve fibers are subdivided into motor vegetative, sensory and motor somatic.

The structure of nerve fibers:
Myelinated nerve fiber contains the following elements (structures):
1) an axial cylinder located in the very center of the nerve fiber,
2) the myelin sheath covering the axial cylinder,
3) the Schwann shell.

The axial cylinder is composed of neurofibrils. The pulp contains a large amount of lipoid substances known as myelin. Myelin provides rapid conduction of nerve impulses. The myelin sheath does not cover the axial cylinder over the entire gap, forming gaps called Ranvier interceptions. In the area of \u200b\u200bRanvier interceptions, the axial cylinder of the nerve fiber is adjacent to the upper - Schwann sheath.

The fiber gap located between two Ranvier interceptions is called a fiber segment. In each such segment, the core of the Schwann shell can be seen on stained preparations. It lies approximately in the middle of the segment and is surrounded by the protoplasm of the Schwann cell, in the loops of which myelin is contained. Between the interceptions of Ranvier, the myelin sheath is also not continuous. In its thickness, the so-called Schmidt-Lanterman notches are found, going in an oblique direction.

The cells of the Schwann membrane, as well as neurons with processes, develop from the ectoderm. They cover the axial cylinder of the nerve fiber of the peripheral nervous system in a similar way to how glial cells cover the nerve fiber in the central nervous system. As a result, they can be called peripheral glial cells.

In the central nervous system, nerve fibers do not have Schwann sheaths. The role of Schwann cells is played by the elements of oligodendroglia. Myelin-free (non-fleshy) nerve fiber lacks myelin sheath and consists only of an axial cylinder and a Schwann sheath.

Function of nerve fibers.
The main function of nerve fibers is to transmit nerve impulses. Currently, two types of nerve transmission have been studied: impulse and impulseless. Pulse transmission is provided by electrolyte and neurotransmitter mechanisms. The speed of transmission of nerve impulses in myelin fibers is much higher than in non-fleshy ones. In its implementation, the most important role belongs to myelin. This substance is able to isolate the nerve impulse, as a result of which the signal transmission along the nerve fiber occurs in an abrupt manner, from one interception of Ranvier to another. Pulseless transmission is carried out by a current of axoplasm through special axon microtubules containing trophogens - substances that have a trophic effect on the innervated organ.

Ganglion (ancient Greek γανγλιον - knot) or nerve knot - an accumulation of nerve cells, consisting of bodies, dendrites and axons, nerve cells and glial cells. Usually the ganglion also has a connective tissue sheath. They are found in many invertebrates and all vertebrates. Often they are connected to each other, forming various structures (nerve plexuses, nerve chains, etc.).

TICKET number 13

1. Bones of the facial skull. Eye socket. The nasal cavity. Messages.

2. Large intestine: departments, their topography, structure, relation to the peritoneum, blood supply and innervation.

3. The medulla oblongata. External and internal structure. Topography of gray and white matter.