Features of nerve cells. Classification, characteristics of nerve cells

Functions of a neuron

background(without stimulation) and caused by(after stimulus) activity.

Spinal nerves

There are 31 pairs of spinal nerves in humans: 8 - cervical, 12 - thoracic, 5 - lumbar, 5 - sacral and 1 pair - coccygeal. They are formed by the fusion of two roots: posterior - sensitive and anterior - motor. Both roots are connected into a single trunk, emerging from the spinal canal through the intervertebral foramen. In the area of ​​the opening lies the spinal ganglion, which contains the bodies of sensory neurons. Short processes enter the posterior horns, long ones end in receptors located in the skin, subcutaneous tissue, muscles, tendons, ligaments, and joints. The anterior roots contain motor fibers from the motor neurons of the anterior horns.

Nerve plexuses

There are cervical, brachial, lumbar and sacral plexuses formed by branches of the spinal nerves.

The cervical plexus is formed by the anterior branches of the 4 upper cervical nerves, lies on the deep muscles of the neck, the branches are divided into motor, mixed and sensory. The motor branches innervate the deep muscles of the neck, the muscles of the neck located below the hyoid bone, the trapezius and sternocleidomastoid muscles.

The mixed branch is the phrenic nerve. Its motor fibers innervate the diaphragm, and its sensory fibers innervate the pleura and pericardium. Sensitive branches innervate the skin of the back of the head, ear, neck, skin under the collarbone and above deltoid muscle.

The brachial plexus is formed by the anterior branches of the 4 lower cervical nerves and the anterior branch of the first thoracic nerve. Innervates the muscles of the chest, shoulder girdle and back. The infraclavicular section of the brachial plexus forms 3 bundles - medial, lateral and posterior. The nerves emerging from these bundles innervate the muscles and skin of the upper limb.

The anterior branches of the thoracic nerves (1-11) do not form plexuses; they run like intercostal nerves. Sensory fibers innervate the skin of the chest and abdomen, motor fibers innervate the intercostal muscles, some muscles of the chest and abdomen.

The lumbar plexus is formed by the anterior branches of the 12th thoracic and 1-4 branches of the lumbar nerves. The branches of the lumbar plexus innervate the muscles of the abdomen, lower back, muscles of the anterior thigh, and muscles of the medial group of the thigh. Sensitive fibers innervate the skin below the inguinal ligament, perineum, and skin of the thigh.

The sacral plexus is formed by the branches of the 4th and 5th lumbar nerves. The motor branches innervate the muscles of the perineum, buttocks, and perineum; sensitive – skin of the perineum and external genitalia. The long branches of the sacral plexus form the sciatic nerve, the largest nerve in the body, innervating the muscles of the lower limb.

3. Classification of nerve fibers.

Based on functional properties (structure, fiber diameter, electrical excitability, speed of development of the action potential, duration of various phases of the action potential, speed of excitation) Erlanger and Gasser divided nerve fibers into fibers of groups A, B and C. Group A is heterogeneous, type A fibers are in turn are divided into subtypes: A-alpha, A-beta, A-gamma, A-delta.

Type A fibers are covered with a myelin sheath. The thickest of them, A-alpha, have a diameter of 12-22 microns and high speed excitation conduction - 70-120 m/s. These fibers conduct excitation from the motor nerve centers of the spinal cord to skeletal muscles(motor fibers) and from muscle proprioceptors to the corresponding nerve centers.

Three other groups of type A fibers (beta, gamma, delta) have a smaller diameter from 8 to 1 μm and a lower excitation velocity from 5 to 70 m/s. The fibers of these groups are predominantly sensitive, conducting excitation from various receptors (tactile, temperature, some pain receptors internal organs) in the central nervous system. The only exceptions are gamma fibers, a significant part of which conduct excitation from spinal cord cells to intrafusal muscle fibers.

Type B fibers are myelinated preganglionic fibers of the autonomic nervous system. Their diameter is 1-μm, and the excitation speed is 3-18 m/s.

Type C fibers include unmyelinated nerve fibers of small diameter - 0.5-2.0 microns. The excitation speed in these fibers is no more than 3 m/s (0.5-3.0 m/s). Most type C fibers are postganglionic fibers of the sympathetic division of the autonomic nervous system, as well as nerve fibers that conduct excitation from pain receptors, some thermoreceptors, and pressure receptors.

4. Laws of conduction of excitation along the nerves.

The nerve fiber has the following physiological properties: excitability, conductivity, lability.

The conduction of excitation along nerve fibers is carried out according to certain laws.

The law of bilateral conduction of excitation along a nerve fiber. Nerves have bilateral conduction, i.e. excitation can spread in any direction from the excited area (the place of its origin), i.e., centripetally and centrifugally. This can be proven if recording electrodes are placed on the nerve fiber at a certain distance from each other, and irritation is applied between them. The excitation will be recorded by electrodes on both sides of the site of irritation. The natural direction of propagation of excitation is: in afferent conductors - from the receptor to the cell, in efferent conductors - from the cell to the working organ.

The law of anatomical and physiological integrity of the nerve fiber. Conduction of excitation along a nerve fiber is possible only if its anatomical and physiological integrity is preserved, i.e. transmission of excitation is possible only through a structurally and functionally unchanged, undamaged nerve (laws of anatomical and physiological integrity). Various factors, affecting the nerve fiber ( narcotic substances, cooling, bandaging, etc.) lead to a violation of physiological integrity, i.e., to a disruption of the mechanisms of excitation transmission. Despite the preservation of its anatomical integrity, the conduction of excitation in such conditions is disrupted.

The law of isolated conduction of excitation along a nerve fiber. As part of a nerve, excitation along a nerve fiber spreads in isolation, without transferring to other fibers present in the nerve. Isolated conduction of excitation is due to the fact that the resistance of the fluid filling the intercellular spaces is significantly lower than the resistance of the nerve fiber membrane. Therefore, the main part of the current arising between the excited and non-excited sections of the nerve fiber passes through the intercellular gaps, without affecting nearby nerve fibers. Isolated excitation conduction has important. The nerve contains a large number of nerve fibers (sensory, motor, autonomic), which innervate effectors (cells, tissues, organs) of various structures and functions. If the excitation inside the nerve spread from one nerve fiber to another, then the normal functioning of the organs would be impossible.

Excitation (action potential) spreads along the nerve fiber without attenuation.

Peripheral nerve practically tireless.

The mechanism of conduction of excitation along the nerve.

Excitation (action potential - AP) propagates in axons, nerve cell bodies, and sometimes in dendrites without a decrease in amplitude and without a decrease in speed (non-decremental). The mechanism of propagation of excitation in different nerve fibers is not the same. When excitation spreads along an unmyelinated nerve fiber, the conduction mechanism includes two components: the irritating effect of the cathelectroton generated by a local PD on the adjacent section of the electrically excitable membrane and the occurrence of PD in this section of the membrane. Local depolarization of the membrane disrupts the electrical stability of the membrane, different size polarization of the membrane in its adjacent areas generates an electromotive force and a local electric current, the field lines of which are closed through ion channels. Ion channel activation increases sodium conductance, following electrotonic achievement critical level depolarization (DUD) in a new section of the membrane, an AP is generated. In turn, this action potential causes local currents, and they generate an action potential in a new area of ​​the membrane. Throughout the entire length of the nerve fiber, a process of new generation of the action potential of the fiber membrane occurs. This type transmission of excitation is called continuous.

The speed of excitation propagation is proportional to the thickness of the fiber and inversely proportional to the resistance of the medium. The conduction of excitation depends on the ratio of the amplitude of the action potential and the value of the threshold potential. This indicator is called guarantee factor(GF) and is equal to 5 - 7, i.e. PD should be 5-7 times higher than the threshold potential. If GF = 1, conduction is unreliable, if GF< 1 проведения нет. Протяженность возбуждённого участка нерва L является произведение времени (длительности) ПД и скорости распространения ПД. Например, в гигантском аксоне кальмара L= 1 мс ´ 25 мм/мс = 25 мм.

Availability at myelinated fibers shell with high electrical resistance, as well as sections of fiber devoid of sheath - nodes of Ranvier - create conditions for a qualitatively new type of conduction of excitation along myelinated nerve fibers. IN myelinated In the fiber, currents are conducted only in areas not covered with myelin - nodes of Ranvier, and in these areas the next AP is generated. Interceptions 1 µm long are located every 1000 - 2000 µm, characterized by high density ion channels, high electrical conductivity and low resistance. AP propagation in myelinated nerve fibers occurs saltatory- jumping from interception to interception, i.e. excitation (AD) seems to “jump” through sections of the nerve fiber covered with myelin, from one interception to another. The speed of this method of excitation is much higher, and it is more economical compared to continuous excitation, since not the entire membrane is involved in the state of activity, but only its small sections in the interception area, thereby reducing the load on the ion pump.

Scheme of the propagation of excitation in unmyelinated and myelinated nerve fibers.

5. Parabiosis.

Nerve fibers have lability- the ability to reproduce a certain number of excitation cycles per unit of time in accordance with the rhythm of existing stimuli. The measure of lability is maximum amount cycles of excitation, which a nerve fiber can reproduce per unit time without transforming the rhythm of stimulation. Lability is determined by the duration of the peak of the action potential, i.e., the phase of absolute refractoriness. Since the duration of absolute refractoriness of the spike potential of a nerve fiber is the shortest, its lability is the highest. A nerve fiber can reproduce up to 1000 impulses per second.

The phenomenon of parabiosis was discovered by the Russian physiologist N.E. Vvedensky in 1901 while studying the excitability of a neuromuscular drug. The state of parabiosis can be caused by various influences– ultra-frequent, ultra-strong stimuli, poisons, drugs and other influences, both normally and in pathology. N. E. Vvedensky discovered that if a section of a nerve is subjected to alteration (i.e., exposure to a damaging agent), then the lability of such a section sharply decreases. Restoration of the initial state of the nerve fiber after each action potential in the damaged area occurs slowly. When this area is exposed to frequent stimuli, it is unable to reproduce the given rhythm of stimulation, and therefore the conduction of impulses is blocked. This state of reduced lability was called parabiosis by N. E. Vvedensky. The state of parabiosis excitable tissue occurs under the influence of strong stimuli and is characterized by phasic disturbances in conductivity and excitability. There are 3 phases: primary, phase most active(optimum) and a phase of reduced activity (pessimum). The third phase combines 3 successively replacing each other stages: equalizing (provisional, transformative - according to N.E. Vvedensky), paradoxical and inhibitory.

The first phase (primum) is characterized by a decrease in excitability and an increase in lability. In the second phase (optimum), excitability reaches a maximum, lability begins to decrease. In the third phase (pessimum), excitability and lability decrease in parallel and 3 stages of parabiosis develop. The first stage - equalizing according to I.P. Pavlov - is characterized by equalization of responses to strong, frequent and moderate irritations. IN equalization phase the magnitude of the response to frequent and rare stimuli is equalized. Under normal conditions of functioning of a nerve fiber, the magnitude of the response of the muscle fibers innervated by it obeys the law of force: the response to rare stimuli is less, and to frequent stimuli it is greater. Under the action of a parabiotic agent and with a rare rhythm of stimulation (for example, 25 Hz), all excitation impulses are conducted through the parabiotic area, since the excitability after the previous impulse has time to recover. With a high stimulation rhythm (100 Hz), subsequent impulses can arrive at a time when the nerve fiber is still in a state of relative refractoriness caused by the previous action potential. Therefore, some impulses are not carried out. If only every fourth excitation is carried out (i.e. 25 impulses out of 100), then the amplitude of the response becomes the same as for rare stimuli (25 Hz) - the response is equalized.

The second stage is characterized by a perverted response - strong irritations cause a smaller response than moderate ones. In this - paradoxical phase there is a further decrease in lability. At the same time, a response occurs to rare and frequent stimuli, but to frequent stimuli it is much less, since frequent stimuli further reduce lability, lengthening the phase of absolute refractoriness. Consequently, there is a paradox - the response to rare stimuli is greater than to frequent ones.

IN braking phase lability is reduced to such an extent that both rare and frequent stimuli do not cause a response. In this case, the nerve fiber membrane is depolarized and does not enter the repolarization stage, i.e., its original state is not restored. Neither strong nor moderate irritations cause a visible reaction; inhibition develops in the tissue. Parabiosis is a reversible phenomenon. If the parabiotic substance does not act for long, then after its action ceases, the nerve exits the state of parabiosis through the same phases, but in the reverse order. However, under the influence of strong stimuli, the inhibitory stage may be followed by a complete loss of excitability and conductivity, and subsequently tissue death.

The works of N.E. Vvedensky on parabiosis played a role important role in the development of neurophysiology and clinical medicine, showing the unity of the processes of excitation, inhibition and rest, changed the prevailing law of force relations in physiology, according to which the stronger the acting stimulus, the greater the reaction.

The phenomenon of parabiosis underlies drug local anesthesia. The influence of anesthetic substances is associated with a decrease in lability and a disruption of the mechanism of excitation along nerve fibers.

Receptive substance.

In cholinergic synapses it is a cholinergic receptor. It has a recognition center that specifically interacts exclusively with acetylcholine. An ion channel is associated with the receptor, which has a gate mechanism and an ion-selective filter, ensuring passage only for certain ions.

Inactivation system.

To restore the excitability of the postsynaptic membrane after the next impulse, inactivation of the transmitter is necessary. Otherwise, with prolonged action of a mediator, a decrease in the sensitivity of receptors to this mediator occurs (receptor desensitization). The inactivation system in the synapse is represented by:

1. An enzyme that destroys a mediator, for example, acetylcholinesterase, which destroys acetylcholine. The enzyme is located on the basement membrane of the synaptic cleft and its destruction by chemical means (eserine, prostigmine) stops the transmission of excitation in the synapse.

2. The system of reverse binding of the transmitter to the presynaptic membrane.

7. Postsynaptic potentials (PSPs)) - local potentials that are not accompanied by refractoriness and do not obey the “all or nothing” law and cause a potential shift on the postsynaptic cell.

general characteristics nerve cells

Neuron is structural unit nervous system. A neuron consists of a soma (body), dendrites, and an axon. The structural and functional unit of the nervous system is a neuron, a glial cell and feeding blood vessels.

Functions of a neuron

The neuron has irritability, excitability, conductivity, and lability. A neuron is capable of generating, transmitting, perceiving the action of a potential, and integrating influences with the formation of a response. Neurons have background(without stimulation) and caused by(after stimulus) activity.

Background activity can be:

Single - generation of single action potentials (AP) at different time intervals.

Burst - generation of series of 2-10 PDs every 2-5 ms with longer time intervals between bursts.

Group - series contain dozens of PDs.

Induced activity occurs:

At the moment the stimulus is turned on, the neuron is “ON”.

At the moment of switching off, "OF" is a neuron.

To turn on and off "ON - OF" - neurons.

Neurons can gradually change their resting potential under the influence of a stimulus.

Nerve cells, extremely diverse in structure and function, form the basis of the central (brain and spinal cord) and peripheral nervous systems. Together with neurons in the description nerve tissue its second important component – ​​glial cells – is considered. They are divided into macroglial cells - astrocytes, oligodendrocytes, ependymocytes and microglial cells.

The main functions of the nervous system performed by neurons are excitation, its conduction and transmission of impulses to effector organs. Neuroglial cells contribute to the performance of these functions by neurons. The activity of the nervous system is based on the principle of functioning of a reflex arc, consisting of neurons connected to each other through specialized contacts - synapses of various types.

Neurons of vertebrates and most invertebrate animals, as a rule, are cells with many long, complex branching processes, some of which perceive excitation. They are called dendrites, and one of the processes, distinguished by its large length and branches in the terminal sections, is called an axon.

The main functional properties of neurons are associated with their structural features plasma membrane containing huge number voltage- and ligand-dependent receptor complexes and ion channels, as well as the ability to release neurotransmitters and neuromodulators in certain areas (synapses). Knowledge of the structural organization of nervous tissue was largely due to the use of special methods staining of neurons and glial cells. Among them, the methods of tissue impregnation with silver salts according to Golgi and Bielschowsky-Gross deserve special attention.

The foundations of classical ideas about the cellular structure of the nervous system were laid in the works of the outstanding Spanish neurohistologist, laureate Nobel Prize, Santiago Ramon y Cajala. A great contribution to the study of nervous tissue was made by the studies of histologists of the Kazan and St. Petersburg-Leningrad schools of neurohistology - K. A. Arnstein, A. S. Dogel, A. E. Smirnov, D. A. Timofeev, A. N. Mislavsky, B. I. Lavrentieva, N. G. Kolosova, A.A. Zavarzina, P.D. Deineki, N.V. Nemilova, Yu.I. Orlova, V.P. Babmindra et al.

The structural and functional polarity of most nerve cells has led to the traditional division of three neuron divisions: body, dendrites and axon. The unique structure of neurons is manifested in the extreme branching of their processes, often reaching very long length, and the presence in cells of a variety of specific protein and non-protein molecules (neurotransmitters, neuromodulators, neuropeptides, etc.) with high biological activity.

The classification of nerve cells according to their structure is based on:

1) body shape - round-oval, pyramidal, basket-shaped, fusiform, pear-shaped, stellate and some other types of cells are distinguished;

2) the number of processes - unipolar, bipolar (as an option - pseudo-unipolar), and multipolar;

3) the nature of dendritic branching and the presence of spines (densely and sparsely branched; spinous and spineless cells);

4) the nature of axon branching (branching only in the terminal part or the presence of collaterals along the entire length, short-axon or long-axon).

Neurons are also divided according to the content of neurotransmitters into: cholinergic, adrenergic, serotonergic, GABA (gammergic), amino acid (glycinergic, glutamatergic, etc.). The presence in one neuron of several neurotransmitters, even those antagonistic in their effects, such as acetylcholine and norepinephrine, makes us treat the unambiguous definition of the neurotransmitter and neuropeptide phenotype of neurons very carefully.

There is also a classical division of neurons (depending on their position in reflex arc) into: afferent (sensitive), intercalary (associative) and efferent (including motor). Sensory neurons have the most variable structural organization endings of dendrites, which fundamentally distinguishes them from the dendrites of other nerve cells. They are often represented by bipolar (sensory ganglia of a number of sensory organs), pseudounipolar (spinal ganglia) or highly specialized neurosensory cells (retinal photoreceptors or olfactory cells). Neurons of the central nervous system that do not generate an action potential (spikeless neurons) and spontaneously excitable oscillatory cells have been found. Analysis of the features of their structural organization and relationship with “traditional” neurons is a promising direction in understanding the activity of the nervous system.

Body (soma). Nerve cell bodies can vary significantly in shape and size. Motor neurons of the anterior horns of the spinal cord and giant pyramids of the cortex cerebral hemispheres- one of the largest cells in the body of vertebrates - the size of the body of the pyramids reaches 130 microns, and vice versa, the granule cells of the cerebellum, having an average diameter of 5-7 microns, are the smallest nerve cells of vertebrates. The cells of the autonomic nervous system also vary in shape and size.

Core. Neurons usually have one nucleus. It is usually large, round, contains one or two nucleoli, chromatin has a low degree of condensation, which indicates high activity of the nucleus. It is possible that some neurons are polyploid cells. The nuclear envelope is represented by two membranes separated by a perinuclear space and having numerous pores. The number of pores in vertebrate neurons reaches 4000 per nucleus. An important component of the core is the so-called. “nuclear matrix” is a complex of nuclear proteins that ensure the structural organization of all components of the nucleus and are involved in the regulation of replication processes, transcription and processing of RNA and their removal from the nucleus.

Cytoplasm (perikaryon). Many, especially large pyramidal neurons, are distinguished by a rich content of granular endoplasmic reticulum(HPP). This is clearly manifested when they are stained with aniline dyes in the form of basophilia of the cytoplasm and the basophilic, or tigroid, substance included in it (Nissl substance). The distribution of basophilic Nissl substance in the cytoplasm of the perikaryon is recognized as one of the criteria for neuron differentiation, as well as an indicator of the functional state of the cell. Neurons also contain big number free ribosomes, usually assembled into rosettes - polysomes. In general, nerve cells contain all the main organelles characteristic of eukaryotic cells. animal cell, although there are a number of features.

The first concerns mitochondria. The intensive work of a neuron is associated with high energy costs, so they contain a lot of mitochondria different types. In the body and processes of neurons there are a few (3-4 pieces) giant mitochondria of the “reticular” and “filamentous” types. The arrangement of the cristae in them is longitudinal, which is also quite rare among mitochondria. In addition, in the body and processes of the neuron there are many small mitochondria of the “traditional” type with transverse cristae. Especially many mitochondria accumulate in the areas of synapses, dendrite branching nodes, and in the initial section of the axon (axon hillock). Due to the intense functioning of mitochondria in a neuron, they usually have a short life cycle (some mitochondria live for about an hour). Mitochondria are renewed through traditional fission or budding of mitochondria and delivered to cell processes through axonal or dendritic transport.

Another one of characteristic features The structure of the cytoplasm of neurons of vertebrate and invertebrate animals is the presence of an intracellular pigment - lipofuscin. Lipofuscin belongs to a group of intracellular pigments, the main components of which are carotenoid yellow or Brown. It is found in small membranous granules scattered throughout the cytoplasm of the neuron. The significance of lipofuscin is actively debated. It is believed that this is the “aging” pigment of the neuron and is associated with the processes of incomplete breakdown of substances in lysosomes.

During the life cycle of nerve cells, the number of lipofuscin granules significantly increases and their distribution in the cytoplasm can indirectly judge the age of the neuron.

There are four morphological stages of neuron “aging”. In young neurons (stage 1 - diffuse) there is little lipofuscin and it is scattered throughout the cytoplasm of the neuron. In mature nerve cells (stage 2, perinuclear), the amount of pigment increases and it begins to accumulate in the nuclear zone. In aging neurons (stage 3 - polar), there is more and more lipofuscin and accumulations of its granules are concentrated near one of the poles of the neuron. And finally, in old neurons (stage 4, bipolar), lipofuscin fills a large volume of cytoplasm and its accumulations are located at opposite poles of the neuron. In some cases, there is so much lipofuscin in the cell that its granules deform the nucleus. The accumulation of lipofuscin during the aging process of neurons and the body is also associated with the property of lipofuscin, as a carotenoid, to bind oxygen. It is believed that in this way the nervous system adapts to the deterioration of oxygen supply to cells that occurs with age.

A special type of endoplasmic reticulum, characteristic of the perikarya of neurons, are subsurface cisterns - one or two flattened membrane vesicles located near the plasma membrane and often associated with it by electron-dense unformed material. In the perikaryon and in the processes (axon and dendrites), multivesicular and multilamellar membranous bodies are often found, represented by clusters of vesicles or fibrillar material with an average diameter of 0.5 μm. They are derivatives of the final stages of lysosome functioning in the processes of physiological regeneration of neuron components and participate in reverse (retrograde) transport.

Neurons(neurocytes, nerve cells themselves) - cells of various sizes (which vary from the smallest in the body, in neurons with a body diameter of 4-5 microns - to the largest with a body diameter of about 140 microns). By birth, neurons lose the ability to divide, so during postnatal life their number does not increase, but, on the contrary, due to the natural loss of cells, gradually decreases. Neuron comprises cell body (perikaryon) and processes that ensure the conduction of nerve impulses - dendrites, bringing impulses to the neuron body, and axon (neurite), carrying impulses from the neuron body.

Neuron body (perikaryon) includes the nucleus and the surrounding cytoplasm (with the exception of those included in the processes). The perikaryon contains the synthetic apparatus of the neuron, and its plasmalemma performs receptor functions, since it contains numerous nerve endings (synapses), carrying excitatory and inhibitory signals from other neurons. Neuron nucleus - usually one, large, round, light, with finely dispersed chromatin (predominance of euchromatin), one, sometimes 2-3 large nucleoli. These features reflect the high activity of transcription processes in the neuron nucleus.

Cytoplasm of a neuron rich in organelles and surrounded by plasmalemma, which has the ability to carrying out nerve impulse due to the local current of Na+ into the cytoplasm and K+ out of it through voltage-dependent membrane ion channels. The plasmalemma contains Na+-K+ pumps that maintain the necessary ion gradients.

Dendrites conduct impulses to the neuron body, receiving signals from other neurons through numerous interneuron contacts (axo-dendritic synapses), located on them in the area of ​​special cytoplasmic protrusions - dendritic spines. Many spines have a special spinous apparatus, consisting of 3-4 flattened tanks separated by areas of dense matter. Spines are labile structures that break down and form again; their number drops sharply with aging, as well as with a decrease in the functional activity of neurons. In most cases, dendrites are numerous, relatively short in length, and highly branched near the neuron body. Large stem dendrites contain all types of organelles; as their diameter decreases, elements of the Golgi complex disappear in them, and the cisternae of the grEPS are preserved. Neurotubules and neurofilaments are numerous and arranged in parallel bundles; they provide dendritic transport, which is carried out from the cell body along the dendrites at a speed of about 3 mm/h.

Axon (neurite)- a long (in humans from 1 mm to 1.5 m) process through which nerve impulses are transmitted to other neurons or cells of working organs (muscles, glands). In large neurons, the axon can contain up to 99% of the volume of cytoplasm. An axon extends from a thickened area of ​​the neuron body that does not contain chromatophilic substance - axon hillock, in which nerve impulses are generated; Almost along its entire length it is covered with a glial membrane. central part axon cytoplasm (axoplasma) contains bundles of neurofilaments oriented along its length; closer to the periphery are bundles of microtubules, ER cisterns, elements of the Golgi complex, mitochondria, membrane vesicles, and a complex network of microfilaments. There are no Nissl bodies in the axon. In the final section, the axon often breaks up into thin branches (telodendria). The axon ends in specialized terminals (nerve endings) on other neurons or cells of working organs.

CLASSIFICATION OF NEURONS

Classification of neurons carried out according to three criteria: morphological, functional and biochemical.

Morphological classification neurons takes into account number of their processes and divides all neurons into three types: unipolar, bipolar and multipolar.

1. Unipolar neurons have one branch. According to most researchers, in nervous system They are not found in humans or other mammals. Some authors still refer to such cells as omacrine neurons retina and interglomerular neurons olfactory bulb.

2. Bipolar neurons have two branches - axon and dendrite. usually extending from opposite poles of cells. They are rare in the human nervous system. These include bipolar cells of the retina, spiral and vestibular ganglia.

Pseudounipolar neurons - a type of bipolar, in which both cell processes (axon and dendrite) extend from the cell body in the form of a single outgrowth, which is then divided in a T-shape. These cells are found in spinal and cranial ganglia.

3. Multipolar neurons have three or more branches: an axon and several dendrites. They are most common in the human nervous system. Up to 80 variants of these cells have been described: spindle-shaped, stellate, pear-shaped, pyramidal, basket-shaped, etc. Based on the length of the axon, they are classified Golgi cells type I(with a long axon) and Golgi cells type II (with short axon).

A. Neuron is a structural and functional unit of nervous tissue. The body of the neuron and its processes are distinguished. The neuron membrane (cell membrane) forms closed space containing protoplasm (cytoplasm and nucleus). Cytoplasm consists of the main substance (cytosol, hyaloplasm) and organelles. Under an electron microscope, hyaloplasm looks like a relatively homogeneous substance and is the internal environment of a neuron. Most of the organelles and the nucleus of a neuron, like any other cell, are enclosed in their own compartments (compartment™), formed by their own (intracellular) membranes, which have selective permeability to individual ions and particles located in the hyaloplasm and organelles. This determines their distinctive composition from each other.

The human brain contains about 25 billion nerve cells, the interaction between which is carried out through many synapses (intercellular connections), the number of which is thousands of times greater than the cells themselves (10 |5 -10 16), since their axons are repeatedly divided dichotomously. Neurons also exert their influence on organs and tissues through synapses. Nerve cells are also present outside the central nervous system: the peripheral part of the autonomic nervous system, afferent neurons of the spinal ganglia and ganglia of the cranial nerves. There are much fewer peripheral nerve cells than central ones. - only about 25 million. Glial cells play an important role in the activity of the first nervous system (see section 2.1, E).

The processes of a neuron represent a large number of dendrites and one axon (Fig. 2.1). Nerve cells have an electrical charge, like other cells of an animal organism and even plants (Fig. 2.2). The resting potential (RP) of a neuron is 60-80 mV, RP - a nerve impulse - 80-110 mV. The soma and dendrites are covered with nerve endings - synaptic boutons and processes of glial cells. On one neuron, the number of synaptic boutons can reach 10,000. The axon starts from the cell body with an axon hillock. The diameter of the cell body is 10-100 microns, the axon - 1-6 km, at the periphery the length of the axon can reach 1 m or more. Neurons of the brain form columns, nuclei and layers that perform certain functions. Cellular accumulations make up Gray matter brain Unmyelinated and myelinated nerve fibers (dendrites and axons of neurons, respectively) pass between the cells.

B. Classification of neurons. Neurons are divided into the following groups.

1. According to the mediator, released in axon terminals, neurons are distinguished as adrenergic, cholinergic, serotonergic, etc.

2. Depending on the part of the central nervous system secrete neurons of the somatic and autonomic nervous systems.

3. Based on the direction of information, the following neurons are distinguished:

Afferent, using receptors to perceive information about the external and internal environment of the body and transmit it to the overlying parts of the central nervous system;

Efferent, transmitting information to the working organs - effectors (nerve cells innervating effectors are sometimes called effectors);

Interneurons (interneurons) provide interaction between neurons of the central nervous system.

4. By influence secrete excitatory and inhibitory neurons.

5. By activity distinguish between background-active and “silent” neurons, excited only in response to stimulation. Background-active neurons differ in the general pattern of impulse generation, since some neurons discharge continuously (rhythmically or arrhythmically), while others discharge in bursts of impulses. The interval between pulses in a burst is milliseconds, and between bursts is seconds. Background-active neurons play an important role in maintaining the tone of the central nervous system and especially the cortex big brain.

6. Based on perceived sensory information neurons are divided into mono-, bi- and polymodal. The neurons of the hearing center in the cerebral cortex are monomodal. Bimodal neurons are found in the secondary visual analyzer zones in the cortex (neurons of the secondary visual analyzer zone in the cerebral cortex respond to light and sound stimuli). Polymodal Neurons are neurons of the associative areas of the brain, the motor cortex; they react to stimulation of receptors of the skin, visual, auditory and other analyzers.

sosomatic, axodendritic, axo-axonal, dendrosomatic, dendrodendritic synapses.

3. By effect distinguish between excitatory and inhibitory synapses. During the activity of the nervous system, individual neurons

are combined into ensembles (modules), neural networks. The latter can include several neurons, tens, thousands of neurons, while the set of neurons forming a module ensures that the module acquires new properties that individual neurons do not possess. The activity of each neuron within a module becomes a function not only of the signals received by it, but also a function of processes determined by a particular design of the module (P.G. Kostyuk).

D. Glial cells (neuroglia - “nerve glue”). These cells are more numerous than neurons, accounting for about 50% of the volume of the central nervous system. They are capable of dividing throughout their lives. The size of glial cells is 3-4 times smaller than nerve cells, their number is huge - reaches 14 * 10 "°, increases with age (the number of neurons decreases). The bodies of neurons, like their axons, are surrounded by glial cells. Glial cells perform several functions: supporting, protective, insulating, metabolic (supplying neurons with nutrients). Microglial cells are capable of phagocytosis, a rhythmic change in their volume (the period of “contraction” is 1.5 minutes, the period of “relaxation” is 4 minutes). Cycles of volume change are repeated every 2-20 hours. It is believed that pulsation promotes the advancement of axoplasm in neurons and affects the flow of intercellular fluid. The membrane potential of neuroglial cells is 70-90 mV, but they do not generate APs; they generate only local currents that spread electrotonically from one cell to another. Excitation processes in neurons and electrical phenomena in glial cells appear to interact.

E. Cerebrospinal fluid (CSF) is a colorless transparent liquid that fills the cerebral ventricles, spinal canal and subarachnoid space. Its origin is associated with the interstitial fluid of the brain. A significant part of the cerebrospinal fluid is formed in specialized plexuses of the ventricles of the brain. Direct nutrient medium brain cells is the interstitial fluid into which cells also secrete their metabolic products. Cerebrospinal fluid is a combination of blood plasma filtrate and interstitial fluid; it contains about 90% water and about 10% solids (2% organic, 8% - inorganic substances). It differs from blood plasma, as does intercellular fluid other tissues, low protein content (0.1 g/l, in plasma - 75 g/l), lower content of amino acids (0.8 and 2 mmol/l, respectively) and glucose (3.9 and about 5 mmol/l, respectively ). Its volume is 100-200 ml (12-14% of the total brain volume), about 600 ml is produced per day. This fluid is renewed 4-8 times a day, the cerebrospinal fluid pressure is 7-14 mm Hg. Art., in a vertical position of the body - 2 times more. Cerebrospinal fluid also performs protective role: is a kind of hydraulic “cushion” of the brain, has bactericidal properties: The cerebrospinal fluid contains immunoglobulins of classes O and A, the complement system, monocytes and lymphocytes. The outflow of cerebrospinal fluid occurs in several ways: 30-40% of it flows through the subarachnoid space into the longitudinal sinus of the cerebral venous system; 10-20% - through the perineural spaces of the cranial and spinal nerves into the lymphatic system; some of the fluid is reabsorbed by the choroid plexuses of the brain.

FUNCTIONS OF NEURONS

The life of an animal organism is concentrated in a cell. Each cell has general (basic) functions, identical with the functions of other cells, and specific ones, characteristic mainly this species cells.

A. Neuron functions are identical general functions any cells of the body.

1. Synthesis of tissue and cellular structures, as well as compounds necessary for life (anabolism). In this case, energy is not only consumed, but also accumulated, as the cell absorbs organic compounds, rich in energy (proteins, fats and carbohydrates entering the body with food). In a cage nutrients come, as a rule, in the form of hydrolysis products of proteins, fats, carbohydrates (monomers) - these are monosaccharides, amino acids, fatty acids and monoglycerides. The synthesis process ensures the restoration of structures undergoing decay.

2. Energy production as a result of catabolism - a set of processes of breakdown of cellular and tissue structures and complex compounds containing energy. Energy is necessary to ensure the functioning of every living cell.

3. Transmembrane transfer of substances, ensuring the entry of necessary substances into the cell and the release from the cell of metabolites and substances used by other cells of the body.

B. Specific functions of nerve cells of the central nervous system and the peripheral nervous system.

1. Perception of change external and internal environment body. This function is carried out primarily with the help of peripheral nerve formations - sensory receptors (see section 1.1.6) and through the spiny apparatus of the dendrites and the body of the neuron (see section 2.1).

2. Signal transmission other nerve cells and effector cells: skeletal muscles, smooth muscles of internal organs, blood vessels, secretory cells. This transmission is realized using synapses (see section 4.3).

3. Recycling arriving to the neuron information through the interaction of excitatory and inhibitory influences of nerve impulses arriving at the neuron (see section 4.5-4.8).

4. Storing information from using memory mechanisms (see section 6.6). Any signal from external and internal environment the body is first transformed into the process of excitation, which is the most characteristic manifestation of the activity of any nerve cell.

5. Nerve impulses provide communication between all cells of the body and regulation of their functions (see section 1.1).

6. With chemical substances nerve cells have trophic influence on effector cells of the body (nutrition; see section 1.1).

The vital activity of the nerve cell itself is ensured by the interaction of all its organelles and the cell membrane (the set of structural elements that form the cell membrane), like any other cell in the body.

Nerve cells communicate with each other through special chemical messengers called neurotransmitters. Medications, including prohibited ones, can inhibit the activity of these molecules. Nerve cells do not have direct contact with each other. Microscopic spaces between sections of cell membranes - synaptic clefts - separate nerve cells and are capable of both emitting signals (presynaptic neuron) and receiving them (guestsynaptic neuron). The presence of a synaptic cleft indicates the impossibility of direct transmission electrical impulse from one nerve cell to another. At the moment when the impulse reaches the synaptic terminal, a sharp change in the potential difference leads to the opening of channels through which calcium ions rush into the presynaptic cell. Human nerve cells, description, characteristics - our topic of publication.

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Photo gallery: Human nerve cells, description, characteristics

Release of neurotransmitters

Calcium ions act on vesicles (small membrane-surrounded vesicles containing chemical transmitters - neurotransmitters) of the nerve ending that approach the presynaptic membrane and merge with it, releasing a gap. The neurotransmitter molecules diffuse (penetrate). After the interaction of the neurotransmitter with a specific receptor on the postsynaptic membrane, it is quickly released and its further fate is twofold. On the one hand, it is possible to completely destroy it under the action of enzymes located in the synaptic cleft; on the other hand, it can be recaptured into presynaptic endings with the formation of new vesicles. This mechanism ensures the short-term action of the neurotransmitter on the receptor molecule. Some illicit drugs, such as cocaine, and some medicinal substances prevent neurotransmitter reuptake (in the case of cocaine, dopamine). At the same time, the period of influence of the latter on the receptors of the postsynaptic membrane is extended, which causes a much more powerful stimulating effect.

Muscle activity

The regulation of muscle activity is carried out by nerve fibers that extend from the spinal cord and end at the neuromuscular junction. When a nerve impulse arrives, it is released from nerve endings neurotransmitter acetylcholine. It penetrates the synaptic cleft and binds to receptors muscle tissue. This triggers a cascade of reactions leading to muscle fiber contraction. In this way, the central nervous system controls the contraction of certain muscles at any given time. This mechanism underlies the regulation of complex movements such as walking. The brain is exclusively complex structure; each of its neurons interacts with thousands of others scattered throughout the nervous system. Since nerve impulses do not differ in strength, information is encoded in the brain based on their frequency, that is, the number of action potentials generated in one second matters. In some ways, this code resembles Morse code. One of the most difficult tasks facing neuroscientists around the world today is trying to understand how this relatively simple system coding; for example, how to explain a person's emotions when a relative or friend dies, or the ability to throw a ball with such accuracy that it hits the target from a distance of 20 meters. It is now becoming apparent that information is not transmitted linearly from one nerve cell to another. On the contrary, one neuron can simultaneously perceive nerve signals from many others (this process is called convergence) and is also able to influence great amount nerve cells, divergence.

Synapses

There are two main types of synapses: in some the postsynaptic neuron is activated, in others it is inhibited (this largely depends on the type of transmitter released). A neuron emits a nerve impulse when the number of excitatory stimuli exceeds the number of inhibitory ones.

Synapse strength

Each neuron receives a huge amount of both excitatory and inhibitory stimuli. Moreover, each synapse has a greater or lesser effect on the probability of an action potential. Synapses with the greatest influence are usually located near the zone of reinforcement of the nerve impulse in the body of the nerve cell.