Nerve cell. The structure of a neuron What nerve cells transmit

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

Development

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

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

During further embryogenesis, the neural plate turns into a tube. Stem ventricular elements are located in the inner layer of its wall. They proliferate and move outwards. In this area, some cells continue to divide. As a result, they are divided into spongioblasts (components of microglia), glioblasts and neuroblasts. Of the latter, nerve cells are formed. There are 3 layers in the wall of the tube:


At 20-24 weeks, blisters begin to form in the cranial segment of the tube, which are the source of brain formation. The remaining sections serve for the development of the spinal cord. The cells involved in the formation of the ridge depart from the edges of the neural trough. It is located between the ectoderm and the tube. Ganglionic plates are formed from the same cells, which serve as the basis for myelocytes (pigmented skin elements), peripheral nerve nodes, melanocytes of the cover, and components of the APUD system.

Components

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


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

Main component of the CNS

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

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

The structure of the nerve cell

The body is distinguished in the element. It has a large light nucleus with one or two nucleoli. The cytoplasm contains all organelles, especially tubules from the granular endoplasmic reticulum. Accumulations of basophilic substance are distributed throughout the cytoplasmic surface. They are formed by ribosomes. In these accumulations, the process of synthesis of all the necessary substances that are transported from the body to the processes takes place. Due to stress, these lumps are destroyed. Thanks to intracellular regeneration, the process of restoration-destruction is constantly taking place.

Impulse formation and reflex activity

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

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

Tasks

According to the function of the nerve cell, the following types of elements are distinguished:

  • Afferent (sensitive). They form 1 link in the reflex arc (spinal nodes). A long dendrite passes to the periphery. It ends there. In this case, a short axon enters the reflex somatic arc in the region of the spinal cord. He is the first to react to the stimulus, resulting in the formation of a nerve impulse.
  • Conductor (plug-in). These are nerve cells in the brain. They form a 2 arc link. These elements are also present in the spinal cord. From them information is received by the motor effector cells of the nervous tissue, branched short dendrites and a long axon reaching the skeletal muscle fiber. An impulse is transmitted through the neuromuscular synapse. Effector (efferent) elements are also distinguished.

reflex arcs

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

fibers

The glial membranes are located independently around the nerve processes. Together, they form nerve fibers. Branches in them are called axial cylinders. There are unmyelinated and myelinated fibers. They differ in the structure of the glial membrane. Myelin-free fibers have a fairly simple device. The axial cylinder approaching the glial cell bends its cytolemma. The cytoplasm closes over it and forms a mesaxon - a double fold. One glial cell may contain several axial cylinders. These are "cable" fibers. Their branches can pass into neighboring glial cells. The impulse travels at a speed of 1-5 m/s. Fibers of this type are found during embryogenesis and in postganglionic areas of the vegetative system. Myelin segments are thick. They are located in the somatic system that innervates the muscles of the skeleton. Lemmocytes (glial cells) pass sequentially, in a chain. They form a heaviness. An axial cylinder runs in the center. The glial sheath contains:

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

Areas of hypersensitivity

In areas where adjacent lemmocytes border, thinning of the nerve fiber occurs and there is no myelin layer. These are places of increased sensitivity. They are considered the most vulnerable. The part of the fiber located between adjacent nodal intercepts is called the internodal segment. Here the impulse passes at a speed of 5-120 m/s.

synapses

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

Varieties

Synapses are divided into:

  • Brake, containing slowing down neurotransmitters (gamma-aminobutyric acid, glycine).
  • Exciting, in which the corresponding components are present (adrenaline, acetylcholine, glutamic acid, norepinephrine).
  • Effector, ending on working cells.

Neuromuscular synapses are formed in the skeletal muscle fiber. They have a presynaptic part formed by the terminal terminal section of the axon from the motor neuron. It is embedded in the fiber. The adjacent site forms the postsynaptic part. It does not contain myofibrils, but there are a large number of mitochondria and nuclei. The postsynaptic membrane is formed by the sarcolemma.

Sensitive endings

They are of great variety:

  • Free are found exclusively in the epidermis. The fiber, passing through the basement membrane and discarding the myelin sheath, freely interacts with epithelial cells. These are pain and temperature receptors.
  • Non-encapsulated captive endings are present in connective tissue. Glia accompanies branches in the axial cylinder. These are tactile receptors.
  • The encapsulated endings are branchings from the axial cylinder, accompanied by the glial inner flask and the outer connective tissue sheath. These are also tactile receptors.

The structure of nerve cells(neurocytus). Neurons have sizes from 4 to 140 microns in diameter, various shapes (pyramidal, stellate, arachnid, round, etc.). At the same time, all neurons have processes ranging in length from a few micrometers to 1.5 m. The processes are divided into 2 types:

1) dendrites that branch; there may be several of them in a neuron, often they are shorter than axons; along them the impulse moves to the cell body;

2) axons, or neurites; neurite in a cell can be only 1; along the axon, the impulse moves from the cell body and is transmitted to the working organ or to another neuron.

Morphological classification of neurocytes(according to the number of branches). Depending on the number of processes, neurocytes are divided into:

1) unipolar if there is only 1 process (axon); occur only in the embryonic period;

2) bipolar, contain 2 processes (axon and dendrite); meet in a retina of an eye and a spiral ganglion of an inner ear;

3) multipolar- have more than 2 processes, one of them is an axon, the rest are dendrites; are found in the brain and spinal cord and peripheral ganglia of the autonomic nervous system;

4) pseudo-unipolar- these are actually bipolar neurons, since the axon and dendrite depart from the cell body in the form of one common process and only then separate and go in different directions; are located in the sensitive nerve ganglia (spinal, sensory ganglia of the head).

By functional classification neurocytes are divided into:

1) sensitive, their dendrites end with receptors (sensitive nerve endings);

2) effector, their axons end in effector (motor or secretory) endings;

3) associative (insert), connect two neurons to each other.

Nuclei neurocytes are round, light, located in the center of the cell or eccentrically, contain dispersed chromatin (euchromatin) and well-defined nucleoli (active nucleus). A neurocyte usually has 1 nucleus. The exception is the neurons of the autonomic ganglions in the cervical and prostate glands.

Neurilemma- plasmolemma of a nerve cell, performs barrier, metabolic, receptor functions and conducts a nerve impulse. A nerve impulse occurs if a mediator acts on the neurolemma, which increases the permeability of the neurolemma, as a result of which Na + ions from the outer surface of the neurolemma enter the inner surface, and potassium ions move from the inner surface to the outer - this is the nerve impulse (depolarization wave) , which quickly moves along the neurolemma.

Neuroplasma- cytoplasm of neurocytes, contains well-developed mitochondria, granular ER, Golgi complex, includes a cell center, lysosomes and special organelles called neurofibrils.

Mitochondria are located in large numbers in the body of neurocytes and processes, especially a lot of them are found in the terminals of nerve endings. The Golgi complex is usually located around the nucleus and has the usual ultramicroscopic structure. The granular ER is very well developed and forms clusters in the body of the neuron and in the dendrites. When staining the nervous tissue with basic dyes (toluidine blue, thionine), the locations of the granular ER are stained basophilically. Therefore, accumulations of granular EPS are called basophilic substance, or chromatophilic substance, or Nissl substance. The chromatophilic substance is contained in the body and dendrites of neurons and is absent in the axons and cones from which axons originate.

With intensive functional activity of neurocytes, a decrease or disappearance of the chromatophilic substance occurs, which is called chromatinolysis.

Neurofibrils stain dark brown on silver impregnation. In the body of the neuron, they have a multidirectional arrangement, and in the processes they are parallel. Neurofibrils consist of neurofilaments 6–10 nm in diameter and neurotubules 20–30 nm in diameter; form the cytoskeleton and participate in intracellular movement. Along neurofibrils the movement of various substances is carried out.

Currents (movement) of neuroplasm- this is the movement of neuroplasm along the processes from the body and to the cell body. There are 4 currents of neuroplasm:

1) slow current along axons from the cell body, characterized by the movement of mitochondria, vesicles, membrane structures and enzymes that catalyze the synthesis of synapse mediators; its speed is 1-3 mm per day;

2) fast current along axons from the cell body, it is characterized by the movement of components from which mediators are synthesized; the speed of this current is 5-10 mm per hour;

3) dendritic current , providing transportation of acetylcholinesterase to the postsynaptic membrane of the synapse at a rate of 3 mm per hour;

4) retrograde current - this is the movement of metabolic products along the processes to the cell body. Rabies viruses move along this path. Each current of movement has its own path along the microtubules. There can be several pathways in one microtubule. Moving along different paths in one direction, the molecules can overtake each other, they can move in the opposite direction. The path of movement along the process from the cell body is called anterograde to the cell body retrograde. Special proteins, dynein and kinesin, take part in the movement of the components.

Neuroglia. It is classified into macroglia and microglia. Microglia is represented by glial macrophages that develop from blood monocytes and perform a phagocytic function. Macrophages have a process shape. Several short processes extend from the body, which branch into smaller ones.

macroglia divided into 3 types:

1) ependymal glia; 2) astrocytic glia; and 3) oligodendroglia.

Ependymal glia, like cells of the surface epithelium, lines the ventricles of the brain and the central canal of the spinal cord. Among ependymocytes, 2 varieties are distinguished: 1) cubic and 2) prismatic. Both have apical and basal surfaces. On the apical surface of the ependymocytes facing the cavity of the ventricles, there are cilia in the embryonic period, which disappear after the birth of the child and remain only in the aqueduct of the midbrain.

A process extends from the basal surface of cylindrical (prismatic) ependymocytes, which penetrates the substance of the brain and on its surface participates in the formation of the outer glial limiting membrane (membrana glialis limitans superficialis). Thus, these ependymocytes perform supporting, delimiting, and barrier functions. Part of the ependymocytes is part of the subcommissural organ and is involved in the secretory function.

Ependymocytes cubic shape line the surface of the vascular plexuses of the brain. There is a basal striation on the basal surface of these ependymocytes. They perform a secretory function, participate in the production of cerebrospinal (cerebrospinal) fluid.

Astrocyte glia is divided into: 1) protoplasmic (gliocytus protoplasmaticus) and 2) fibrous (gliocytus fibrosus).

Protoplasmic astrocytes are located mainly in the gray matter of the brain and spinal cord. Short thick processes extend from their body, from which secondary processes extend.

Fibrous astrocytes are located mainly in the white matter of the brain and spinal cord. Numerous long, almost non-branching processes extend from their round or oval body, which reach the surface of the brain and participate in the formation of glial boundary surface membranes. The processes of these astrocytes approach the blood vessels and form glial limiting perivascular membranes (membrana glialis limitans perivascularis) on their surface, thus participating in the formation of the blood-brain barrier.

The functions of protoplasmic and fibrous astrocytes are numerous:

1) support;

2) barrier;

3) participate in the exchange of mediators;

4) participate in water-salt metabolism;

5) secrete neurocyte growth factor.

Oligodendrogliocytes are located in the medulla of the brain and spinal cord, accompany the processes of neurocytes. The composition of the nerve trunks, nerve ganglia and nerve endings are neurolemmocytes that develop from the neural crest. Depending on where the oligodendrocytes are localized, they have a different shape, structure, and perform different functions. In particular, in the brain and spinal cord they have an oval or angular shape, a few short processes extend from their body. In the event that they accompany the processes of nerve cells in the composition of the brain and spinal cord, their shape is flattened. They're called neurolemmocytes. Neurolemmocytes, or Schwann cells, form sheaths around the processes of nerve cells that are part of the peripheral nerves. Here they perform trophic and delimiting functions and take part in the regeneration of nerve fibers when they are damaged. In the peripheral nerve nodes, neurolemmocytes acquire a round or oval shape, surround the bodies of neurons. They are called node gliocytes(gliocyti ganglii). Here they form sheaths around nerve cells. In peripheral nerve endings, neurolemmocytes are called sensitive cells.

Nerve fibers(neurofibra). These are processes of nerve cells (dendrites or axons) covered with a sheath consisting of neurolemmocytes. A process in a nerve fiber is called axial cylinder(cylindraxis). Depending on the structure of the membrane, nerve fibers are divided into non-myelinated (neurofibra amyelinata) and myelinated (neurofibra myelinata). If the sheath of a nerve fiber includes a layer of myelin, then such a fiber is called myelin; if there is no myelin layer in the shell - unmyelinated.

unmyelinated nerve fibers located mainly in the peripheral autonomic nervous system. Their shell is a cord of neurolemmocytes, in which axial cylinders are immersed. An unmyelinated fiber containing several axial cylinders is called fiber cable type. Axial cylinders from one fiber can pass into the next one.

Education process unmyelinated nerve fiber happens as follows. When a process appears in a nerve cell, a strand of neurolemmocytes appears next to it. The process of the nerve cell (axial cylinder) begins to sink into the strand of neurolemmocytes, dragging the plasmolemma deep into the cytoplasm. The double plasmalemma is called mesaxon. Thus, the axial cylinder is located at the bottom of the mesaxon (suspended on the mesaxon). Outside, the non-myelinated fiber is covered with a basement membrane.

myelinated nerve fibers are located mainly in the somatic nervous system, have a much larger diameter compared to unmyelinated ones - up to 20 microns. The axle cylinder is also thicker. Myelin fibers are stained with osmium in a black-brown color. After staining, 2 layers are visible in the fiber sheath: the inner myelin and the outer, consisting of the cytoplasm, nucleus and plasmolemma, which is called neurolemma. An uncolored (light) axial cylinder runs in the center of the fiber.

Oblique light notches (incisio myelinata) are visible in the myelin layer of the shell. Along the fiber, there are constrictions through which the myelin sheath layer does not pass. These narrowings are called nodal intercepts (nodus neurofibra). Only the neurilemma and the basement membrane surrounding the myelin fiber pass through these intercepts. Nodal nodes are the boundary between two adjacent lemmocytes. Here, short outgrowths with a diameter of about 50 nm depart from the neurolemmocyte, extending between the ends of the same processes of the adjacent neurolemmocyte.

The section of myelin fiber located between two nodal interceptions is called the internodal, or internodal, segment. Only 1 neurolemmocyte is located within this segment.

myelin sheath layer- this is a mesaxon screwed onto the axial cylinder.

Myelin fiber formation. Initially, the process of myelin fiber formation is similar to the process of myelin-free fiber formation, i.e., the axial cylinder is immersed in the strand of neurolemmocytes and mesaxon is formed. After that, the mesaxon lengthens and wraps around the axial cylinder, pushing the cytoplasm and nucleus to the periphery. This mesaxon, screwed onto the axial cylinder, is the myelin layer, and the outer layer of the membrane is the nucleus and cytoplasm of neurolemmocytes pushed to the periphery.

Myelinated fibers differ from unmyelinated fibers in structure and function. In particular, the speed of the impulse along the non-myelinated nerve fiber is 1-2 m per second, along the myelinated one - 5-120 m per second. This is explained by the fact that along the myelin fiber the impulse moves in somersaults (jumps). This means that within the nodal interception, the impulse moves along the neurolemma of the axial cylinder in the form of a depolarization wave, i.e., slowly; within the internodal segment, the impulse moves like an electric current, i.e., quickly. At the same time, the impulse along the unmyelinated fiber moves only in the form of a wave of depolarization.

The electron diffraction pattern clearly shows the difference between the myelinated fiber and the non-myelinated fiber - the mesaxon is screwed in layers onto the axial cylinder.

Regeneration of neurons. After damage, nerve cells cannot regenerate, however, after damage to the processes of nerve cells in the composition of nerve fibers, recovery occurs. When a nerve is damaged, the nerve fibers passing through it are torn. After the fiber breaks, 2 ends are formed in it - the end that is connected to the body of the neuron is called central; the end that is not connected to the nerve cell is called peripheral.

At the peripheral end, 2 processes occur: 1) degeneration and 2) regeneration. Initially, the process of degeneration occurs, which consists in the swelling of neurolemmocytes, the myelin layer dissolves, the axial cylinder is fragmented, drops (ovoids) are formed, consisting of myelin and a fragment of the axial cylinder. By the end of the 2nd week, resorption of the ovoids occurs, leaving only the neurilemma of the fiber sheath. Neurolemmocytes continue to multiply, ribbons (strands) are formed from them.

After resorption of the ovoids, the axial cylinder of the central end thickens and a growth flask is formed, which begins to grow, sliding along the ribbons of neurolemmocytes. By this time, a neuroglial-connective tissue scar is formed between the broken ends of the nerve fibers, which is an obstacle to the advancement of the growth flask. Therefore, not all axial cylinders can pass to the opposite side of the resulting scar. Consequently, after damage to the nerves, the innervation of organs or tissues is not fully restored. Meanwhile, part of the axial cylinders, equipped with growth flasks, makes its way to the opposite side of the neuroglial scar, immersed in the strands of neurolemmocytes. Then the mesaxon wraps around these axial cylinders, forming the myelin sheath of the nerve fiber. In the place where the nerve ending is located, the growth of the axial cylinder stops, the ending terminals and all its components are formed.

Neuron- structural and functional unit of the nervous system, is an electrically excitable cell that processes and transmits information through electrical and chemical signals.

neuron development.

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

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

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



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

A nerve cell - a neuron - is a structural and functional unit of the nervous system. A neuron is a cell capable of perceiving irritation, becoming excited, generating nerve impulses and transmitting them to other cells. The neuron consists of a body and processes - short, branching (dendrites) and long (axon). Impulses always move along the dendrites towards the cell, and along the axon - away from the cell.

Types of neurons

Neurons that transmit impulses to the central nervous system (CNS) are called sensory or afferent. motor, or efferent, neurons transmit impulses from the CNS to effectors, such as muscles. Those and other neurons can communicate with each other using intercalary neurons (interneurons). The last neurons are also called contact or intermediate.

Depending on the number and location of processes, neurons are divided into unipolar, bipolar and multipolar.

The structure of a neuron

A nerve cell (neuron) is made up of body (pericarion) with a kernel and several processes(Fig. 33).

Pericarion is the metabolic center in which most of the synthetic processes take place, in particular, the synthesis of acetylcholine. The cell body contains ribosomes, microtubules (neurotubules) and other organelles. Neurons are formed from neuroblast cells that do not yet have outgrowths. Cytoplasmic processes depart from the body of the nerve cell, the number of which may be different.

short branching processes, conducting impulses to the cell body, are called dendrites. Thin and long processes that conduct impulses from the perikaryon to other cells or peripheral organs are called axons. When axons regrow during the formation of nerve cells from neuroblasts, the ability of nerve cells to divide is lost.

The terminal sections of the axon are capable of neurosecretion. Their thin branches with swellings at the ends are connected to neighboring neurons in special places - synapses. The swollen endings contain small vesicles filled with acetylcholine, which plays the role of a neurotransmitter. There are vesicles and mitochondria (Fig. 34). Branched outgrowths of nerve cells permeate the entire body of the animal and form a complex system of connections. At synapses, excitation is transmitted from neuron to neuron or to muscle cells. Material from the site http://doklad-referat.ru

Functions of neurons

The main function of neurons is the exchange of information (nerve signals) between parts of the body. Neurons are susceptible to stimulation, that is, they are able to be excited (generate excitation), conduct excitations and, finally, transmit it to other cells (nerve, muscle, glandular). Electrical impulses pass through neurons, and this makes communication possible between receptors (cells or organs that perceive stimulation) and effectors (tissues or organs that respond to stimulation, such as muscles).

Nerve cells or neurons are electrically excitable cells that process and transmit information using electrical impulses. These signals are transmitted between neurons through synapses. Neurons can communicate with each other in neural networks. Neurons are the main material of the brain and spinal cord of the human central nervous system, as well as ganglia of the human peripheral nervous system.

Neurons come in several types depending on their functions:

  • Sensory neurons that respond to stimuli such as light, sound, touch, and other stimuli that affect sensory cells.
  • Motor neurons that send signals to muscles.
  • Interneurons that connect one neuron to another in the brain, spinal cord, or neural networks.

A typical neuron consists of a cell body ( catfish), dendrites and axon. Dendrites are thin structures extending from the cell body, they have reusable branching and are several hundred micrometers in size. The axon, which in its myelinated form is also called a nerve fiber, is a specialized cellular extension originating from the cell body from a place called the axon hillock (tubercle), extending up to one meter. Often, nerve fibers are bundled into bundles and into the peripheral nervous system, forming nerve threads.

The cytoplasmic part of the cell containing the nucleus is called the cell body or soma. Usually, the body of each cell has dimensions from 4 to 100 microns in diameter, it can be of various shapes: spindle-shaped, pear-shaped, pyramidal, and also much less often star-shaped. The body of the nerve cell contains a large spherical central nucleus with many Nissl granules with a cytoplasmic matrix (neuroplasm). Nissl granules contain ribonucleoprotein and take part in protein synthesis. Neuroplasm also contains mitochondria and Golgi bodies, melanin and lipochromic pigment granules. The number of these cell organelles depends on the functional characteristics of the cell. It should be noted that the cell body exists with a non-functional centrosome, which does not allow neurons to divide. That is why the number of neurons in an adult is equal to the number of neurons at birth. Along the entire length of the axon and dendrites, there are fragile cytoplasmic filaments called neurofibrils, originating from the cell body. The cell body and its appendages are surrounded by a thin membrane called the neural membrane. The cell bodies described above are present in the gray matter of the brain and spinal cord.

Short cytoplasmic appendages of the cell body that receive impulses from other neurons are called dendrites. Dendrites conduct nerve impulses to the cell body. Dendrites have an initial thickness of 5 to 10 microns, but gradually their thickness decreases and they continue with abundant branching. Dendrites receive an impulse from the axon of a neighboring neuron through the synapse and conduct the impulse to the cell body, which is why they are called receptive organs.

A long cytoplasmic appendage of the cell body that transmits impulses from the cell body to the neighboring neuron is called an axon. The axon is much larger than the dendrites. The axon originates at the conical height of the cell body, called the axon hillock, devoid of Nissl granules. The length of the axon is variable and depends on the functional connection of the neuron. The axon cytoplasm or axoplasm contains neurofibrils, mitochondria, but there are no Nissl granules in it. The membrane that covers the axon is called the axolemma. The axon can produce processes called accessory along its direction, and towards the end of the axon has an intense branching, ending in a brush, its last part has an increase to form a bulb. Axons are present in the white matter of the central and peripheral nervous systems. Nerve fibers (axons) are covered by a thin, lipid-rich membrane called the myelin sheath. The myelin sheath is formed by Schwann cells that cover the nerve fibers. The part of the axon not covered by the myelin sheath is a knot of adjacent myelinated segments called the node of Ranvier. The function of an axon is to transmit an impulse from the cell body of one neuron to the dendron of another neuron through the synapse. Neurons are specifically designed to transmit intercellular signals. The diversity of neurons is associated with the functions they perform; the size of the soma of neurons varies from 4 to 100 microns in diameter. The soma nucleus has dimensions from 3 to 18 microns. The dendrites of a neuron are cellular appendages that form entire dendritic branches.

The axon is the thinnest structure of the neuron, but its length can exceed the diameter of the soma by hundreds or thousands of times. The axon carries nerve signals from the soma. The place where the axon exits the soma is called the axon hillock. The length of the axons can be different and in some parts of the body reach a length of more than 1 meter (for example, from the base of the spine to the tip of the toe).

There are some structural differences between axons and dendrites. Thus, typical axons almost never contain ribosomes, with the exception of some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes that decrease with distance from the cell body.

The human brain has a very large number of synapses. Thus, each of the 100 billion neurons contains an average of 7,000 synaptic connections with other neurons. It has been established that the brain of a three-year-old child has about 1 quadrillion synapses. The number of these synapses decreases with age and stabilizes in adults. An adult has between 100 and 500 trillion synapses. According to research, the human brain contains about 100 billion neurons and 100 trillion synapses.

Types of neurons

Neurons come in several shapes and sizes and are classified according to their morphology and function. For example, the anatomist Camillo Golgi divided neurons into two groups. To the first group, he attributed neurons with long axons, which transmit signals over long distances. To the second group, he attributed neurons with short axons, which could be confused with dendrites.

Neurons are classified according to their structure into the following groups:

  • Unipolar. The axon and dendrites emerge from the same appendage.
  • Bipolar. The axon and a single dendrite are located on opposite sides of the soma.
  • Multipolar. At least two dendrites are located separately from the axon.
  • Golgi type I. The neuron has a long axon.
  • Golgi type II. Neurons with axons located locally.
  • Anaxon neurons. When the axon is indistinguishable from the dendrites.
  • basket cages- interneurons that form densely woven endings throughout the soma of target cells. Present in the cerebral cortex and cerebellum.
  • Betz cells. They are large motor neurons.
  • Lugaro cells- interneurons of the cerebellum.
  • Medium spiky neurons. Present in the striatum.
  • Purkinje cells. They are large multipolar neurons of the cerebellum of the Golgi type I.
  • pyramidal cells. Neurons with a triangular soma of the Golgi II type.
  • Renshaw Cells. Neurons connected at both ends to alpha motor neurons.
  • Unipolar racemose cells. Interneurons that have unique dendritic endings in the form of a brush.
  • Cells of the anterior horn. They are motor neurons located in the spinal cord.
  • Spindle cages. Interneurons connecting distant regions of the brain.
  • Afferent neurons. Neurons that transmit signals from tissues and organs to the central nervous system.
  • Efferent neurons. Neurons that transmit signals from the central nervous system to effector cells.
  • interneurons that connect neurons in specific areas of the central nervous system.

Action of neurons

All neurons are electrically excitable and maintain voltage across their membranes by metabolically conductive ion pumps coupled with ion channels that are embedded in the membrane to generate ion differentials such as sodium, chloride, calcium, and potassium. Voltage changes in the cross-membrane lead to a change in the functions of voltage-dependent ionic feces. When the voltage changes at a sufficiently high level, the electrochemical impulse causes the generation of an active potential, which quickly moves along the cells of the axon, activating synaptic connections with other cells.

Most nerve cells are the basic type. A certain stimulus causes an electrical discharge in the cell, a discharge similar to that of a capacitor. This produces an electrical impulse of about 50-70 millivolts, which is called the active potential. An electrical impulse propagates along the fiber, along the axons. The pulse propagation speed depends on the fiber, it is about tens of meters per second on average, which is noticeably lower than the propagation speed of electricity, which is equal to the speed of light. As soon as the impulse reaches the axon bundle, it is transmitted to neighboring nerve cells under the action of a chemical mediator.

A neuron acts on other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect of a postsynaptic neuron is determined not by the presynaptic neuron or neurotransmitter, but by the type of receptor that is activated. The neurotransmitter is like a key, and the receptor is a lock. In this case, one key can be used to open "locks" of various types. Receptors, in turn, are classified into excitatory (increasing the rate of transmission), inhibitory (slowing down the rate of transmission) and modulating (causing long-term effects).

Communication between neurons is carried out through synapses, in this place is the end of the axon (axon terminal). Neurons such as Purkinje cells in the cerebellum can have over a thousand dendritic junctions, communicating with tens of thousands of other neurons. Other neurons (the large neuronal cells of the supraoptic nucleus) have only one or two dendrites, each receiving thousands of synapses. Synapses can be either excitatory or inhibitory. Some neurons communicate with each other through electrical synapses, which are direct electrical connections between cells.

In a chemical synapse, when the action potential reaches the axon, a voltage opens in the calcium channel, which allows calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to penetrate the membrane, releasing the contents into the synaptic cleft. There is a process of diffusion of mediators through the synaptic cleft, which in turn activate receptors on the postsynaptic neuron. In addition, highly cytosolic calcium in the axon terminal induces mitochondrial calcium uptake, which in turn activates mitochondrial energy metabolism to produce ATP, which maintains continuous neurotransmission.

1) always alone;

2) from one to several;

3) from two to several;

4) always several.

How many dendrites can one neuron have?

1) always alone;

2) from one to several;

3) from two to several;

4) always several.

8. Small thickenings on the surface of dendrites, which are supposedly places of synaptic contacts, are called:

1) axons;

2) microtubules;

3) spines;

4) dendritic tubercles.

9. Neurons of this type transmit information in the direction from the periphery to the central nervous system:

1) afferent;

2) efferent;

3) insertion;

4) brake.

10. Neurons of this type transmit information in the direction from the central nervous system to the periphery:

1) afferent;

2) efferent;

3) insertion;

4) brake.

11. Neurons of this type transmit information within the nervous system from one department to another:

1) afferent;

2) efferent;

3) insertion;

4) brake.

12. Nissl substance (tigroid) is:

1) stained elements of the neuron cytoskeleton;

2) stained Golgi complex;

3) colored granular EPS;

4) stained hyaloplasm.

13. Neurons with only one process, according to their structure, are:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

Neurons with closely spaced axons

and dendrite, as a result of which the impression of having only one process is visually created, according to the structure they are:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

15. Neurons of this type have one axon and one dendrite located at different poles of the cell:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

16. Neurons of this type have many processes:

1) unipolar;

2) pseudo-unipolar;

3) bipolar;

4) multipolar.

17. Specify the type of glial cells that resemble a star in shape, and their processes form “legs” that surround the outer surface of the blood capillaries of the nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

18. This type of glial cells forms myelin in the central nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

19. Specify the cells that form the myelin sheath in the peripheral nervous system:

1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

20. These phagocytic cells are small in size, their main function is protective:



1) astrocytes;

2) oligodendrogliocytes;

3) microgliocytes;

4) Schwann cells.

21. Indicate the function that is primarily characteristic of astrocytes:

2) myelin formation;

3) phagocytosis;

4) formation of cerebrospinal fluid.

22. Indicate the function that is primarily characteristic of Schwann cells:

1) trophic supply and support of neurons;

2) myelin formation;

3) phagocytosis;

4) the formation of cerebrospinal fluid.

23. Specify the function that is primarily characteristic of microglial cells:

1) trophic supply and support of neurons;

2) myelin formation;

3) phagocytosis;

4) the formation of cerebrospinal fluid.

24. Specify the function that is primarily characteristic of ependymal glia cells:

1) trophic supply and support of neurons;

2) myelin formation;

3) phagocytosis;

4) participation in the formation of cerebrospinal fluid.

25. As a rule, the larger the diameter of the nerve fiber, the faster the conduction of excitation through it:

3) diameter doesn't matter.

As a rule, the smaller the diameter of the nerve fiber, the faster the conduction of excitation.

by it:

3) diameter doesn't matter.

27. The spread of excitation along the unmyelinated nerve fiber goes:

1) saltatory;

2) continuously.

28. The spread of excitation along the myelinated nerve fiber goes:

1) saltatory;

2) continuously.

29. A small section of the exposed nerve fiber membrane between two adjacent myelin-forming cells is called:

1) Schmidt-Langhans notch;

2) the interception of Ranvier;

3) the Kuiper belt;

4) tight contact.

Which processes of a neuron undergo myelination?

1) only axons;

2) only dendrites;

3) both axons and dendrites.

What law does the following formulation refer to: "Excitation along the nerve fiber spreads in both directions from the place of its origin"?

1) the law of bilateral excitation;



2) the law of isolated conduction of excitation;

3) the law of strength-duration;

4) Pfluger's law.

32. What law does the following wording refer to:

« As part of a nerve, excitation propagates along the nerve fiber without passing

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