Topic 17. NERVE TISSUE

Structural and functional features of the nervous tissue:

1) consists of two main types of cells - neurocytes and neuroglia;

2) there is no intercellular substance;

3) nervous tissue is not divided into morphological subgroups;

4) the main source of origin is the neuroectoderm.

Structural components of the nervous tissue:

1) nerve cells (neurocytes or neurons);

2) glial cells - gliocytes.

Functions of nervous tissue:

1) perception of various stimuli and their transformation into nerve impulses;

2) conduction of nerve impulses, their processing and transmission to the working organs.

These functions are performed by neurocytes - the functionally leading structural components of the nervous tissue. Neuroglial cells contribute to the implementation of these functions.

Sources and stages of development of nervous tissue

The main source is the neuroectoderm. Some cells, glial cells, develop from microglia and from mesenchyme (from blood monocytes).

Stages of development:

1) neural plate;

2) neural groove;

3) neural tube, ganglion plate, neural placodes.

Nervous tissue develops from the neural tube, mainly from the organs of the central nervous system (spinal cord and brain). From the ganglion plate develops the nervous tissue of some organs of the peripheral nervous system (vegetative and spinal ganglia). Cranial nerve ganglia develop from neural placodes. In the process of development of the nervous tissue, two types of cells are first formed:

1) neuroblasts;

2) glioblasts.

Then, various types of neurocytes differentiate from neuroblasts, and various types of macroglial cells (ependymocytes, astrocytes, oligodendrocytes) differentiate from glioblasts.

Characterization of neurocytes

Morphologically, all differentiated neurocytes are process cells. Conventionally, two parts are distinguished in each nerve cell:

1) cell body (pericaryon);

2) processes.

The processes of neurocytes are divided into two types:

1) an axon (neurite), which conducts impulses from the cell body to other nerve cells or working organs;

2) a dendrite that conducts impulses to the cell body.

In any nerve cell there is only one axon, there can be one or more dendrites. The processes of nerve cells end with terminal devices of various types (effector, receptor, synaptic).

The structure of the perikaryon of a nerve cell. In the center, usually one nucleus is localized, containing mainly euchromatin, and 1–2 distinct nucleoli, which indicates a high functional stress of the cell.

The most developed organelles of the cytoplasm are the granular ER and the lamellar Golgi complex.

When staining neurocytes with basic dyes (according to the Nissl method), granular EPS is detected in the form of basophilic clumps (Nissl clumps), and the cytoplasm has a spotted appearance (the so-called tigroid substance).

The processes of nerve cells are elongated sections of nerve cells. They contain neuroplasm, as well as single mitochondria, neurofilaments and neurotubules. In the processes, there is a movement of neuroplasm from the perikaryon to the nerve endings (direct current), as well as from the terminals to the pericarinone (retrograde current). At the same time, direct fast transport (5–10 mm/h) and direct slow transport (1–3 mm/day) are distinguished in axons. Transport of substances in dendrites - 3 mm/h.

The most common method for detecting and studying nerve cells is the silver nitrate impregnation method.

Classification of neurocytes

Nerve cells are classified:

1) by morphology;

2) by function.

According to morphology, according to the number of processes, they are divided into:

1) unipolar (pseudo-unipolar) - with one process;

2) bipolar - with two processes;

3) multipolar - more than two processes.

By function, they are divided into:

1) afferent (sensitive);

2) efferent (motor, secretory);

3) associative (insert);

4) secretory (neuroendocrine).

Structural and functional characteristics of glial cells

Neuroglia cells are auxiliary cells of the nervous tissue and perform the following functions:

1) support;

2) trophic;

3) delimiting;

4) secretory;

5) protective, etc.

Glial cells in their morphology are also process cells, not identical in size, shape, and number of processes. On the basis of size, they are divided primarily into macroglia and microglia. In addition, macroglial cells have an ectodermal source of origin (from the neuroectoderm), microglial cells develop from the mesenchyme.

Ependymocytes have a strictly limited localization: they line the cavities of the central nervous system (central canal of the spinal cord, ventricles and cerebral aqueduct). In their morphology, they somewhat resemble epithelial tissue, since they form the lining of the brain cavities. Ependymocytes have an almost prismatic shape, and they distinguish between apical and basal poles. They are interconnected by their lateral surfaces by means of desmosomal junctions. On the apical surface of each epindimocyte there are cilia, due to the vibrations of which the movement of cerebrospinal fluid in the brain cavities is ensured.

Thus, ependymocytes perform the following functions of the nervous system:

1) delimiter (forming a lining of the brain cavities);

2) secretory;

3) mechanical (ensure the movement of cerebral fluid);

4) support (for neurocytes);

5) barrier (participate in the formation of the superficial glial boundary membrane).

Astrocytes are cells with numerous processes that together resemble the shape of a star, hence their name. According to the structural features of their processes, astrocytes are divided into:

1) protoplasmic (short, but wide and strongly branching processes);

2) fibrous (thin, long, slightly branching processes).

Protoplasmic astrocytes perform supporting and trophic functions for gray matter neurocytes.

Fibrous astrocytes carry out a supporting function for neurocytes and their processes, since their long, thin processes form glial fibers. In addition, the terminal extensions of the processes of fibrous astrocytes form perivascular (circumvascular) glial boundary membranes, which are one of the structural components of the blood-brain barrier.

Oligodendrocytes are small cells, the most common population of gliocytes. They are localized mainly in the peripheral nervous system and, depending on the area of ​​localization, are divided into:

1) mantle gliocytes (surround the bodies of nerve cells in the nerve and autonomic ganglia;

2) lemmocytes, or Schwann cells (surround the processes of nerve cells, together with which they form nerve fibers);

3) terminal gliocytes (accompany the terminal branching of the dendrites of sensitive nerve cells).

All varieties of oligodendrocytes, surrounding the bodies, processes and endings of nerve cells, perform supporting, trophic, and barrier functions for them, isolating nerve cells from lymphocytes.

The fact is that the antigens of nerve cells are foreign to their own lymphocytes. Therefore, nerve cells and their various parts are distinguished from blood lymphocytes and connective tissue:

1) perivascular boundary glial membranes;

2) superficial glial boundary membrane;

3) lemmocytes and terminal gliocytes (on the periphery).

When these barriers are violated, autoimmune reactions occur.

Microglia is represented by small process cells that perform a protective function - phagocytosis. Based on this, they are called glial macrophages. Most researchers believe that glial macrophages (like any other macrophages) are cells of mesenchymal origin.

Nerve fibers

Nerve fibers are not independent structural elements of the nervous tissue, but are complex formations that include the following elements:

1) processes of nerve cells (axial cylinders);

2) glial cells (lemmocytes, or Schwann cells);

3) connective tissue plate (knitting plate).

The main function of nerve fibers is to conduct nerve impulses. In this case, the processes of nerve cells (axial cylinders) conduct nerve impulses, and glial cells (lemmocytes) contribute to this conduction.

According to the structural features and function, nerve fibers are divided into two types:

1) unmyelinated;

2) myelin.

The structure and functional features of an unmyelinated nerve fiber. An unmyelinated nerve fiber is a chain of lemmocytes into which several (5–20) axial cylinders are pressed. Each axial cylinder bends the cytolemma of the lemmocyte and, as it were, sinks into its cytoplasm. In this case, the axial cylinder is surrounded by the cytolemma of the lemmocyte, and its contiguous areas constitute the mesaxon.

Mesaxone in unmyelinated nerve fibers does not play a significant functional role, but is an important structural and functional formation in the myelinated nerve fiber.

In their structure, unmyelinated nerve fibers are cable-type fibers. Despite this, they are thin (5 - 7 microns) and conduct nerve impulses very slowly (1 - 2 m / s).

The structure of the myelinated nerve fiber. The myelinated nerve fiber has the same structural components as the unmyelinated one, but differs in a number of features:

1) the axial cylinder is one and plunges into the central part of the lemmocyte chain;

2) the mesaxon is long and twisted around the axial cylinder, forming a myelin layer;

3) the cytoplasm and nucleus of lemmocytes are shifted to the periphery and constitute the neurolemma of the myelin nerve fiber;

4) the basal plate is located on the periphery.

On the cross section of the myelinated nerve fiber, the following structural elements are visible:

1) axial cylinder;

2) myelin layer;

3) neurolemma;

4) basal plate.

Since the basis of any cytolemma is the bilipid layer, the myelin sheath of the myelin nerve fiber (twisted mesaxon) is formed by layers of lipid layers, intensely stained black with osmic acid.

Along the course of the myelinated nerve fiber, the boundaries of neighboring lemmocytes are visible - nodal intercepts (Ranvier intercepts), as well as areas between two intercepts (internodal segments), each of which corresponds to the length of one lemmocyte. In each internodal segment, myelin notches are clearly visible - transparent areas that contain the cytoplasm of the lemmocyte between the turns of the mesaxon.

The high speed of conduction of nerve impulses along myelinated nerve fibers is explained by the saltatory method of conducting nerve impulses: jumps from one intercept to another.

The reaction of nerve fibers to rupture or intersection. After a rupture or intersection of a nerve fiber, the processes of degeneration and regeneration are carried out in it.

Since the nerve fiber is a combination of nerve and glial cells, after its damage, a reaction is noted (both in nerve and glial cells). After crossing, the most noticeable changes appear in the distal section of the nerve fiber, where the collapse of the axial cylinder is noted, i.e., degeneration of the part of the nerve cell cut off from the body. Lemmocytes surrounding this area of ​​the axial cylinder do not die, but round, proliferate and form a strand of glial cells along the disintegrated nerve fiber. At the same time, these glial cells phagocytize fragments of the disintegrated axial cylinder and its myelin sheath.

In the perikaryon of a nerve cell with a cut-off process, signs of irritation appear: swelling of the nucleus and its shift to the periphery of the cell, expansion of the perinuclear space, degranulation of the membranes of the granular ER, vacuolization of the cytoplasm, etc.

In the proximal part of the nerve fiber at the end of the axial cylinder, an expansion is formed - a growth flask, which gradually grows into the strand of glial cells at the site of the dead distal section of the same fiber. Glial cells surround the growing axial cylinder and gradually transform into lemmocytes. As a result of these processes, the regeneration of the nerve fiber occurs at a rate of 1–4 mm per day. The axial cylinder, growing up to the terminal gliocytes of the disintegrated nerve ending, branches and forms the terminal apparatus (motor or sensory ending) with the help of glial cells. As a result of the regeneration of the nerve fiber and nerve ending, the innervation of the damaged area (reinnervation) is restored, which leads to the restoration of its functions. It should be emphasized that a necessary condition for the regeneration of the nerve fiber is a clear comparison of the proximal and distal sections of the damaged nerve fiber. This is achieved by suturing the end of the cut nerve.

The concepts of "nerve fiber" and "nerve" should not be confused.

The nerve is a complex formation, consisting of:

1) nerve fibers;

2) loose fibrous connective tissue that forms the nerve sheath.

Among the sheaths of the nerve are distinguished:

1) endoneurium (connective tissue surrounding individual nerve fibers);

2) perineurium (connective tissue surrounding bundles of nerve fibers);

3) epineurium (connective tissue surrounding the nerve trunk).

In these membranes are blood vessels that provide trophism of nerve fibers.

Nerve endings (or terminal nerve apparatus). They are the endings of nerve fibers. If the axial cylinder of a nerve fiber is a dendrite of a sensitive nerve cell, then its terminal apparatus forms a receptor. If the axial cylinder is an axon of a nerve cell, then its terminal apparatus forms an effector or synaptic ending. Therefore, nerve endings are divided into three main groups:

1) effector (motor or secretory);

2) prescription (sensitive);

3) synaptic.

The motor nerve ending is the terminal apparatus of the axon on a striated muscle fiber or on a myocyte. A motor nerve ending on a striated muscle fiber is also called a motor plaque. It has three parts:

1) nerve pole;

2) synaptic cleft;

3) muscular pole.

Each terminal branch of the axon contains the following structural elements:

1) presynaptic membrane;

2) synaptic vesicles with a mediator (acetylcholine);

3) accumulation of mitochondria with longitudinal cristae.

The muscle pole (or motor plaque sheets) includes:

1) postsynaptic membrane - a specialized section of the myosymplast plasmolemma containing acetylcholine receptor proteins;

2) a section of the sarcoplasm of the myosymplast, which lacks myofibrils and contains an accumulation of nuclei and sarcosomes.

The synaptic cleft is a 50 nm space between pre- and postsynaptic membranes that contains the enzyme acetylcholinesterase.

Receptor endings (or receptors). They are specialized end devices of dendrites of sensory neurons, mainly pseudo-unipolar nerve cells of the spinal ganglia and cranial nerves, as well as some autonomic neurins (Dogel type II cells).

Receptor nerve endings are classified according to several criteria:

1) by localization:

a) interoroceptors (receptors of internal organs);

b) extrareceptors (perceive external stimuli: repeaters of the skin, sensory organs);

c) proprioceptors (localized in the apparatus of movement);

2) according to the specificity of perception (by modality):

a) chemoreceptors;

b) mechanoreceptors;

c) baroreceptors;

d) thermoreceptors (thermal, cold);

3) by structure:

a) free;

b) non-free (encapsulated, non-encapsulated).

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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