Histologic structure of the nervous system

The nervous system has a complex histological structure. It consists of nerve cells (neurons) with their processes (fibers), neuroglia and connective tissue elements. The basic structural and functional unit of the nervous system is the neuron (neurocyte). Depending on the number of processes extending from the cell body, there are 3 types of neurons - multipolar, bipolar and unipolar. Most neurons in the central nervous system are bipolar cells with one axon and a large number of dichotomously branching dendrites. A more detailed classification takes into account the features of the shape (pyramidal, spindle-shaped, basket-shaped, stellate) and size - from very small to giant [for example, the length of gigantic pyramidal neurons (Betz cells) in the motor zone of the cortex is 4-120 μm]. The total number of such neurons in the cortex of both hemispheres of the brain alone reaches 10 billion.

Bipolar cells, which have an axon and one dendrite, are also quite common in various parts of the CNS. Such cells are characteristic of the visual, auditory and olfactory systems - specialized sensory systems.

Unipolar (pseudounipolar) cells are found much less frequently. They are located in the mesencephalic nucleus of the trigeminal nerve and in the spinal ganglia (ganglia of the posterior roots and sensory cranial nerves). These cells provide certain types of sensitivity - pain, temperature, tactile, as well as a sense of pressure, vibration, stereognosis and perception of the distance between the places of two point touches on the skin (two-dimensional spatial sense). Such cells, although called unipolar, actually have 2 processes (axon and dendrite), which merge near the cell body. Cells of this type are characterized by the presence of a unique, very dense internal capsule of glial elements (satellite cells), through which the cytoplasmic processes of ganglion cells pass. The outer capsule around the satellite cells is formed by connective tissue elements. True unipolar cells are found only in the mesencephalic nucleus of the trigeminal nerve, which conducts proprioceptive impulses from the masticatory muscles to the cells of the thalamus.

The function of dendrites is to conduct impulses toward the cell body (afferent, cellulopetal) from its receptive areas. In general, the cell body, including the axon hillock, can be considered part of the receptive area of the neuron, since the axon endings of other cells form synaptic contacts on these structures in the same way as on the dendrites. The surface of dendrites receiving information from the axons of other cells is significantly increased by small outgrowths (typicon).

The axon conducts impulses efferently - from the cell body and dendrites. When describing the axon and dendrites, we proceed from the possibility of conducting impulses in only one direction - the so-called law of dynamic polarization of the neuron. Unilateral conduction is characteristic only of synapses. Along the nerve fiber, impulses can spread in both directions. In stained sections of nervous tissue, the axon is recognized by the absence of tigroid substance in it, whereas in dendrites, at least in their initial part, it is revealed.

The cell body (perikaryon), with the participation of its RNA, performs the function of a trophic center. It may not have a regulating effect on the direction of impulse movement.

Nerve cells have the ability to perceive, conduct and transmit nerve impulses. They synthesize mediators involved in their conduction (neurotransmitters): acetylcholine, catecholamines, as well as lipids, carbohydrates and proteins. Some specialized nerve cells have the ability to neurocrinia (synthesize protein products - octapeptides, for example, antidiuretic hormone, vasopressin, oxytocin in the rivets of the supraoptic and paraventricular nuclei of the hypothalamus). Other neurons, which are part of the basal sections of the hypothalamus, produce so-called releasing factors that affect the function of the adenohypophysis.

All neurons are characterized by a high metabolic rate, so they need a constant supply of oxygen, glucose and other substances.

The body of a nerve cell has its own structural features, which are determined by the specificity of its function.

The neuron's body, in addition to the outer shell, has a three-layer cytoplasmic membrane consisting of two layers of phospholipids and proteins. The membrane performs a barrier function, protecting the cell from the entry of foreign substances, and a transport function, ensuring the entry of substances necessary for its vital activity into the cell. A distinction is made between passive and active transport of substances and ions through the membrane.

Passive transport is the transfer of substances in the direction of decreasing electrochemical potential along the concentration gradient (free diffusion through the lipid bilayer, facilitated diffusion - transport of substances through the membrane).

Active transport is the transfer of substances against the gradient of electrochemical potential using ion pumps. Cytosis is also distinguished - a mechanism for the transfer of substances through the cell membrane, which is accompanied by reversible changes in the membrane structure. Not only is the entry and exit of substances regulated through the plasma membrane, but information is also exchanged between the cell and the extracellular environment. Nerve cell membranes contain many receptors, the activation of which leads to an increase in the intracellular concentration of cyclic adenosine monophosphate (nAMP) and cyclic guanosine monophosphate (nGMP), which regulate cellular metabolism.

The nucleus of a neuron is the largest of the cellular structures visible with light microscopy. In most neurons, the nucleus is located in the center of the cell body. The cell plasma contains chromatin granules, which are a complex of deoxyribonucleic acid (DNA) with simple proteins (histones), non-histone proteins (nucleoproteins), protamines, lipids, etc. Chromosomes become visible only during mitosis. In the center of the nucleus is the nucleolus, which contains a significant amount of RNA and proteins; ribosomal RNA (rRNA) is formed in it.

The genetic information contained in the chromatin DNA is transcribed into messenger RNA (mRNA). Then the mRNA molecules penetrate the pores of the nuclear membrane and enter the ribosomes and polyribosomes of the granular endoplasmic reticulum. There, protein molecules are synthesized; amino acids brought by special transfer RNA (tRNA) are used. This process is called translation. Some substances (cAMP, hormones, etc.) can increase the rate of transcription and translation.

The nuclear membrane consists of two membranes - internal and external. The pores through which the exchange between the nucleoplasm and cytoplasm takes place occupy 10% of the surface of the nuclear membrane. In addition, the external nuclear membrane forms protrusions from which the strands of the endoplasmic reticulum with ribosomes attached to them (granular reticulum) arise. The nuclear membrane and the membrane of the endoplasmic reticulum are morphologically close to each other.

In the bodies and large dendrites of nerve cells, clumps of basophilic substance (Nissl substance) are clearly visible under light microscopy. Electron microscopy revealed that basophilic substance is a part of the cytoplasm saturated with flattened cisterns of the granular endoplasmic reticulum containing numerous free and membrane-attached ribosomes and polyribosomes. The abundance of rRNA in ribosomes determines the basophilic staining of this part of the cytoplasm, visible under light microscopy. Therefore, basophilic substance is identified with the granular endoplasmic reticulum (ribosomes containing rRNA). The size of clumps of basophilic granularity and their distribution in neurons of different types are different. This depends on the state of the impulse activity of neurons. In large motor neurons, clumps of basophilic substance are large and the cisterns are compactly located in it. In the granular endoplasmic reticulum, new cytoplasmic proteins are continuously synthesized in ribosomes containing rRNA. These proteins include proteins involved in the construction and restoration of cell membranes, metabolic enzymes, specific proteins involved in synaptic conduction, and enzymes that inactivate this process. Newly synthesized proteins in the neuron cytoplasm enter the axon (and also the dendrites) to replace the spent proteins.

If the axon of a nerve cell is cut not too close to the perikaryon (so as not to cause irreversible damage), then redistribution, reduction and temporary disappearance of basophilic substance (chromatolysis) occur and the nucleus moves to the side. During axon regeneration in the body of the neuron, movement of basophilic substance towards the axon is observed, the amount of granular endoplasmic reticulum and mitochondria increases, protein synthesis increases and processes may appear at the proximal end of the cut axon.

The lamellar complex (Golgi apparatus) is a system of intracellular membranes, each of which is a series of flattened cisterns and secretory vesicles. This system of cytoplasmic membranes is called the agranular reticulum due to the absence of ribosomes attached to its cisterns and vesicles. The lamellar complex is involved in the transport of certain substances from the cell, in particular proteins and polysaccharides. A significant portion of the proteins synthesized in the ribosomes on the membranes of the granular endoplasmic reticulum, upon entering the lamellar complex, are converted into glycoproteins, which are packaged into secretory vesicles and then released into the extracellular environment. This indicates the presence of a close connection between the lamellar complex and the membranes of the granular endoplasmic reticulum.

Neurofilaments can be found in most large neurons, where they are located in the basophilic substance, as well as in myelinated axons and dendrites. Neurofilaments are structurally fibrillar proteins with an unclear function.

Neurotubules are visible only with electron microscopy. Their role is to maintain the shape of the neuron, especially its processes, and participate in the axoplasmic transport of substances along the axon.

Lysosomes are vesicles bounded by a simple membrane and providing phagocytosis of the cell. They contain a set of hydrolytic enzymes capable of hydrolyzing substances that have entered the cell. In the event of cell death, the lysosomal membrane ruptures and autolysis begins - hydrolases released into the cytoplasm break down proteins, nucleic acids and polysaccharides. A normally functioning cell is reliably protected by the lysosomal membrane from the action of hydrolases contained in lysosomes.

Mitochondria are structures in which oxidative phosphorylation enzymes are localized. Mitochondria have external and internal membranes and are located throughout the cytoplasm of the neuron, forming clusters in the terminal synaptic extensions. They are a kind of energy stations of cells in which adenosine triphosphate (ATP) is synthesized - the main source of energy in a living organism. Thanks to mitochondria, the process of cellular respiration is carried out in the body. The components of the tissue respiratory chain, as well as the ATP synthesis system, are localized in the internal membrane of the mitochondria.

Among other various cytoplasmic inclusions (vacuoles, glycogen, crystalloids, iron-containing granules, etc.) there are also some pigments of black or dark brown color, similar to melanin (in the cells of the substantia nigra, blue spot, dorsal motor nucleus of the vagus nerve, etc.). The role of pigments has not been fully clarified. However, it is known that a decrease in the number of pigmented cells in the substantia nigra is associated with a decrease in the content of dopamine in its cells and the caudate nucleus, which leads to parkinsonism syndrome.

The axons of nerve cells are enclosed in a lipoprotein sheath that begins at some distance from the cell body and ends at a distance of 2 µm from the synaptic terminal. The sheath is located outside the axon's boundary membrane (axolemma). Like the cell body sheath, it consists of two electron-dense layers separated by a less electron-dense layer. Nerve fibers surrounded by such lipoprotein sheaths are called myelinated.With light microscopy it was not always possible to see such an "insulating" layer around many peripheral nerve fibers, which for this reason were classified as unmyelinated (non-myelinated). However, electron microscopic studies have shown that these fibers are also enclosed in a thin myelin (lipoprotein) sheath (thinly myelinated fibers).

Myelin sheaths contain cholesterol, phospholipids, some cerebrosides and fatty acids, as well as protein substances intertwined in the form of a network (neurokeratin). The chemical nature of the myelin of peripheral nerve fibers and the myelin of the central nervous system is somewhat different. This is due to the fact that in the central nervous system myelin is formed by oligodendroglia cells, and in the peripheral nervous system - by lemmocytes. These two types of myelin also have different antigenic properties, which is revealed in the infectious-allergic nature of the disease. The myelin sheaths of nerve fibers are not continuous, but are interrupted along the fiber by gaps called interceptions of the node (interceptions of Ranvier). Such interceptions exist in nerve fibers of both the central and peripheral nervous systems, although their structure and periodicity in different parts of the nervous system are different. The branches of the nerve fiber usually depart from the site of the interception of the node, which corresponds to the place of closure of two lemmocytes. At the end of the myelin sheath at the level of the node interception, a slight narrowing of the axon is observed, the diameter of which decreases by 1/3.

Myelination of the peripheral nerve fiber is carried out by lemmocytes. These cells form an outgrowth of the cytoplasmic membrane, which spirally wraps the nerve fiber. Up to 100 spiral layers of myelin of regular structure can be formed. In the process of wrapping around the axon, the cytoplasm of the lemmocyte is displaced toward its nucleus; this ensures the convergence and close contact of adjacent membranes. Electron microscopically, the myelin of the formed sheath consists of dense plates about 0.25 nm thick, which are repeated in the radial direction with a period of 1.2 nm. Between them there is a light zone, divided into two by a less dense intermediate plate of irregular outline. The light zone is a highly water-saturated space between the two components of the bimolecular lipid layer. This space is available for the circulation of ions. The so-called "non-myelinated" fibers of the autonomic nervous system are covered by a single spiral of the lemmocyte membrane.

The myelin sheath provides isolated, non-decremental (without a drop in potential amplitude) and faster conduction of excitation along the nerve fiber. There is a direct relationship between the thickness of this sheath and the speed of impulse conduction. Fibers with a thick myelin layer conduct impulses at a speed of 70-140 m/s, while conductors with a thin myelin sheath at a speed of about 1 m/s and even slower 0.3-0.5 m/s - "non-myelin" fibers.

The myelin sheaths around axons in the central nervous system are also multilayered and formed by processes of oligodendrocytes. The mechanism of their development in the central nervous system is similar to the formation of myelin sheaths in the periphery.

The cytoplasm of the axon (axoplasm) contains many filiform mitochondria, axoplasmic vesicles, neurofilaments, and neurotubules. Ribosomes are very rare in the axoplasm. Granular endoplasmic reticulum is absent. This leads to the fact that the neuron body supplies the axon with proteins; therefore, glycoproteins and a number of macromolecular substances, as well as some organelles such as mitochondria and various vesicles, must move along the axon from the cell body.

This process is called axonal, or axoplasmic, transport.

Certain cytoplasmic proteins and organelles move along the axon in several streams with different speeds. Antegrade transport moves at two speeds: a slow stream goes along the axon at a speed of 1-6 mm/day (lysosomes and some enzymes necessary for the synthesis of neurotransmitters in the endings of axons move this way), and a fast stream from the cell body at a speed of about 400 mm/day (this stream transports components necessary for synaptic function - glycoproteins, phospholipids, mitochondria, dopamine hydroxylase for the synthesis of adrenaline). There is also a retrograde movement of axoplasm. Its speed is about 200 mm/day. It is maintained by contraction of surrounding tissues, pulsation of adjacent vessels (this is a kind of axon massage) and blood circulation. The presence of retrograde axo transport allows some viruses to enter the bodies of neurons along the axon (for example, the tick-borne encephalitis virus from the site of a tick bite).

Dendrites are usually much shorter than axons. Unlike axons, dendrites branch dichotomously. In the CNS, dendrites do not have a myelin sheath. Large dendrites also differ from axons in that they contain ribosomes and cisterns of granular endoplasmic reticulum (basophilic substance); there are also many neurotubules, neurofilaments, and mitochondria. Thus, dendrites have the same set of organelles as the body of a nerve cell. The surface of dendrites is significantly increased by small outgrowths (spines), which serve as sites of synpaptic contact.

The parenchyma of brain tissue includes not only nerve cells (neurons) and their processes, but also neuroglia and elements of the vascular system.

Nerve cells connect to each other only by contact - a synapse (Greek synapsis - touching, grasping, connecting). Synapses can be classified by their location on the surface of the postsynaptic neuron. A distinction is made between: axodendritic synapses - the axon ends on the dendrite; axosamatic synapses - contact is formed between the axon and the neuron body; axo-axonal - contact is established between axons. In this case, the axon can form a synapse only on the unmyelinated part of another axon. This is possible either in the proximal part of the axon or in the area of the terminal button of the axon, since in these places the myelin sheath is absent. There are also other types of synapses: dendro-dendritic and dendrosomatic. Approximately half of the entire surface of the neuron body and almost the entire surface of its dendrites are dotted with synaptic contacts from other neurons. However, not all synapses transmit nerve impulses. Some of them inhibit the reactions of the neuron with which they are connected (inhibitory synapses), while others, located on the same neuron, excite it (excitatory synapses). The combined effect of both types of synapses on one neuron leads at any given moment to a balance between the two opposite types of synaptic effects. Excitatory and inhibitory synapses are structured identically. Their opposite action is explained by the release of different chemical neurotransmitters in the synaptic endings, which have different abilities to change the permeability of the synaptic membrane for potassium, sodium, and chlorine ions. In addition, excitatory synapses more often form axodendritic contacts, while inhibitory synapses form axosomatic and axo-axonal contacts.

The part of the neuron through which impulses enter the synapse is called the presynaptic terminal, and the part that receives the impulses is called the postsynaptic terminal. The cytoplasm of the presynaptic terminal contains many mitochondria and synaptic vesicles containing neurotransmitter. The axolemma of the presynaptic part of the axon, which is closest to the postsynaptic neuron, forms the presynaptic membrane in the synapse. The part of the plasma membrane of the postsynaptic neuron that is closest to the presynaptic membrane is called the postsynaptic membrane. The intercellular space between the pre- and postsynaptic membranes is called the synaptic cleft.

The structure of neuron bodies and their processes is very diverse and depends on their functions. There are receptor (sensory, vegetative), effector (motor, vegetative) and combinational (associative) neurons. Reflex arcs are built from a chain of such neurons. Each reflex is based on the perception of stimuli, their processing and transfer to the responding organ-executor. The set of neurons necessary for the implementation of a reflex is called a reflex arc. Its structure can be both simple and very complex, including both afferent and efferent systems.

Afferent systems are ascending conductors of the spinal cord and brain that conduct impulses from all tissues and organs. The system, including specific receptors, conductors from them and their projections in the cerebral cortex, is defined as an analyzer. It performs the functions of analysis and synthesis of stimuli, i.e. the primary decomposition of the whole into parts, units and then the gradual addition of the whole from units, elements.

Efferent systems originate from many parts of the brain: the cerebral cortex, subcortical ganglia, subthalamic region, cerebellum, and brainstem structures (in particular, from those parts of the reticular formation that influence the segmental apparatus of the spinal cord). Numerous descending conductors from these brain structures approach the neurons of the segmental apparatus of the spinal cord and then proceed to the executive organs: striated muscles, endocrine glands, vessels, internal organs, and skin.

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