Neurocytes (neurons) are able to perceive, analyze irritation, come into a state of excitement, generate nerve impulses, transmit them to other neurons or working organs. The number of neurons in human nervous tissue reaches one trillion.

Neuron classifications

It is carried out according to three main groups of traits: morphological, functional and biochemical.

1. Morphological classification of neurons(by structural features). By the number of processes neurons are divided into unipolar(with one process), bipolar ( with two branches ) , pseudo-unipolar(falsely unipolar), multipolar(have three or more processes). (Figure 8-2). The latter are the most in the nervous system.

Rice. 8-2. Types of nerve cells.

1. Unipolar neuron.

2. Pseudo-unipolar neuron.

3. Bipolar neuron.

4. Multipolar neuron.

In the cytoplasm of neurons, neurofibrils are visible.

(According to Yu.A. Afanasyev and others).

Pseudo-unipolar neurons are called because, moving away from the body, the axon and dendrite first fit tightly to each other, giving the impression of one process, and only then diverge in a T-shape (these include all receptor neurons of the spinal and cranial ganglia). Unipolar neurons are found only in embryogenesis. Bipolar neurons are bipolar cells of the retina, spiral and vestibular ganglia. By form up to 80 variants of neurons are described: stellate, pyramidal, pear-shaped, fusiform, arachnid, etc.

2. Functional(depending on the function performed and the place in the reflex arc): receptor, effector, intercalary and secretory. Receptor(sensitive, afferent) neurons with the help of dendrites perceive the effects of the external or internal environment, generate a nerve impulse and transmit it to other types of neurons. They are found only in the spinal ganglia and sensory nuclei of the cranial nerves. Effective(efferent) neurons transmit excitation to the working organs (muscles or glands). They are located in the anterior horns of the spinal cord and autonomic nerve ganglia. Interlocking(associative) neurons are located between receptor and effector neurons; by their number most of all, especially in the central nervous system. Secretory neurons(neurosecretory cells) is specialized neurons that function like endocrine cells... They synthesize and secrete neurohormones into the blood, located in the hypothalamic region of the brain. They regulate the activity of the pituitary gland, and through it, and many peripheral endocrine glands.

3. Mediator(according to the chemical nature of the released mediator):

- cholinergic neurons (acetylcholine mediator);

- aminergic (mediators - biogenic amines, for example norepinephrine, serotonin, histamine);

- GABAergic (mediator - gamma-aminobutyric acid);

- aminoergic (mediators - amino acids such as glutamine, glycine, aspartate);

- peptidergic (mediators - peptides, for example, opioid peptides, substance P, cholecystokinin, etc.);

- purinergic (mediators - purine nucleotides, for example adenine), etc.

Internal structure of neurons

Core neuron is usually large, round, with fine chromatin, 1-3 large nucleoli. This reflects the high intensity of transcription processes in the neuron nucleus.

Cell membrane neuron is able to generate and conduct electrical impulses. This is achieved by changing the local permeability of its ion channels for Na + and K +, changing the electrical potential and quickly moving it along the cytolemma (depolarization wave, nerve impulse).

All general-purpose organelles are well developed in the cytoplasm of neurons. Mitochondria are numerous and provide high energy requirements of the neuron associated with the significant activity of synthetic processes, the conduction of nerve impulses, the operation of ion pumps. They are characterized by fast wear and tear (Figure 8-3). Golgi complex very well developed. It is no coincidence that this organelle was first described and demonstrated in the course of cytology in neurons. With light microscopy, it is revealed in the form of rings, threads, grains located around the nucleus (dictyosome). Numerous lysosomes provide constant intensive destruction of the worn components of the cytoplasm of the neuron (autophagy).

R
is. 8-3. Ultrastructural organization of the neuron body.

D. Dendrites. A. Axon.

1. Nucleus (the nucleolus is shown by the arrow).

2. Mitochondria.

3. Golgi complex.

4. Chromatophilic substance (areas of the granular cytoplasmic reticulum).

5. Lysosomes.

6. Axon mound.

7. Neurotubules, neurofilaments.

(According to V.L.Bykov).

For normal functioning and renewal of neuron structures, the protein-synthesizing apparatus must be well developed in them (Fig. 8-3). Granular cytoplasmic reticulum forms clusters in the cytoplasm of neurons, which are well stained with basic dyes and are visible under light microscopy in the form of lumps chromatophilic substance(basophilic, or tiger substance, Nissl's substance). The term "Nissl's substance" is preserved in honor of the scientist Franz Nissl, who first described it. Lumps of chromatophilic substance are located in the perikaryons of neurons and dendrites, but never occur in axons, where the protein synthesizing apparatus is poorly developed (Fig. 8-3). With prolonged irritation or damage to the neuron, these accumulations of the granular cytoplasmic reticulum disintegrate into separate elements, which at the light-optical level is manifested by the disappearance of Nissl's substance ( chromatolysis, tigrolysis).

Cytoskeleton neurons are well developed, forms a three-dimensional network, represented by neurofilaments (6-10 nm thick) and neurotubules (20-30 nm in diameter). Neurofilaments and neurotubules are connected to each other by transverse bridges, when fixed, they stick together into bundles 0.5-0.3 μm thick, which are stained with silver salts. At the light-optical level, they are described under the name neurofibrill. They form a network in the perikarya of neurocytes, and lie parallel in the processes (Fig. 8-2). The cytoskeleton maintains the shape of cells, and also provides a transport function - it is involved in the transport of substances from the perikaryon to the processes (axonal transport).

Inclusions in the cytoplasm of a neuron are represented by lipid drops, granules lipofuscin- "aging pigment" - yellow-brown lipoprotein nature. They are residual bodies (telolysosomes) with products of undigested neuron structures. Apparently, lipofuscin can also accumulate at a young age, with intensive functioning and damage of neurons. In addition, there are pigment inclusions in the cytoplasm of the neurons of the substantia nigra and the blue spot of the brain stem. melanin... Inclusions are found in many neurons of the brain glycogen.

Neurons are not capable of division, and with age their number gradually decreases due to natural death. In degenerative diseases (Alzheimer's, Huntington's, Parkinson's disease), the intensity of apoptosis increases and the number of neurons in certain parts of the nervous system decreases sharply.

The structure of the main divisions of neurons

Like other cells, neurons are composed of cytoplasm and nucleus. The neuron secretes perikaryon or the cell body (part of the cytoplasm around the nucleus), appendages and nerve endings (terminal branches)... The sizes of perikarions vary from 4 µm in cerebellar granule cells to 130 µm in ganglionic neurons of the cerebral cortex. The processes can be up to 1 m in length (for example, the processes of the neurons of the spinal cord and spinal nodes reach the tips of the fingers and toes (Fig. 8-1).

Rice. 8-1. General principles of neuron structure. 1. The body of the neuron. 2. Axon. 3. Dendrites. 4. Interception of Ranvier. 5. Nervous ending. (After Stevens, 1979).

The processes of neurons are divided into two types: axons (neurites) and dendrites. There is always one axon in a nerve cell; it removes a nerve impulse from the body of a neuron and transmits it to other neurons or cells of working organs (muscles, glands). There are one or more dendrites (from the Greek dendron - tree) in the nerve cell, they bring impulses to the body of the neuron. Dendrites increase the receptor, perceiving surface of the neuron thousands of times (Figure 8-1).

A neuron is an independent structural and functional unit, but with the help of its processes it interacts with other neurons, forming reflex arcs- the neural circuits from which the nervous system is built.

In the human body, a nerve impulse is transmitted from one neuron to another, or to a working organ not directly, but through a chemical mediator - mediator.

In the nervous system of animals and humans, about a hundred different mediators have been found, and, accordingly, neurons of various mediator nature.

Axonal and dendritic transport

Axonal transport

Axonal transport (axotoc) is the movement of substances from the body of the neuron to the processes ( anterograde axotoc) and in the opposite direction ( retrograde axotoc). Distinguish slow axonal flow of substances (1-5 mm per day) and quick(up to 1-5 m per day). Both transport systems are present in both axons and dendrites.

Axonal transport ensures the unity of the neuron. It creates a permanent connection between the body of the neuron (trophic center) and processes. The main synthetic processes take place in the perikaryon. The organelles necessary for this are concentrated here. In the shoots, synthetic processes are weak.

Anterograde fast system transports to the nerve endings proteins and organelles necessary for synaptic functions (mitochondria, membrane fragments, vesicles, enzyme proteins involved in the exchange of neurotransmitters, as well as neurotransmitter precursors). Retrograde system returns used and damaged membranes and proteins to the perikarion for degradation in lysosomes and renewal, brings information about the state of the periphery, nerve growth factors.

Slow transport Is an anterograde system that conducts proteins and other substances to renew the axoplasm of mature neurons and ensure the growth of processes during their development and regeneration.

Retrograde transport can play a role in pathology. Due to it, neurotropic viruses (herpes, rabies, poliomyelitis) can move from the periphery to the central nervous system.

These processes extend from opposite ends of the cell, and it usually has a fusiform shape (see Fig.).

They are often found in specialized sensory organs - the retina of the eye, the olfactory epithelium and bulb, the auditory and vestibular ganglia. Bipolar cells are involved, in particular, in the transmission of impulses from sensory cells to the central parts of the analyzers. One typical example of bipolar neurons is retinal bipolar cells. Sensitive neurons of the spinal ganglia of vertebrates are also bipolar at certain stages of embryonic development (later they turn into pseudo-unipolar neurons).

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Neurons (Gray matter) Afferent Nerve / Sensory neuron Efferent Nerve / Motor neuron Synapse Sensory receptor Neuroglia Myelin (White matter) Connective tissue

Excerpt Characterizing Bipolar Neurons

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It is assumed that the human CNS consists of approximately 10 "neurons. Their shapes and sizes are varied, but all neurons have some common structural features (Fig. 1.1). The external structure of a neuron is the soma (body) and processes: axon and dendrites. The axon is a long process that conducts excitation from the cell body to other neurons or to peripheral organs. The axon departs from the soma at a place called the axonal hillock. For several tens of microns, the axon does not have a myelin sheath. This section of the axon, together with the axonal hillock, is called the initial segment.

Scheme 1. Departments of the nervous system

Further, the axon can be covered with a myelin sheath. The myelin sheath consists of a protein-lipid complex - myelin and is formed as a result of multiple wrapping of the axon by Schwann cells (a type of oligodendroglial cells).

Along the course of the myelin sheath, there are nodal interceptions of Ranvier, corresponding to the boundaries between the Schwann cells. The myelin sheath performs an insulating, supporting, barrier and, apparently, trophic and transport functions. The speed of impulse conduction in myelinated (pulp) fibers is higher than in unmyelinated (non-fleshy) fibers, since the propagation of a nerve impulse in them occurs abruptly from interception to interception, where the extracellular fluid is in direct contact with the axon membrane. The evolutionary meaning of the myelin sheath is to save the metabolic energy of the neuron. The pulp fibers are part of the sensory and motor nerves that supply the sensory organs and skeletal muscles, and belong mainly to the sympathetic division of the autonomic nervous system.

Rice. 1.1.

Spinal cord motor neuron. The functions of individual structural elements of the neuron are indicated (according to R. Eckert, D. Randall,

J. Augustine, 1991)

Short processes (dendrites) of the neuron branch out around the cell body. Their function is to perceive nerve impulses coming from other neurons, and the subsequent conduction of excitation to the soma. The bodies of neurons (somas) in the central nervous system are concentrated in the gray matter of the cerebral hemispheres, in the subcortical nuclei, in the brain stem, in the cerebellum and in the spinal cord. Non-fleshy fibers innervate the muscles, they are also part of the autonomic nervous system. Myelinated fibers form the white matter of various parts of the spinal cord and brain. The shape and size of the bodies of neurons and their processes, even in the same parts of the central nervous system, can differ significantly. Thus, the diameter of the grain cells of the cerebral cortex does not exceed 4 microns, and the diameter of the giant pyramidal cells in the cerebral cortex or in the anterior horns of the spinal cord can range from 50 to 100 microns or more.

The course, length and branching of the processes of nerve cells are also very variable. Thus, the axons of most cells have branching only at the level of the initial segment (axon collateral) and at the end when approaching another cell or an innervated organ. For the most part, they do not branch, in contrast to dendrites, which branch very intensively and mainly closer to the cell body. The length of the axons of various cells can be measured both in microns (in the gray matter of the cerebral hemispheres) and tens of centimeters (in the pathways of the spinal cord).

The morphological classification of neurons takes into account the number of processes in neurons and subdivides all neurons into the following types (Fig. 1.2):

• unipolar neurons have one process; noted in humans during early embryonic development, and in postnatal ontogenesis they are found only in the mesencephalic nucleus of the trigeminal nerve, providing proprioceptive sensitivity of the masticatory muscles;
• bipolar neurons have two processes (axon and dendrite), usually extending from different poles of the cell. In humans, this type of neuron is usually found in the peripheral parts of the auditory, visual and olfactory sensory systems (bipolar cells of the spiral ganglion, retina). Bipolar cells are associated with a receptor by a dendrite, and an axon with a neuron of an overlying level. A variety of bipolar neurons are pseudo-unipolar neurons. The axon and dendrite of these cells extend from the soma in the form of a T-shaped outgrowth, which is further divided into two processes. One of them (dendrite) is directed to the receptors, and the second (axon) to the central nervous system. This type of cells is noted in the sensory spinal and cranial ganglia and provides the perception of temperature, proprioceptive, pain, tactile, baroreceptive and vibration sensitivity;
• multipolar neurons have one axon and more than two dendrites. They are widespread in the human nervous system.

According to their functions, the cells of the central nervous system are divided into afferent(sensitive), efferent(effector), intercalary(intermediate) neurons.

Rice. 1.2. Types of neurons depending on the number of processes: 1 - unipolar; 2 - bipolar; 3 - multipolar;

4 - pseudo-unipolar

The soma of afferent neurons has a simple rounded shape with one process, which is divided into two fibers in a T-shape. One fiber goes to the periphery and forms there sensitive endings (in the skin, muscles, tendons), the second goes to the central nervous system (to the centers of the spinal cord or brain stem), where it branches into endings that end on other cells. The peripheral process is most likely a modified dendrite, and the process that is directed to the central nervous system is an axon. The soma of a sensitive neuron is located outside the central nervous system in the spinal ganglia or in the ganglia of the cranial nerves. Sensitive neurons include some neurons in the central nervous system, which receive impulses not directly from receptors, but through other neurons located below, an example is the neurons of the optic hillock.

The structure of efferent neurons is similar to the structure of afferent ones. However, through their axons, excitation is carried out to the periphery. Those of the efferent neurons that form the motor nerve fibers that go to the skeletal muscles are called motoneurons. Their bodies lie in the medulla oblongata, in the anterior horns of the spinal cord. Many efferent neurons transmit excitation not directly to the periphery, but through the cells located below. For example, efferent neurons of the cerebral hemispheres or the red nucleus of the midbrain, whose impulses go to the motor neurons of the spinal cord.

Insertion (intermediate) neurons are a special type of neurons. Their main difference from afferent and efferent neurons is that they are located inside the central nervous system and their processes do not leave its limits. These neurons do not establish direct communication with sensory or effector structures. They seem to be inserted between the sensory and motor cells and unite them together, sometimes through very long switching chains. The variety of their shapes and sizes is great, but in general, their structure corresponds to the structure of afferent and efferent neurons. The differences are mainly determined by the shape of the soma, as well as the length and degree of branching of the processes. Some classifications include up to 10 or more types of intercalary neurons. According to these characteristics, pyramidal, stellate, basket-like, fusiform, polymorphic neurons, grain cells, etc. are distinguished.

Morphological polarization of neurons (dendrite - soma - axon) is associated with their functional polarization. It manifests itself in the fact that only the axon of the cell has structures on its branches designed to transfer activity to other cells. There are no such structures on the surface of the soma and dendrites. Therefore, in a system of neurons connected to each other, excitation is transmitted only in one direction through the processes of their neurons.

The axons of each neuron, approaching other nerve cells, branch out, forming numerous endings on the dendrites of these cells, on their bodies and on the terminal branches - the germinals of the axons. On the body of a large pyramidal cell of the cerebral cortex, up to a thousand nerve endings formed by the nerve processes of other neurons can be located, and one nerve fiber can form up to 10 thousand such contacts on many nerve cells. Using the method of electron microscopy, the researchers studied in detail the areas of communication between nerve cells (intercellular contacts), which were called synapses (synaptic connections) by C. Sherrington in 1897.