Neurons are divided into receptor, effector and intercalary.

The complexity and diversity of the functions of the nervous system are determined by the interaction between neurons. This interaction is a set of different signals transmitted between neurons or muscles and glands. Signals are emitted and propagated by ions. Ions generate an electrical charge (action potential) that moves through the body of the neuron.

Of great importance to science was the invention of the Golgi method in 1873, which allowed individual neurons to be stained. The term "neuron" (German Neuron) to refer to nerve cells was introduced by G. W. Waldeyer in 1891.

The structure of neurons

cell body

The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), bounded externally by a membrane of lipid bilayer. Lipids are composed of hydrophilic heads and hydrophobic tails. Lipids are arranged in hydrophobic tails to each other, forming a hydrophobic layer. This layer allows only fat-soluble substances (eg oxygen and carbon dioxide) to pass through. There are proteins on the membrane: in the form of globules on the surface, on which outgrowths of polysaccharides (glycocalix) can be observed, due to which the cell perceives external irritation, and integral proteins penetrating the membrane through, in which there are ion channels.

The neuron consists of a body with a diameter of 3 to 130 microns. The body contains a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). The cytoskeleton of a neuron consists of fibrils of different diameters: Microtubules (D = 20-30 nm) - consist of the protein tubulin and stretch from the neuron along the axon, up to the nerve endings. Neurofilaments (D = 10 nm) - together with microtubules provide intracellular transport of substances. Microfilaments (D = 5 nm) - consist of actin and myosin proteins, they are especially pronounced in growing nerve processes and in neuroglia. ( neuroglia, or just glia (from other Greek. νεῦρον - fiber, nerve + γλία - glue), - a set of auxiliary cells of the nervous tissue. It makes up about 40% of the volume of the CNS. The number of glial cells in the brain is approximately equal to the number of neurons).

In the body of the neuron, a developed synthetic apparatus is revealed, the granular endoplasmic reticulum of the neuron stains basophilically and is known as the "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon. Neurons differ in shape, number of processes and functions. Depending on the function, sensitive, effector (motor, secretory) and intercalary are distinguished. Sensory neurons perceive stimuli, convert them into nerve impulses and transmit them to the brain. Effector (from lat. effectus - action) - they develop and send commands to the working bodies. Intercalary - carry out a connection between sensory and motor neurons, participate in information processing and command generation.

A distinction is made between anterograde (away from the body) and retrograde (towards the body) axon transport.

Dendrites and axon

Action Potential Creation and Conduction Mechanism

In 1937, John Zachary Jr. determined that the squid giant axon could be used to study the electrical properties of axons. Squid axons were chosen because they are much larger than human ones. If you insert an electrode inside the axon, you can measure its membrane potential.

The axon membrane contains voltage-gated ion channels. They allow the axon to generate and conduct electrical signals through its body called action potentials. These signals are generated and propagated by electrically charged sodium (Na +), potassium (K +), chlorine (Cl -), calcium (Ca 2+) ions.

Pressure, stretch, chemical factors, or a change in membrane potential can activate a neuron. This happens due to the opening of ion channels that allow ions to cross the cell membrane and, accordingly, change the membrane potential.

Thin axons use less energy and metabolic substances to conduct an action potential, but thick axons allow it to be conducted faster.

In order to conduct action potentials more quickly and less energy-intensive, neurons can use special glial cells to coat axons called oligodendrocytes in the CNS or Schwann cells in the peripheral nervous system. These cells do not completely cover the axons, leaving gaps on the axons open to extracellular material. In these intervals, an increased density of ion channels. They are called intercepts of Ranvier. Through them, the action potential passes through the electric field between the gaps.

Classification

Structural classification

Based on the number and arrangement of dendrites and axons, neurons are divided into non-axonal, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.

Afferent neurons(sensitive, sensory, receptor or centripetal). Neurons of this type include primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free endings.

Efferent neurons(effector, motor, motor or centrifugal). Neurons of this type include final neurons - ultimatum and penultimate - not ultimatum.

Associative neurons(intercalary or interneurons) - a group of neurons communicates between efferent and afferent.

• unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain;
• pseudo-unipolar cells grouped near the spinal cord in the intervertebral ganglia;
• bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
• multipolar neurons (have one axon and several dendrites), predominant in the CNS.

Development and growth of a neuron

The issue of neuronal division is currently debatable. According to one version, the neuron develops from a small precursor cell, which stops dividing even before it releases its processes. The axon begins to grow first, and the dendrites form later. A thickening appears at the end of the developing process of the nerve cell, which 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 microspines 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. 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.

Microtubules and neurofilaments are elongated mainly by 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 growth cone advance rate is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs at its far end during the growth of the neuron process. New membrane material is added at the end. The growth cone is an area of ​​rapid exocytosis and endocytosis, as evidenced by the many vesicles found 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 synthesized in the body of the neuron is 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.

Properties and functions of neurons

Properties:

• The presence of a transmembrane potential difference(up to 90 mV), the outer surface is electropositive with respect to the inner surface.
• Very high sensitivity to certain chemicals and electrical current.
• The ability to neurosecrete, that is, to the synthesis and release of special substances (neurotransmitters) into the environment or the synaptic cleft.

• High power consumption, a high level of energy processes, which necessitates a constant supply of the main energy sources - glucose and oxygen, necessary for oxidation.

Functions:

• receiving function(synapses are contact points, we receive information in the form of an impulse from receptors and neurons).
• Integrative function(information processing, as a result, a signal is formed at the output of the neuron, carrying the information of all the summed signals).
• Conductor function(from the neuron along the axon there is information in the form of an electric current to the synapse).
• Transfer function(a nerve impulse, having reached the end of the axon, which is already part of the structure of the synapse, causes the release of a mediator - a direct transmitter of excitation to another neuron or executive organ).

The structure of the neuron, its properties.

Neurons are excitable cells of the nervous system. Unlike glial cells, they are able to be excited (generate action potentials) and conduct excitation. Neurons are highly specialized cells and do not divide during life.

In a neuron, a body (soma) and processes are distinguished. The soma of a neuron has a nucleus and cellular organelles. The main function of the soma is to carry out cell metabolism.

Fig.3. The structure of a neuron. 1 - soma (body) of the neuron; 2 - dendrite; 3 - the body of the Schwan cell; 4 - myelinated axon; 5 - axon collateral; 6 - axon terminal; 7 - axon mound; 8 - synapses on the body of a neuron

Number processes neurons are different, but according to their structure and function they are divided into two types.

1. Some are short, strongly branching processes, which are called dendrites(from dendro- tree, branch). A nerve cell carries from one to many dendrites. The main function of dendrites is to collect information from many other neurons. A child is born with a limited number of dendrites (interneuronal connections), and the increase in brain mass that occurs at the stages of postnatal development is realized due to an increase in the mass of dendrites and glial elements.

2. Another type of processes of nerve cells are axons. The axon in the neuron is one and is a more or less long process, branching only at the end farthest from the soma. These branches of the axon are called axon terminals (terminals). The place of the neuron from which the axon starts has a special functional significance and is called axon hillock. Here, an action potential is generated - a specific electrical response of an excited nerve cell. The function of the axon is to conduct the nerve impulse to the axon terminals. Along the course of the axon, branches can form.

Part of the axons of the central nervous system is covered with a special electrically insulating substance - myelin . Axon myelination is carried out by cells glia . In the central nervous system, this role is performed by oligodendrocytes, in the peripheral nervous system - Schwann cells, which are a type of oligodendrocytes. The oligodendrocyte wraps around the axon, forming a multilayer sheath. Myelination is not subjected to the area of ​​the axon hillock and axon terminal. The cytoplasm of the glial cell is squeezed out of the intermembrane space during the "wrapping" process. Thus, the axon myelin sheath consists of densely packed, interspersed lipid and protein membrane layers. The axon is not completely covered with myelin. There are regular breaks in the myelin sheath - interceptions of Ranvier . The width of such interception is from 0.5 to 2.5 microns. The function of interceptions of Ranvier is the rapid hopping propagation of action potentials, which occurs without attenuation.

In the central nervous system, the axons of various neurons heading towards the same structure form ordered bundles - pathways. In such a conducting bundle, axons are guided in a "parallel course" and often one glial cell forms a sheath for several axons. Since myelin is a white substance, the pathways of the nervous system, consisting of densely lying myelinated axons, form white matter brain. AT gray matter brain cell bodies, dendrites and unmyelinated parts of axons are localized.

Fig. 4. The structure of the myelin sheath 1 - the connection between the body of the glial cell and the myelin sheath; 2 - oligodendrocyte; 3 - scallop; 4 - plasma membrane; 5 - cytoplasm of an oligodendrocyte; 6 - neuron axon; 7 - interception of Ranvier; 8 - mesaxon; 9 - loop of the plasma membrane

It is very difficult to reveal the configuration of an individual neuron because they are densely packed. All neurons are usually divided into several types depending on the number and shape of processes extending from their bodies. There are three types of neurons: unipolar, bipolar and multipolar.

Rice. 5. Types of neurons. a - sensory neurons: 1 - bipolar; 2 - pseudo-bipolar; 3 - pseudo-unipolar; b - motor neurons: 4 - pyramidal cell; 5 - motor neurons of the spinal cord; 6 - neuron of the double nucleus; 7 - neuron of the nucleus of the hypoglossal nerve; c - sympathetic neurons: 8 - neuron of the stellate ganglion; 9 - neuron of the superior cervical ganglion; 10 - neuron of the lateral horn of the spinal cord; d - parasympathetic neurons: 11 - neuron of the node of the muscular plexus of the intestinal wall; 12 - neuron of the dorsal nucleus of the vagus nerve; 13 - ciliary node neuron

Unipolar cells. Cells, from the body of which only one process departs. In fact, when leaving the soma, this process is divided into two: an axon and a dendrite. Therefore, it is more correct to call them pseudo-unipolar neurons. These cells are characterized by a certain localization. They belong to non-specific sensory modalities (pain, temperature, tactile, proprioceptive).

bipolar cells are cells that have one axon and one dendrite. They are characteristic of the visual, auditory, olfactory sensory systems.

Multipolar cells have one axon and many dendrites. Most neurons of the CNS belong to this type of neurons.

Based on the shape of these cells, they are divided into spindle-shaped, basket-shaped, stellate, pyramidal. Only in the cerebral cortex there are up to 60 variants of the forms of neuron bodies.

Information about the shape of neurons, their location and the direction of the processes is very important, because they allow us to understand the quality and quantity of connections coming to them (the structure of the dendritic tree), and the points to which they send their processes.

This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure. There are over one hundred billion neurons in the human body.

Review

The complexity and diversity of the functions of the nervous system are determined by the interaction between neurons, which, in turn, are a set of different signals transmitted as part of the interaction of neurons with other neurons or muscles and glands. Signals are emitted and propagated by ions, which generate an electrical charge that travels along the neuron.

Structure

The neuron consists of a body with a diameter of 3 to 130 microns, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as processes. There are two types of processes: dendrites and. The neuron has a developed and complex cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). The cytoskeleton of a neuron consists of fibrils of different diameters: Microtubules (D = 20-30 nm) - consist of the protein tubulin and stretch from the neuron along the axon, up to the nerve endings. Neurofilaments (D = 10 nm) - together with microtubules provide intracellular transport of substances. Microfilaments (D = 5 nm) - consist of actin and myosin proteins, are especially pronounced in growing nerve processes and in. In the body of the neuron, a developed synthetic apparatus is revealed, the granular ER of the neuron stains basophilically and is known as the "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon.

A distinction is made between anterograde (away from the body) and retrograde (towards the body) axon transport.

Dendrites and axon

An axon is usually a long process adapted to conduct from the body of a neuron. Dendrites are, as a rule, short and highly branched processes that serve as the main site for the formation of excitatory and inhibitory synapses that affect the neuron (different neurons have a different ratio of the length of the axon and dendrites). A neuron may have several dendrites and usually only one axon. One neuron can have connections with many (up to 20 thousand) other neurons.

Dendrites divide dichotomously, while axons give rise to collaterals. The branch nodes usually contain mitochondria.

Dendrites do not have a myelin sheath, but axons can. The place of generation of excitation in most neurons is the axon hillock - a formation at the place where the axon leaves the body. In all neurons, this zone is called the trigger zone.

Synapse(Greek σύναψις, from συνάπτειν - hug, embrace, shake hands) - the place of contact between two neurons or between a neuron and the effector cell receiving the signal. Serves for transmission between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. Some synapses cause neuron depolarization, others hyperpolarization; the former are excitatory, the latter are inhibitory. Usually, to excite a neuron, stimulation from several excitatory synapses is necessary.

The term was introduced in 1897 by the English physiologist Charles Sherrington.

Classification

Structural classification

Based on the number and arrangement of dendrites and axons, neurons are divided into non-axonal, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.

Axonless neurons- small cells, grouped close in the intervertebral ganglia, having no anatomical signs of division of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of axonless neurons is poorly understood.

Unipolar neurons- neurons with one process, are present, for example, in the sensory nucleus of the trigeminal nerve in.

bipolar neurons- neurons with one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia.

Multipolar neurons- Neurons with one axon and several dendrites. This type of nerve cells predominates in.

Pseudo-unipolar neurons- are unique in their kind. One process departs from the body, which immediately divides in a T-shape. This entire single tract is covered with a myelin sheath and structurally represents an axon, although along one of the branches, excitation goes not from, but to the body of the neuron. Structurally, dendrites are ramifications at the end of this (peripheral) process. The trigger zone is the beginning of this branching (that is, it is located outside the cell body). Such neurons are found in the spinal ganglia.

Functional classification

By position in the reflex arc, afferent neurons (sensitive neurons), efferent neurons (some of them are called motor neurons, sometimes this is not a very accurate name applies to the entire group of efferents) and interneurons (intercalary neurons) are distinguished.

Afferent neurons(sensitive, sensory or receptor). Neurons of this type include primary cells and pseudo-unipolar cells, in which dendrites have free endings.

Efferent neurons(effector, motor or motor). Neurons of this type include final neurons - ultimatum and penultimate - not ultimatum.

Associative neurons(intercalary or interneurons) - a group of neurons communicates between efferent and afferent, they are divided into intrusion, commissural and projection.

secretory neurons- neurons that secrete highly active substances (neurohormones). They have a well-developed Golgi complex, the axon ends in axovasal synapses.

Morphological classification

The morphological structure of neurons is diverse. In this regard, when classifying neurons, several principles are used:

• take into account the size and shape of the body of the neuron;
• the number and nature of branching processes;
• the length of the neuron and the presence of specialized membranes.

According to the shape of the cell, neurons can be spherical, granular, stellate, pyramidal, pear-shaped, fusiform, irregular, etc. The size of the neuron body varies from 5 microns in small granular cells to 120-150 microns in giant pyramidal neurons. The length of a human neuron ranges from 150 microns to 120 cm.

According to the number of processes, the following morphological types of neurons are distinguished:

• unipolar (with one process) neurocytes present, for example, in the sensory nucleus of the trigeminal nerve in;
• pseudo-unipolar cells grouped nearby in the intervertebral ganglia;
• bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
• multipolar neurons (have one axon and several dendrites), predominant in the CNS.

Development and growth of a neuron

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 microspines 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 found in the body of the 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.

Neuron is the basic structural and functional unit of the nervous system. A neuron is a nerve cell with processes (color. Table III, BUT). It distinguishes cell body, or soma, one long, slightly branching process - axon and many (from 1 to 1000) short, strongly branching processes - dendrites. The length of the axon reaches a meter or more, its diameter ranges from hundredths of a micron (µm) to 10 µm; the length of the dendrite can reach 300 microns, and its diameter - 5 microns.

The axon, leaving the soma of the cell, gradually narrows, separate processes depart from it - collaterals. During the first 50-100 microns from the cell body, the axon is not covered by the myelin sheath. The part of the cell body adjacent to it is called axon hillock. The part of the axon that is not covered by the myelin sheath, together with the axon hillock, is called the initial segment of the axon.These areas differ in a number of morphological and functional features.

Through dendrites, excitation comes from receptors or other neurons to the cell body, and the axon transmits excitation from one neuron to another or working organ. The dendrites have lateral processes (spikes) that increase their surface and are the places of greatest contact with other neurons. The end of the axon branches strongly, one axon can contact 5 thousand nerve cells and create up to 10 thousand contacts (Fig. 26, BUT).

The point of contact of one neuron with another is called synapse(from the Greek word "synapto" - to contact). In appearance, synapses are shaped like buttons, bulbs, loops, etc.

The number of synaptic contacts is not the same on the body and processes of the neuron and is very variable in different parts of the central nervous system. The body of a neuron is 38% covered with synapses, and there are up to 1200-1800 synapses per neuron. There are many synapses on dendrites and spines, their number is small on the axon hillock.

All neurons central nervous system connect with each other basically in one direction: the axon ramifications of one neuron are in contact with the cell body and dendrites of another neuron.

The body of a nerve cell in different parts of the nervous system has a different size (its diameter ranges from 4 to 130 microns) and shape (rounded, flattened, polygonal, oval). It is covered with a complex membrane and contains organelles characteristic of any other cell: in the cytoplasm there is a nucleus with one or more nucleoli, mitochondria, ribosomes, the Golgi apparatus, the endoplasmic reticulum, etc.

characteristic feature structure of the nerve cell is the presence of granular reticulum with a large number of ribosomes and neurofibrils. Ribosomes in nerve cells are associated with a high level of metabolism, protein and RNA synthesis.

The nucleus contains genetic material - deoxyribonucleic acid (DNA), which regulates the composition of the RNA of the soma of the neuron. RNA, in turn, determines the amount and type of protein synthesized in the neuron.

neurofibrils are the thinnest fibers that cross the cell body in all directions (Fig. 26, B) and continuing into shoots.

Neurons are distinguished by structure and function. According to the structure (depending on the number of processes extending from the cell body), they are distinguished unipolar(with one branch), bipolar(with two processes) and multipolar(with many processes) neurons.

According to their functional properties, they distinguish afferent(or centripetal) neurons that carry impulses from receptors to the central nervous system efferent, motor, motoneurons(or centrifugal), transmitting excitation from the central nervous system to the innervated organ, and plug-in, contact or intermediate neurons connecting afferent and efferent pathways.

Afferent neurons are unipolar, their bodies lie in the spinal ganglia. The process extending from the cell body is divided in a T-shape into two branches, one of which goes to the central nervous system and performs the function of an axon, and the other approaches the receptors and is a long dendrite.

Most efferent and intercalary neurons are multipolar. Multipolar intercalary neurons are located in large numbers in the posterior horns of the spinal cord, and are also found in all other parts of the central nervous system. Οʜᴎ are also bipolar, such as retinal neurons, which have a short branching dendrite and a long axon. Motor neurons are located mainly in the anterior horns of the spinal cord.

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

1. Morphological classification of neurons(according to the features of the structure). By number of shoots neurons are divided into unipolar(with one branch), bipolar ( with two processes ) , pseudo-unipolar(false 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.

Neurofibrils are visible in the cytoplasm of neurons.

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

Pseudo-unipolar neurons are called because, moving away from the body, the axon and dendrite first fit tightly to each other, creating the impression of one process, and only then diverge in a T-shaped way (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 shape up to 80 variants of neurons have been 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. Effector(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. Insertion(associative) neurons are located between the receptor and effector neurons; by their number most, especially in the central nervous system. secretory neurons(neurosecretory cells) specialized neurons that function like endocrine cells. They synthesize and secrete neurohormones into the blood and are located in the hypothalamic region of the brain. They regulate the activity of the pituitary gland, and through it many peripheral endocrine glands.

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

Cholinergic neurons (mediator acetylcholine);

Aminergic (mediators - biogenic amines, such as norepinephrine, serotonin, histamine);

GABAergic (mediator - gamma-aminobutyric acid);

Amino acid-ergic (mediators - amino acids such as glutamine, glycine, aspartate);

Peptidergic (mediators - peptides, such as opioid peptides, substance P, cholecystokinin, etc.);

Purinergic (mediators - purine nucleotides, such as adenine), etc.

The internal structure of neurons

Core neurons are usually large, rounded, with finely dispersed chromatin, 1-3 large nucleoli. This reflects the high intensity of transcription processes in the neuron nucleus.

Cell wall A neuron is capable of generating and conducting 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).

In the cytoplasm of neurons, all general-purpose organelles are well developed. Mitochondria are numerous and provide high energy needs of the neuron, associated with a significant activity of synthetic processes, the conduction of nerve impulses, and the operation of ion pumps. They are characterized by rapid 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 detected in the form of rings, filaments, grains located around the nucleus (dictyosomes). Numerous lysosomes provide constant intensive destruction of wearable components of the neuron cytoplasm (autophagy).

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

D. Dendrites. A. Axon.

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

2. Mitochondria.

3. Golgi complex.

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

5. Lysosomes.

6. Axon hillock.

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 in the cytoplasm of neurons forms clusters that are well stained with basic dyes and are visible under light microscopy in the form of clumps chromatophilic substance(basophilic, or tiger substance, Nissl substance). The term "Nissl substance" has been preserved in honor of the scientist Franz Nissl, who first described it. Lumps of chromatophilic substance are located in the perikarya of neurons and dendrites, but are never found in axons, where the protein-synthesizing apparatus is poorly developed (Fig. 8-3). With prolonged stimulation or damage to a neuron, these accumulations of the granular cytoplasmic reticulum break up into separate elements, which at the light-optical level is manifested by the disappearance of the Nissl substance ( chromatolysis, tigrolysis).

cytoskeleton neurons is 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 neurofibrils. They form a network in the perikaryons of neurocytes, and in the processes they lie parallel (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 the neuron are represented by lipid drops, granules lipofuscin- "aging pigment" - yellow-brown color of lipoprotein nature. They are residual bodies (telolisosomes) with products of undigested neuron structures. Apparently, lipofuscin can also accumulate at a young age, with intensive functioning and damage to 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. Many neurons in the brain contain inclusions glycogen.

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