Neurons are capable. Do nerve cells recover

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 microspinules are 0.1 to 0.2 µm thick and can be up to 50 µm in length; the wide and flat area of the growth cone is about 5 µm wide and long, although its shape may vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others elongate, deviate in different directions, touch the substrate and can stick to it.

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

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 for science was the invention of the Golgi method in 1873, which made it possible to stain individual neurons. The term "neuron" (German Neuron) to refer to nerve cells was introduced by G. W. Waldeyer in 1891.

Encyclopedic YouTube

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✪ Structure of a neuron

Subtitles

Now we know how a nerve impulse is transmitted. Let everything begin with the excitation of dendrites, for example, this outgrowth of the body of a neuron. Excitation means opening the ion channels of the membrane. Through the channels, ions enter the cell or come out of the cell. This can lead to inhibition, but in our case, the ions act electrotonically. They change the electrical potential on the membrane, and this change in the region of the axon hillock may be enough to open sodium ion channels. Sodium ions enter the cell, the charge becomes positive. This opens potassium channels, but this positive charge activates the next sodium pump. Sodium ions re-enter the cell, thus the signal is transmitted further. The question is, what happens at the junction of neurons? We agreed that it all began with the excitation of the dendrites. As a rule, the source of excitation is another neuron. This axon will also transmit excitation to some other cell. It could be a muscle cell or another nerve cell. How? Here is the axon terminal. And here there may be a dendrite of another neuron. This is another neuron with its own axon. His dendrite is excited. How does this happen? How does the impulse from the axon of one neuron pass to the dendrite of another? Transmission from axon to axon, from dendrite to dendrite, or from axon to cell body is possible, but most often the impulse is transmitted from axon to neuron dendrites. Let's take a closer look. We are interested in what is happening in that part of the picture, which I will circle in a box. The axon terminal and dendrite of the next neuron fall into the frame. So here's the axon terminal. It looks something like this under magnification. This is the axon terminal. Here is its internal contents, and next to it is the dendrite of a neighboring neuron. This is what the dendrite of a neighboring neuron looks like under magnification. Here's what's inside the first neuron. The action potential moves across the membrane. Finally, somewhere on the axon terminal membrane, the intracellular potential becomes positive enough to open the sodium channel. Before the arrival of the action potential, it is closed. Here is the channel. It lets sodium ions into the cell. This is where it all starts. Potassium ions leave the cell, but as long as the positive charge remains, it can open other channels, not just sodium ones. There are calcium channels at the end of the axon. I'll paint pink. Here is the calcium channel. It is usually closed and does not allow divalent calcium ions to pass through. This is a voltage-gated channel. Like sodium channels, it opens when the intracellular potential becomes positive enough to let calcium ions into the cell. Divalent calcium ions enter the cell. And this moment is amazing. These are cations. There is a positive charge inside the cell due to sodium ions. How does calcium get there? The calcium concentration is created using an ion pump. I have already talked about the sodium-potassium pump, there is a similar pump for calcium ions. These are protein molecules embedded in the membrane. The membrane is phospholipid. It consists of two layers of phospholipids. Like this. It's more like a real cell membrane. Here the membrane is also two-layered. This is obvious, but I'll clarify just in case. Here, too, there are calcium pumps that function similarly to sodium-potassium pumps. The pump receives an ATP molecule and a calcium ion, splits off the phosphate group from ATP and changes its conformation, pushing calcium out. The pump is designed in such a way that it pumps calcium out of the cell. It consumes the energy of ATP and provides a high concentration of calcium ions outside the cell. At rest, the concentration of calcium outside is much higher. When an action potential is received, calcium channels open, and calcium ions from the outside enter the axon terminal. There, calcium ions bind to proteins. And now let's see what is actually happening in this place. I have already mentioned the word "synapse". The point of contact between the axon and the dendrite is the synapse. And there is a synapse. It can be considered a place where neurons connect to each other. This neuron is called presynaptic. I'll write it down. You need to know the terms. presynaptic. And this is postsynaptic. Postsynaptic. And the space between these axon and dendrite is called the synaptic cleft. synaptic cleft. It's a very, very narrow gap. Now we are talking about chemical synapses. Usually, when people talk about synapses, they mean chemical ones. There are also electric ones, but we won’t talk about them yet. Consider a conventional chemical synapse. In a chemical synapse, this distance is only 20 nanometers. The cell, on average, has a width of 10 to 100 microns. A micron is 10 to the minus sixth power of meters. It's 20 times 10 to the minus ninth power. This is a very narrow gap, if we compare its size with the size of the cell. There are vesicles inside the axon terminal of the presynaptic neuron. These vesicles are connected to the cell membrane from the inside. Here are the bubbles. They have their own lipid bilayer membrane. Bubbles are containers. There are many of them in this part of the cell. They contain molecules called neurotransmitters. I'll show them in green. Neurotransmitters inside the vesicles. I think this word is familiar to you. Many medications for depression and other mental health problems act specifically on neurotransmitters. Neurotransmitters Neurotransmitters within the vesicles. When voltage-gated calcium channels open, calcium ions enter the cell and bind to proteins that hold the vesicles. The vesicles are held on the presynaptic membrane, that is, this part of the membrane. They are retained by proteins of the SNARE group. Proteins of this family are responsible for membrane fusion. That's what these proteins are. Calcium ions bind to these proteins and change their conformation so that they pull the vesicles so close to the cell membrane that the vesicle membranes fuse with it. Let's look at this process in more detail. After calcium binds to SNARE family proteins on the cell membrane, they pull the vesicles closer to the presynaptic membrane. Here is the bubble. This is how the presynaptic membrane goes. Between themselves, they are connected by proteins of the SNARE family, which attracted the bubble to the membrane and are located here. The result was membrane fusion. This leads to the fact that neurotransmitters from the vesicles enter the synaptic cleft. This is how neurotransmitters are released into the synaptic cleft. This process is called exocytosis. Neurotransmitters leave the cytoplasm of the presynaptic neuron. You have probably heard their names: serotonin, dopamine, adrenaline, which is both a hormone and a neurotransmitter. Norepinephrine is both a hormone and a neurotransmitter. All of them are probably familiar to you. They enter the synaptic cleft and bind to the surface structures of the membrane of the postsynaptic neuron. postsynaptic neuron. Let's say they bind here, here, and here to specific proteins on the surface of the membrane, as a result of which ion channels are activated. Excitation occurs in this dendrite. Let's say the binding of neurotransmitters to the membrane leads to the opening of sodium channels. Membrane sodium channels open. They are transmitter dependent. Due to the opening of sodium channels, sodium ions enter the cell, and everything repeats again. An excess of positive ions appears in the cell, this electrotonic potential spreads to the region of the axon hillock, then to the next neuron, stimulating it. This is how it happens. It is possible otherwise. Suppose instead of opening sodium channels, potassium ion channels will open. In this case, potassium ions will go out along the concentration gradient. Potassium ions leave the cytoplasm. I will show them as triangles. Due to the loss of positively charged ions, the intracellular positive potential decreases, as a result of which the generation of an action potential in the cell is difficult. I hope this is understandable. We started with excitement. An action potential is generated, calcium enters, the contents of the vesicles enter the synaptic cleft, sodium channels open, and the neuron is stimulated. And if you open potassium channels, the neuron will slow down. Synapses are very, very, very many. There are trillions of them. The cerebral cortex alone is thought to contain between 100 and 500 trillion synapses. And that's just the bark! Each neuron is capable of forming many synapses. In this picture, synapses could be here, here, and here. Hundreds and thousands of synapses on every nerve cell. With one neuron, another, third, fourth. A huge number of connections ... huge. Now you see how complicated everything that has to do with the human mind is arranged. Hope you find it useful. Subtitles by the Amara.org community

The structure of neurons

cell body

The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), bounded on the outside 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 simply 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 is on average 10-50 times greater than that of neurons.)

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. 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 (Ca2+) 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 gaps, there is an increased density of ion channels. They are called intercepts Ranvier. Through them, the action potential passes through the electric field between the gaps.

Classification

Structural classification

Axonless neurons- small cells, grouped near the spinal cord in the intervertebral ganglia, which do not have anatomical signs of separation 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 the midbrain. Many morphologists believe that unipolar neurons are not found in the human body and higher vertebrates.

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

Functional classification

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, 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. 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;
axon length and the presence of specialized sheaths.

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 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 microspinules are 0.1 to 0.2 µm thick and can be up to 50 µm in length; the wide and flat area of the growth cone is about 5 µm wide and long, although its shape may vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others elongate, deviate in different directions, touch the substrate and can stick to it.

The growth cone is filled with small, sometimes interconnected, irregularly shaped membranous vesicles. 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 sources of energy - 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).

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 (spines) 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 branches 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.

Cells in the human body are differentiated depending on the species. In fact, they are structural elements of various tissues. Each is maximally adapted to a certain type of activity. The structure of the neuron is a clear confirmation of this.

Nervous system

Most body cells have a similar structure. They have a compact form enclosed in a shell. Inside the nucleus and a set of organelles that perform the synthesis and metabolism of necessary substances. However, the structure and functions of the neuron are different. It is the structural unit of the nervous tissue. These cells provide communication between all body systems.

The CNS is based on the brain and spinal cord. These two centers secrete gray and white matter. The differences are related to the functions performed. One part receives a signal from the stimulus and processes it, while the other part is responsible for carrying out the necessary response command. Outside the main centers, the nervous tissue forms bundles of clusters (nodes or ganglia). They branch out, spreading a signal-conducting network throughout the body (peripheral nervous system).

Nerve cells

To provide multiple connections, the neuron has a special structure. In addition to the body, in which the main organelles are concentrated, processes are present. Some of them are short (dendrites), usually there are several of them, the other (axon) is one, and its length in individual structures can reach 1 meter.

The structure of the nerve cell of a neuron is designed to provide the best interchange of information. The dendrites are highly branched (like the crown of a tree). With their endings, they interact with the processes of other cells. The place where they meet is called a synapse. There is a reception and transmission of impulses. Its direction: receptor - dendrite - cell body (soma) - axon - reacting organ or tissue.

The internal structure of the neuron in terms of the composition of organelles is similar to other structural units of tissues. It contains a nucleus and a cytoplasm bounded by a membrane. Inside are mitochondria and ribosomes, microtubules, the endoplasmic reticulum, the Golgi apparatus.

In most cases, several thick branches (dendrites) depart from the soma of the cell (base). They do not have a clear boundary with the body and are covered by a common membrane. As they move away, the trunks become thinner, their branching occurs. As a result, their thinnest parts look like pointed threads.

The special structure of the neuron (thin and long axon) suggests the need to protect its fiber throughout its entire length. Therefore, on top it is covered with a sheath of Schwann cells that form myelin, with nodes of Ranvier between them. This structure provides additional protection, isolates passing impulses, additionally feeds and supports the threads.

The axon originates from a characteristic elevation (knoll). The process eventually also branches, but this does not occur along its entire length, but closer to the end, at the junctions with other neurons or with tissues.

Classification

Neurons are divided into types depending on the type of mediator (mediator of the conductive impulse) released at the endings of the axon. It can be choline, adrenaline, etc. From their location in the central nervous system, they can refer to somatic neurons or to autonomic ones. Distinguish between perceiving cells (afferent) and transmitting reverse signals (efferent) in response to irritation. Between them there may be interneurons responsible for the exchange of information within the CNS. According to the type of response, cells can inhibit excitation or, conversely, increase it.

According to the state of their readiness, they distinguish: “silent”, which begin to act (transmit an impulse) only in the presence of a certain type of irritation, and background ones, which constantly monitor (continuous generation of signals). Depending on the type of information received from the sensors, the structure of the neuron also changes. In this regard, they are classified into bimodal, with a relatively simple response to irritation (two interrelated types of sensation: an injection and, as a result, pain, and polymodal. This is a more complex structure - polymodal neurons (specific and ambiguous reaction).

Features, structure and functions of a neuron

The surface of the neuron membrane is covered with small outgrowths (thorns) to increase the contact area. In total, they can occupy up to 40% of the cell area. The nucleus of a neuron, like in other types of cells, carries hereditary information. Nerve cells do not divide by mitosis. If the connection of the axon with the body is broken, the process dies off. However, if the soma has not been damaged, it is able to generate and grow a new axon.

The fragile structure of the neuron suggests the presence of additional "guardianship". Protective, supporting, secretory and trophic (nutrition) functions are provided by neuroglia. Her cells fill all the space around. To a certain extent, it helps to restore broken connections, and also fights infections and generally “takes care” of neurons.

cell membrane

This element provides a barrier function, separating the internal environment from the external neuroglia. The thinnest film consists of two layers of protein molecules and phospholipids located between them. The structure of the neuron membrane suggests the presence in its structure of specific receptors responsible for the recognition of stimuli. They have selective sensitivity and, if necessary, are “switched on” in the presence of a counterparty. The connection between the internal and external environments occurs through tubules that allow calcium or potassium ions to pass through. At the same time, they open or close under the action of protein receptors.

Thanks to the membrane, the cell has its own potential. When it is transmitted along the chain, the innervation of the excitable tissue occurs. The contact of the membranes of neighboring neurons occurs at synapses. Maintaining the constancy of the internal environment is an important component of the vital activity of any cell. And the membrane finely regulates the concentration of molecules and charged ions in the cytoplasm. In this case, they are transported in the required quantities for the metabolic reactions to proceed at the optimal level.

Neuron Mouse cerebral cortex pyramidal neuron, expressive green fluorescent protein (GFP)

Classification

Structural classification

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

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 the central nervous system.

Functional classification

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

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 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, 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.

Literature

Polyakov G. I., On the principles of neuronal organization of the brain, M: Moscow State University, 1965
Kositsyn N. S. Microstructure of dendrites and axodendritic connections in the central nervous system. M.: Nauka, 1976, 197 p.
Nemechek S. et al. Introduction to Neurobiology, Avicennum: Prague, 1978, 400 pp.
Bloom F., Leizerson A., Hofstadter L. Brain, Mind and Behavior
Brain (collection of articles: D. Hubel, C. Stevens, E. Kandel and others - issue of Scientific American (September 1979)). M.: Mir, 1980
Savelyeva-Novosyolova N. A., Savelyev A. V. A device for modeling a neuron. A. s. No. 1436720, 1988
Saveliev A.V. Sources of variations in the dynamic properties of the nervous system at the synaptic level // Journal “Artificial Intelligence”, National Academy of Sciences of Ukraine. - Donetsk, Ukraine, 2006. - No. 4. - S. 323-338.

Histology : Nervous tissue

Neurons
(Gray matter)

Soma Axon (Axon hillock, Axon terminal, Axoplasm, Axolemma, Neurofilaments)

PNS: Schwann cells Neurolemma Noxus of Ranvier/Internodal segment Myelin notch