Where are the first interneurons located. Intercalary neurons

04.06.2020

A connecting neuron that sits between sensory (afferent) and motor (efferent) neurons. It is located in the central nervous system. Also called intermediate neuron, and in older texts - associative neuron.

Watch value Intercalary Neuron in other dictionaries

Insert App. - 1. Designed for insertion, insertion.
Efremova's explanatory dictionary

Neuron M. - 1. The same as: neuron.
Efremova's explanatory dictionary

Intercalary - (shn), insert, insert. Adj. to insert.
Ushakov's Explanatory Dictionary

Neuron - neuron, m. (Greek neuron - fiber, nerve) (anat.). Nerve cell.
Ushakov's Explanatory Dictionary

Neuron - -and; m. [from the Greek. neuron - nerve] Spec. The nerve cell with all the processes extending from it.
Explanatory dictionary Kuznetsov

Insert Disc - (discus intercalatus, LNH) is a general name for microscopic structures at the point of contact of adjacent muscle cells of the myocardium, ensuring their connection into muscle complexes and transmission ........
Large Medical Dictionary

Motor Neuron -, a nerve cell that conducts information to EFFECTS (usually muscles) from the CENTRAL NERVOUS SYSTEM (CNS), thus causing the appropriate response. Axons (processes, ........

Neuron - (nerve cell), the basic structural and functional unit of the NERVOUS SYSTEM, carrying out the rapid transmission of NERVOUS IMPULSES between different organs. Consists........
Scientific and technical encyclopedic Dictionary

Sensory Neuron - (sensitive neuron), a nerve cell that conducts information from RECEPTORS in any part of the body to the CENTRAL NERVOUS SYSTEM (CNS). Their nerve endings are located at ........
Scientific and technical encyclopedic dictionary

Neuron - (neuronum, neurocytus, LNH; Greek neuron vein, nerve; synonym: nerve cell, neurocyte, neurocyte) a cell capable of perceiving irritation, coming into a state of excitement, producing ........
Large Medical Dictionary

Neuron Amacrine - (n. Amacrinum, LNH) N., located in the inner granular layer of the retina and providing communication between the neurons of this layer.
Large Medical Dictionary

Neuron associative - see Intercalary neuron.
Large Medical Dictionary

Neuron Afferent - (n. Afferens, n. Sensorium: syn.: N. receptor, N. sensory, N. sensitive) N., carrying out perception and transmission of excitation from receptors to other N. central nervous system.
Large Medical Dictionary

Neuron Bipolar - (n. Bipolare, LNH) N., which has two processes - an axon and a dendrite.
Large Medical Dictionary

Neuron Vegetative - the general name of N., which are part of the ganglia, plexuses, and nerves of the autonomic nervous system.
Large Medical Dictionary

Neuron Fusiform - (n. Fusiforme, LNH) an elongated multipolar intercalary N., which is found in the molecular plate of the cerebral cortex.
Large Medical Dictionary

Neuron Fusiform Horizontal - (n. Fusiforme horizontale, LNH) multipolar N. elongated, found mainly between the layer of piriform neurons and the granular layer of the cerebellar cortex.
Large Medical Dictionary

Neuron Internal - (n. Internum, LNH) N. internal departments the anterior horn of the spinal cord, the axon of which passes through the white commissure to the opposite half of the spinal cord.
Large Medical Dictionary

Neuron Intercalary - (n. Intercalatum; synonym: N. associative, N. intermediate) N., participating in the transmission of excitation from afferent N. to efferent.
Large Medical Dictionary

Neuron Input - a formal neuron that performs the function of an input in a specific system of neurons (neural network), that is, it receives signals only from the external environment for this system.
Large Medical Dictionary

Neuron Giant Pyramidal - (n. Gigantopyramidale, LNH; syn.: Betza cell, giant pyramidal cell) large pyramidal N. of the inner pyramidal plate of the cerebral cortex; N.'s axons form ........
Large Medical Dictionary

Neuron Horizontal - (n. Horizomale, LNH) 1) N. of the inner granular layer of the retina, the processes of which are in contact with the central endings of photoreceptor cells, realizing redistribution ........
Large Medical Dictionary

Neuron Piriform - (n. Piriforme, LNH; syn. Purkinje cell) efferent N. of the cerebellar cortex located in its ganglionic layer and having a pear-shaped form.
Large Medical Dictionary

Neuron Motor - see Motoneuron.
Large Medical Dictionary

Neuron Longaxon - (n. Longiaxonicum, LNH; syn. Dogel type I cell) multipolar vegetative N., the axon of which transmits impulses to smooth or cardiac muscle tissue.
Large Medical Dictionary

Neuron Star - (n. Stellatum, LNH) intercalary N. star-shaped.
Large Medical Dictionary

Neuron Stellate Longaxon - (n. Stellatum longiaxonicum, LNH) N. z., Located in the granular layer of the cerebellar cortex, having an axon extending into the white matter.
Large Medical Dictionary

Neuron Stellate Short Axon - (n. Stellatum breviaxonicum, LNH) H. h. the granular layer of the cerebellar cortex, which has an axon going to the glomeruli of the cerebellum.
Large Medical Dictionary

Neuron Granular - (n. Granulare, LNH) the general name of small N. of round, angular and pyramidal shape, located in the outer granular plate of the cerebral cortex, the dendrites of which rise ........
Large Medical Dictionary

Neuron Granular Large - (granoneurocytus magnus, LNH) is the general name for large N. located in the molecular layer of the cerebellar cortex, the dendrites of which are distributed in the molecular layer, and the axons go into the granular ........
Large Medical Dictionary

The function of the nervous system is

1) management of the activities of various systems that make up an integral organism,

2) coordination of the processes occurring in it,

3) the establishment of the relationship between the body and the environment.

The activity of the nervous system is of a reflex nature. Reflex (Latin reflexus - reflected) is the body's response to any impact. It can be an external or internal influence (from the external environment or from your own body).

The structural and functional unit of the nervous system is neuron(nerve cell, neurocyte).A neuron has two parts - body and offshoots... The processes of a neuron, in turn, are of two types - dendrites and axons... The processes along which the nerve impulse is brought to the body of the nerve cell are called dendrites. The process along which a nerve impulse from the body of a neuron is directed to another nerve cell or to a working tissue is called axon. Nervecageable to skip nervousimpulse in one direction onlynii - from the dendrite through the cell body toaxon.

Neurons in the nervous system form chains along which nerve impulses are transmitted (moved). The transmission of a nerve impulse from one neuron to another occurs at the places of their contacts and is provided by a special kind of anatomical structures, called interneuronal synapsesowls.

In the nerve chain, different neurons perform different functions. In this regard, there are three main types of neurons:

1. sensitive (afferent) neuron.

2. intercalary neuron.

3. effector (efferent) neuron.

Sensitive, (receptor,orafferent) neurons. The main characteristics of sensory neurons:

and) teating sensitive neurons always lie in the nodes (spinal), outside the brain or spinal cord;

b) a sensitive neuron has two processes - one dendrite and one axon;

in) dendrite of a sensitive neuron follows to the periphery to this or that organ and ends there with a sensitive ending - receptor. Receptor it is an organ which is able to transform the energy of external influence (irritation) into a nerve impulse;

d) sensory neuron axon is sent to the central nervous system, to the spinal cord or to the brain stem, as part of the posterior roots of the spinal nerves or the corresponding cranial nerves.

A receptor is an organ that is able to convert the energy of external influences (irritation) into a nerve impulse. It is located at the end of the dendrite of a sensitive neuron

There are the following kinds of recipetori depending on localization:

1) Exteroceptorsperceive irritation from the external environment. They are located in the outer covers of the body, in the skin and mucous membranes, in the sense organs;

2) Interoceptors get irritated from the internal environment of the body, they are located in the internal organs;

3) Proprioceptors perceive irritations from the musculoskeletal system (in muscles, tendons, ligaments, fascia, joint capsules.

Sensitive neuron function - perception of an impulse from a receptor and its transmission to the central nervous system. Pavlov attributed this phenomenon to the beginning of the analysis process.

Intercalary, (associative, closure, or conductor, neuron ) carries out the transfer of excitation from a sensitive (afferent) neuron to efferent ones. Circumferential (intercalary) neurons lie within the central nervous system.

Effective, (efferent)neuron. There are two types of efferent neurons. it dvigastric neuron,andsecretory neuron.Basic properties motor neurons:

(nerve cell) - the main structural and functional unit of the nervous system; a neuron generates, perceives and transmits nerve impulses, thus transmitting information from one part of the body to another (see Fig.). Each neuron has a large body (cell body) (or perikaryon (...

Psychological encyclopedia

Nerve cell, the basic structural and functional unit of the nervous system. Although they differ in a wide variety of shapes and sizes and are involved in a wide range of functions, all neurons consist of a cell body, or soma, containing a nucleus and nerve processes: an axon and ...

In general, depending on the tasks and responsibilities assigned to neurons, they are divided into three categories:

- Sensory (sensory) neurons receive and transmit impulses from receptors "to the center", i.e. the central nervous system. Moreover, the receptors themselves are specially trained cells of the sense organs, muscles, skin and joints that are able to detect physical or chemical changes inside and outside our body, convert them into impulses and happily transmit them to sensory neurons. Thus, the signals go from the periphery to the center.

Next type:

- Motor (motor) neurons, which rumbling, snorting and beeping, carry signals from the brain or spinal cord to the executive organs, which are muscles, glands, etc. Yeah, so the signals go from the center to the periphery.

well and intermediate (intercalary) neurons, in simple terms, they are "extension cords", i.e. receive signals from sensory neurons and send these impulses further to other intermediate neurons, or directly to motor neurons.

In general, this is what happens: in sensory neurons, dendrites are connected to receptors, and axons - to other neurons (intercalary). In motor neurons, on the contrary, dendrites are connected to other neurons (intercalary), and axons are connected to some effector, i.e. stimulant of contraction of any muscle or gland secretion. Well, and, accordingly, in interneurons and dendrites and axons are connected with other neurons.

It turns out that the simplest path that a nerve impulse can take will consist of three neurons: one sensory, one intercalary, and one motor.

Aha, and now let's remember the uncle - a very “nervous pathologist”, with a malicious smile, knocking his “magic” hammer on the knee. Sound familiar? So, this is the simplest reflex: when he hits the knee tendon, the muscle attached to it stretches and the signal from the sensory cells (receptors) located in it is transmitted through sensory neurons to the spinal cord. And already in it, sensory neurons contact either through intercalary or directly with motor neurons, which in response send impulses back to the same muscle, forcing it to contract, and the leg to straighten.

The spinal cord itself is nestled comfortably inside our spine. It is soft and vulnerable, and therefore hides in the vertebrae. The spinal cord is only 40-45 centimeters long, with a little finger (about 8 mm) thick and weighs some 30 grams! But, for all its frailty, the spinal cord is the control center of a complex network of nerves that spread throughout the body. Almost like a mission control center! :) Without it, neither the musculoskeletal system, nor the main vital organs can in any way function and work.

The spinal cord takes its origin at the level of the occipital foramen of the skull, and ends at the level of the first or second lumbar vertebrae. But already below the spinal cord in the spinal canal there is such a dense bundle of nerve roots, coolly called the cauda equina, apparently for the similarity with it. So, the cauda equina is a continuation of the nerves that leave the spinal cord. They are responsible for the innervation of the lower limbs and pelvic organs, i.e. transmit signals from the spinal cord to them.

The spinal cord is surrounded by three membranes: soft, arachnoid, and hard. And the space between the soft and arachnoid membranes is filled with more cerebrospinal fluid. Through the intervertebral openings from the spinal cord spinal nerves depart: 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 or 2 coccygeal. Why steam? Yes, because the spinal nerve comes out with two roots: the posterior (sensory) and the anterior (motor), connected into one trunk. So, each such pair controls a certain part of the body. That is, for example, if you accidentally grabbed a hot pot (God forbid! Pah-pah-pah!), Then a pain signal immediately appears at the endings of the sensory nerve, immediately entering the spinal cord, and from there - into paired motor nerve, which transmits the order: “Akhtung-akhtung! Remove your hand immediately! " Moreover, believe me, this happens very quickly - even before the brain registers the pain impulse. As a result, you have time to pull your hand away from the pan before you even feel pain. Of course, this reaction saves us from severe burns or other damage.

In general, almost all of our automatic and reflex actions are controlled by the spinal cord, well, with the exception of those monitored by the brain itself. Well, for example: we perceive what we see with the help of the optic nerve going to the brain, and at the same time we turn our gaze in different directions with the help of the eye muscles, which are already controlled by the spinal cord. Yes, and we cry the same on the orders of the spinal cord, which "manages" the lacrimal glands.

We can say that our conscious actions come from the brain, but as soon as we begin to perform these actions automatically and reflexively, they are transferred to the management of the spinal cord. So, when we are just learning to do something, then, of course, we consciously think over and think over and comprehend every movement, which means we use the brain, but over time we can already do it automatically, and this means that the brain transfers the "reins" by this action to the spinal cord, it just became bored and uninteresting ... because our brain is very inquisitive, inquisitive and loves to learn!

Well, it's time for us to be curious ... ...

The peripheral nervous system (systerna nervosum periphericum) is a conditionally distinguished part of the nervous system, the structures of which are located outside the brain and spinal cord. The peripheral nervous system includes 12 pairs of cranial nerves that travel from the spinal cord and brain to the periphery and 31 pairs of spinal nerves.
Cranial nerves include: Olfactory nerve (nervus olfactorius) - 1st pair, refers to the nerves of special sensitivity. It starts from the olfactory receptors of the nasal mucosa in the superior turbinate. Represents 15 - 20 thin nerve filaments formed by non-fleshy fibers. The filaments do not form a common trunk, but penetrate into the cranial cavity through the ethmoid plate of the ethmoid bone, where they attach to the cells of the olfactory bulb. The fibers of the olfactory pathway conduct an impulse to the subcortical, or primary, centers of smell, from where part of the fibers is directed to the cerebral cortex. Oculomotor nerve (nervus oculomotorius) - 3rd pair, is a mixed nerve. Nerve fibers come out of brain stem on the inner surfaces of the legs of the brain and form a relatively large nerve that goes forward in the outer wall of the cavernous sinus. On the way, the nerve fibers of the sympathetic plexus of the internal carotid artery join it. The branches of the oculomotor nerve approach the levator upper eyelid, upper, internal and lower rectus muscles, and the lower oblique muscle of the eyeball.
Block nerve (nervus trochlearis) - 4th pair, refers to motor nerves. The core of the block nerve is located in the midbrain. Bending around the leg of the brain from the lateral side, the nerve exits to the base of the brain, passing between the leg and the temporal lobe. Then, together with the oculomotor nerve, it passes from the skull to the orbit and innervates the superior oblique muscle of the eyeball.

In general, depending on the tasks and responsibilities assigned to neurons, they are divided into three categories:

Next type:

It turns out that the simplest path that a nerve impulse can take will consist of three neurons: one sensory, one intercalary, and one motor.

Yeah, but now let's remember the uncle - a very "nervous pathologist", with a malicious smile, knocking his "magic" hammer on the knee. Sound familiar? So, this is the simplest reflex: when he hits the knee tendon, the muscle attached to it stretches and the signal from the sensory cells (receptors) located in it is transmitted through sensory neurons to the spinal cord. And already in it, sensory neurons contact either through intercalary or directly with motor neurons, which in response send impulses back to the same muscle, forcing it to contract, and the leg to straighten.

The spinal cord takes its origin at the level of the occipital foramen of the skull, and ends at the level of the first or second lumbar vertebrae. But already below the spinal cord in the spinal canal there is such a dense bundle of nerve roots, coolly called the cauda equina, apparently for the similarity with it. So, the cauda equina is a continuation of the nerves that leave the spinal cord. They are responsible for innervation lower limbs and the pelvic organs, i.e. transmit signals from the spinal cord to them.

The spinal cord is surrounded by three membranes: soft, arachnoid, and hard. And the space between the soft and arachnoid shells is filled with more cerebrospinal fluid... Through the intervertebral openings from the spinal cord spinal nerves depart: 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 or 2 coccygeal. Why steam? Yes, because the spinal nerve comes out with two roots: the posterior (sensory) and the anterior (motor), connected into one trunk. So, each such pair controls a certain part of the body. That is, for example, if you accidentally grabbed a hot pot (God forbid! Pah-pah-pah!), Then a pain signal immediately appears at the endings of the sensory nerve, immediately entering the spinal cord, and from there - into paired motor nerve, which transmits the order: “Akhtung-akhtung! Remove your hand immediately! " Moreover, believe me, this happens very quickly - even before the brain registers the pain impulse. As a result, you have time to pull your hand away from the pan before you even feel pain. Of course, this reaction saves us from severe burns or other damage.

Well, it's time for us to be curious ... ...

A neuron is a specific, electrically excitable cell in the human nervous system and has unique characteristics. Its functions are to process, store and transmit information. Neurons are characterized by a complex structure and narrow specialization. They are also divided into three types. This article details the interneuron and its role in the action of the central nervous system.

Classification of neurons

The human brain has approximately 65 billion neurons that constantly communicate with each other. These cells are divided into several types, each of which performs its own special functions.

The sensitive neuron plays the role of a transmitter of information between the sense organs and the central parts of the human nervous system. It perceives various stimuli, which it converts into nerve impulses, and then transmits the signal to the human brain.

Motor - sends impulses to various organs and tissues. Basically, this type is involved in controlling the reflexes of the spinal cord.

An intercalary neuron is responsible for processing and switching impulses. The functions of this type of cells are to receive and process information from sensory and motor neurons, between which they are located. Moreover, intercalated (or intermediate) neurons occupy 90% of the human central nervous system, and are also found in large numbers in all areas of the brain and spinal cord.

The structure of intermediate neurons

An interneuron consists of a body, axon, and dendrites. Each part has its own specific functions and is responsible for a specific action. His body contains all the components from which cellular structures are created. The important role of this part of the neuron is to generate nerve impulses and perform trophic function. The oblong process, which carries the signal from the cell body, is called an axon. It is divided into two types: myelinated and non-myelinated. There are various synapses at the end of the axon. The third component of neurons is dendrites. They are short branches that branch out in different directions. Their function is to deliver impulses to the body of the neuron, which provides communication between different types of neurons in the central nervous system.

Scope of influence

What determines the area of \u200b\u200binfluence of the intercalary neuron? First of all, its own structure. Basically, cells of this type have axons, whose synapses end on neurons of the same center, which ensures their union. Some intermediate neurons are activated by others, from other centers, and then deliver information to their neuronal center. Such actions enhance the effect of the signal, which is repeated in parallel paths, thereby lengthening the storage life of information data in the center. As a result, the place where the signal was delivered increases the reliability of the influence on the executive structure. Other interneurons can receive activation from the motor “brothers” connections from their center. Then they become transmitters of information back to their center, thereby creating feedback. Thus, the insertion neuron plays an important role in the formation of special closed networks that prolong the storage life of information in the nerve center.

Excitatory type of intermediate neurons

Interneurons are divided into two types: excitatory and inhibitory. When the former are activated, the transfer of data from one neural group to another is facilitated. This task is performed by "slow" neurons, which have the ability to long-term activation. They transmit signals for quite a long time. In parallel with these actions, intermediate neurons activate their "fast" "colleagues". When the activity of "slow" neurons increases, the reaction time of "fast" ones decreases. At the same time, the latter somewhat slow down the work of the "slow" ones.

Inhibitory type of intermediate neurons

The interneuron of the inhibitory type comes into an active state due to direct signals that come to their center or come from it. This action takes place by feedbacks... Direct excitation of this type of intercalary neurons is characteristic of the intermediate centers of the sensory pathways of the spinal cord. And in the motor centers of the cerebral cortex, there is an activation of intercalary neurons due to feedback.

The role of interneurons in the functioning of the spinal cord

In the work of the human spinal cord, an important role is played by the pathways that are located outside of the bundles that perform the conductive function. It is along these paths that the impulses that are sent by the insertion and sensory neurons move. Signals travel up and down these pathways, transmitting different information to the corresponding parts of the brain. The interneurons of the spinal cord are located in the intermediate-medial nucleus, which, in turn, is located in the posterior horn. Intermediate neurons are an important anterior part of the spinal cord. On the back of the horn of the spinal cord are fibers consisting of intercalated neurons. They form the lateral dorsal thalamic tract, which has a special function. He is a conductor, that is, it transmits signals about pain and temperature sensitivity, first in the diencephalon, and then in the cerebral cortex itself.

More information on interneurons

In the human nervous system, intercalary neurons perform a special and extremely important function. They connect different groups of nerve cells to each other, transmit a signal from the brain to the spinal cord. Although this particular type is the smallest in size. In the shape of the intercalary neurons, they resemble a star. The bulk of these elements is located in the gray matter of the brain, and their processes do not protrude beyond the human central nervous system.

Nerve tissue - the main structural element of the nervous system. IN composition of nervous tissue includes highly specialized nerve cells - neurons, and neuroglia cellsperforming supporting, secretory and protective functions.

Neuron Is the basic structural and functional unit of nervous tissue. These cells are able to receive, process, encode, transmit and store information, establish contacts with other cells. The unique features of the neuron are the ability to generate bioelectric discharges (impulses) and transmit information along the processes from one cell to another using specialized endings -.

The functioning of a neuron is facilitated by the synthesis in its axoplasm of transmitter substances - neurotransmitters: acetylcholine, catecholamines, etc.

The number of neurons in the brain is approaching 10 11. One neuron can have up to 10,000 synapses. If these elements are considered as cells for storing information, then we can come to the conclusion that the nervous system can store 10 19 units. information, i.e. is able to accommodate almost all the knowledge accumulated by humanity. Therefore, the idea that human brain during his life he remembers everything that happens in the body and during his communication with the environment. However, the brain cannot extract from all the information that is stored in it.

For various structures the brain is characterized by certain types of neural organization. The neurons regulating a single function form the so-called groups, ensembles, columns, nuclei.

Neurons vary in structure and function.

By structure (depending on the number of processes extending from the body) unipolar (with one process), bipolar (with two processes) and multipolar (with many processes) neurons.

By functional properties allocate afferent (or centripetal) neurons carrying excitation from receptors in, efferent, motor, motoneurons (or centrifugal), transmitting excitement from the central nervous system to the innervated organ, and intercalary, contact or intermediate neurons connecting afferent and efferent neurons.

Afferent neurons are unipolar; their bodies lie in the spinal ganglia. The outgrowth from the cell body is T-shapedly divided 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 of the efferent and intercalary neurons are multipolar (Fig. 1). Multipolar interneurons are located in large numbers in the posterior horns of the spinal cord, as well as in all other parts of the central nervous system. They can also be bipolar, for example, retinal neurons with a short branching dendrite and a long axon. Motor neurons are located mainly in the anterior horns of the spinal cord.

Figure: 1. The structure of the nerve cell:

1 - microtubules; 2 - a long process of a nerve cell (axon); 3 - endoplasmic reticulum; 4 - core; 5 - neuroplasm; 6 - dendrites; 7 - mitochondria; 8 - nucleolus; 9 - myelin sheath; 10 - interception of Ranvier; 11 - the end of the axon

Neuroglia

Neuroglia, or glia, - a set of cellular elements of the nervous tissue, formed by specialized cells of various shapes.

It was discovered by R. Virkhov and named by him neuroglia, which means "nerve glue". Neuroglial cells fill the space between neurons, accounting for 40% of the brain volume. Glial cells are 3-4 times smaller than nerve cells; their number in the central nervous system of mammals reaches 140 billion. With age, the number of neurons in the human brain decreases, while the number of glial cells increases.

It has been established that neuroglia are related to the metabolism in the nervous tissue. Some neuroglial cells secrete substances that affect the state of neuronal excitability. It is noted that for different mental states the secretion of these cells changes. FROM functional state neuroglia associate long-term trace processes in the central nervous system.

Glial cell types

By the nature of the structure of glial cells and their location in the central nervous system, there are:

astrocytes (astroglia);
oligodendrocytes (oligodendroglia);
microglial cells (microglia);
schwann cells.

Glial cells perform supporting and protective functions for neurons. They are part of the structure. Astrocytes are the most numerous glial cells that fill the spaces between neurons and cover. They prevent the spread of neurotransmitters into the central nervous system that diffuse from the synaptic cleft. Astrocytes contain receptors for neurotransmitters, the activation of which can cause fluctuations in the membrane potential difference and changes in astrocyte metabolism.

Astrocytes tightly surround the capillaries of the blood vessels of the brain, located between them and the neurons. On this basis, it is assumed that astrocytes play an important role in the metabolism of neurons, adjusting capillary permeability for certain substances.

One of the important functions of astrocytes is their ability to absorb excess K + ions, which can accumulate in the intercellular space with high neural activity. In the areas of dense adhesion of astrocytes, gap junctions are formed, through which astrocytes can exchange various small-sized ions and, in particular, K + ions.This increases the possibility of absorption of K + ions by them.Uncontrolled accumulation of K + ions in the interneuronal space would lead to an increase in the excitability of neurons. Thus, astrocytes, absorbing excess K + ions from the interstitial fluid, prevent an increase in neuronal excitability and the formation of foci of increased neuronal activity. The appearance of such foci in the human brain may be accompanied by the fact that their neurons generate a series of nerve impulses, which are called convulsive discharges.

Astrocytes take part in the removal and destruction of neurotransmitters entering extrasynaptic spaces. Thus, they prevent the accumulation of neurotransmitters in the interneuronal spaces, which could lead to dysfunction of the brain.

Neurons and astrocytes are separated by intercellular gaps of 15-20 microns, called the interstitial space. Interstitial spaces occupy up to 12-14% of the brain volume. An important property of astrocytes is their ability to absorb CO2 from the extracellular fluid of these spaces, and thereby maintain a stable brain pH.

Astrocytes are involved in the formation of interfaces between the nerve tissue and the vessels of the brain, the nervous tissue and the membranes of the brain during the growth and development of the nervous tissue.

Oligodendrocytes characterized by the presence of a small number of short processes. One of their main functions is the formation of the myelin sheath of nerve fibers within the central nervous system... These cells are also located in the immediate vicinity of the bodies of neurons, but functional significance this fact is unknown.

Microglia cells make up 5-20% of the total number of glial cells and are scattered throughout the central nervous system. It was found that their surface antigens are identical to those of blood monocytes. This indicates their origin from the mesoderm, penetration into the nervous tissue during embryonic development and subsequent transformation into morphologically recognizable microglial cells. In this regard, it is generally accepted that the most important function of microglia is to protect the brain. It has been shown that damage to the nervous tissue in it increases the number of phagocytic cells due to blood macrophages and activation of the phagocytic properties of microglia. They remove dead neurons, glial cells and their structural elements, phagocytose foreign particles.

Schwann cells form the myelin sheath of peripheral nerve fibers outside the central nervous system. The membrane of this cell is repeatedly wrapped around, and the thickness of the formed myelin sheath can exceed the diameter nerve fiber... The length of the myelinated areas of the nerve fiber is 1-3 mm. In the intervals between them (Ranvier's interceptions), the nerve fiber remains covered only by a surface membrane that has excitability.

One of the most important properties of myelin is its high resistance to electric current. It is due to the high content of sphingomyelin and other phospholipids in myelin, which give it current-insulating properties. In areas of the nerve fiber covered with myelin, the process of generating nerve impulses is impossible. Nerve impulses are generated only on the membrane of Ranvier's interceptions, which provides a higher speed of nerve impulses conduction to myelinated nerve fibers in comparison with unmyelinated ones.

It is known that the structure of myelin can be easily disrupted during infectious, ischemic, traumatic, toxic damage to the nervous system. In this case, the process of demyelination of nerve fibers develops. Especially often demyelination develops with a disease multiple sclerosis... As a result of demyelination, the rate of conduction of nerve impulses along nerve fibers decreases, the rate of delivery of information to the brain from receptors and from neurons to executive organs decreases. This can lead to impaired sensory sensitivity, movement disorders, and work regulation. internal organs and other serious consequences.

Structure and function of neurons

Neuron (nerve cell) is a structural and functional unit.

The anatomical structure and properties of the neuron ensure its implementation main functions: the implementation of metabolism, the receipt of energy, the perception of various signals and their processing, the formation or participation in response reactions, the generation and conduction of nerve impulses, the unification of neurons into neural circuits that provide both the simplest reflex reactions and the higher integrative functions of the brain.

Neurons consist of a nerve cell body and processes - an axon and dendrites.

Figure: 2. The structure of the neuron

Nerve cell body

Body (perikarion, catfish) the neuron and its processes are covered with a neuronal membrane throughout. The cell body membrane differs from the membrane of the axon and dendrites by the content of various receptors, the presence on it.

In the body of a neuron there is a neuroplasm and a nucleus separated from it by membranes, a rough and smooth endoplasmic reticulum, the Golgi apparatus, mitochondria. The chromosomes of the nucleus of neurons contain a set of genes encoding the synthesis of proteins necessary for the formation of the structure and implementation of the functions of the neuron body, its processes and synapses. These are proteins that perform the functions of enzymes, carriers, ion channels, receptors, etc. Some proteins perform functions while in the neuroplasm, while others are embedded in the membranes of organelles, soma and neuron processes. Some of them, for example, enzymes required for the synthesis of neurotransmitters, are delivered to the axonal terminal by axonal transport. In the cell body, peptides are synthesized that are necessary for the vital activity of axons and dendrites (for example, growth factors). Therefore, when the body of a neuron is damaged, its processes degenerate and are destroyed. If the body of the neuron is preserved, and the process is damaged, then its slow restoration (regeneration) and restoration of the innervation of denervated muscles or organs occurs.

The site of protein synthesis in the bodies of neurons is the rough endoplasmic reticulum (tigroid granules or Nissl bodies) or free ribosomes. Their content in neurons is higher than in glial or other cells of the body. In the smooth endoplasmic reticulum and the Golgi apparatus, proteins acquire their characteristic spatial conformation, are sorted and sent to transport streams to the structures of the cell body, dendrites or axons.

In numerous mitochondria of neurons, as a result of oxidative phosphorylation processes, ATP is formed, the energy of which is used to maintain the vital activity of the neuron, the operation of ion pumps and the maintenance of asymmetry of ion concentrations on both sides of the membrane. Consequently, the neuron is in constant readiness not only for the perception of various signals, but also for the response to them - the generation of nerve impulses and their use to control the functions of other cells.

In the mechanisms of perception by neurons of various signals, molecular receptors of the cell body membrane, sensory receptors formed by dendrites, and sensitive cells of epithelial origin are involved. Signals from other nerve cells can reach the neuron through multiple synapses formed on the neuron's dendrites or gel.

Nerve cell dendrites

Dendrites neurons form a dendritic tree, the nature of branching and the size of which depend on the number of synaptic contacts with other neurons (Fig. 3). There are thousands of synapses on the dendrites of a neuron, formed by axons or dendrites of other neurons.

Figure: 3. Synaptic contacts of the interneuron. The arrows on the left show the arrival of afferent signals to the dendrites and the body of the interneuron, on the right - the direction of propagation of the efferent signals of the interneuron to other neurons

Synapses can be heterogeneous both in function (inhibitory, excitatory) and in the type of neurotransmitter used. The membrane of dendrites, involved in the formation of synapses, is their postsynaptic membrane, which contains receptors (ligand-dependent ion channels) for the neurotransmitter used in this synapse.

Excitatory (glutamatergic) synapses are located mainly on the surface of dendrites, where there are eminences, or outgrowths (1-2 μm), called spines. There are channels in the membrane of the spines, the permeability of which depends on the transmembrane potential difference. In the cytoplasm of dendrites in the area of \u200b\u200bspines, secondary messengers of intracellular signal transmission, as well as ribosomes, on which protein is synthesized in response to synaptic signals, were found. The exact role of spines remains unknown, but it is clear that they increase the surface area of \u200b\u200bthe dendritic tree for synapse formation. Spines are also neuron structures for receiving input signals and processing them. Dendrites and spines provide the transfer of information from the periphery to the body of the neuron. The dendrite membrane in mowing is polarized due to the asymmetric distribution of mineral ions, the operation of ion pumps and the presence of ion channels in it. These properties underlie the transfer of information through the membrane in the form of local circular currents (electrotonically) that arise between postsynaptic membranes and the adjacent sections of the dendrite membrane.

Local currents, when they propagate through the dendrite membrane, attenuate, but turn out to be sufficient in magnitude to transmit to the membrane of the neuron body signals received through synaptic inputs to the dendrites. No voltage-gated sodium and potassium channels have yet been identified in the dendrite membrane. She does not have excitability and the ability to generate action potentials. However, it is known that an action potential arising on the membrane of the axonal hillock can propagate along it. The mechanism of this phenomenon is unknown.

It is assumed that dendrites and spines are part of the neural structures involved in memory mechanisms. The number of spines is especially high in the dendrites of neurons in the cerebellar cortex, basal ganglia, and cerebral cortex. The area of \u200b\u200bthe dendritic tree and the number of synapses decrease in some areas of the cerebral cortex of the elderly.

Neuron axon

Axon - an outgrowth of a nerve cell that is not found in other cells. Unlike dendrites, the number of which is different for a neuron, all neurons have one axon. Its length can reach up to 1.5 m. In the place where the axon leaves the body of the neuron, there is a thickening - an axonal mound, covered with a plasma membrane, which is soon covered with myelin. The area of \u200b\u200bthe axonal hillock not covered by myelin is called the initial segment. The axons of neurons, up to their terminal ramifications, are covered with a myelin sheath, interrupted by Ranvier's interceptions - microscopic myelin-free areas (about 1 μm).

Throughout the axon (myelinated and unmyelinated fiber) is covered with a bilayer phospholipid membrane with embedded protein molecules that carry out the functions of transporting ions, voltage-gated ion channels, etc. Proteins are evenly distributed in the membrane of the unmyelinated nerve fiber, and in the membrane of the myelinated nerve fiber they are located predominantly in the area of \u200b\u200binterceptions of Ranvier. Since there is no rough reticulum and ribosomes in the axoplasm, it is obvious that these proteins are synthesized in the neuron body and delivered to the axon membrane by axonal transport.

Properties of the membrane covering the body and axon of a neuron, are different. This difference concerns primarily the permeability of the membrane to mineral ions and is due to the content of various types. If the content of ligand-dependent ion channels (including postsynaptic membranes) prevails in the membrane of the body and dendrites of the neuron, then in the membrane of the axon, especially in the area of \u200b\u200bRanvier interceptions, there is a high density of voltage-dependent sodium and potassium channels.

The membrane of the initial segment of the axon has the lowest polarization value (about 30 mV). In areas of the axon more distant from the cell body, the transmembrane potential is about 70 mV. The low value of the polarization of the membrane of the initial segment of the axon determines that in this area the membrane of the neuron has the greatest excitability. It is here that postsynaptic potentials that have arisen on the membrane of the dendrites and the cell body as a result of the transformation of information signals received by the neuron in the synapses are propagated along the membrane of the neuron body with the help of local circular electric currents. If these currents cause depolarization of the membrane of the axonal hillock to a critical level (E k), then the neuron will respond to the receipt of signals from other nerve cells by generating its own action potential (nerve impulse). The resulting nerve impulse is then carried along the axon to other nerve, muscle or glandular cells.

On the membrane of the initial segment of the axon, there are spines on which GABAergic inhibitory synapses are formed. The arrival of signals along these from other neurons can prevent the generation of a nerve impulse.

Classification and types of neurons

The classification of neurons is carried out both by morphological and functional characteristics.

By the number of processes, multipolar, bipolar and pseudo-unipolar neurons are distinguished.

By the nature of connections with other cells and the function performed, they are distinguished sensory, insertion and motor neurons. Sensory neurons are also called afferent neurons, and their processes are centripetal. The neurons that carry out the function of transmitting signals between nerve cells are called intercalary, or associative.Neurons whose axons form synapses on effector cells (muscle, glandular) are referred to as motor,or efferent, their axons are called centrifugal.

Afferent (sensory) neurons they perceive information by sensory receptors, convert it into nerve impulses and conduct it to the brain and spinal cord. The bodies of sensory neurons are found in the spinal and cranial. These are pseudo-unipolar neurons, the axon and dendrite of which extend from the body of the neuron together and then separate. The dendrite follows to the periphery to organs and tissues as part of sensory or mixed nerves, and the axon as part of the dorsal roots enters the dorsal horns of the spinal cord or as part of the cranial nerves into the brain.

Interlocking, or associative, neurons perform the functions of processing incoming information and, in particular, provide the closure of reflex arcs. The bodies of these neurons are located in the gray matter of the brain and spinal cord.

Efferent neurons also perform the function of processing the received information and transmitting efferent nerve impulses from the brain and spinal cord to the cells of the executive (effector) organs.

Integrative activity of the neuron

Each neuron receives a huge number of signals through numerous synapses located on its dendrites and body, as well as through the molecular receptors of plasma membranes, cytoplasm and nucleus. Signaling uses many different types of neurotransmitters, neuromodulators, and other signaling molecules. Obviously, in order to form a response to the simultaneous arrival of multiple signals, a neuron must be able to integrate them.

The set of processes that ensure the processing of incoming signals and the formation of a neuron response to them is included in the concept integrative activity of the neuron.

Perception and processing of signals arriving at a neuron is carried out with the participation of dendrites, the cell body, and the axonal hillock of the neuron (Fig. 4).

Fig. 4. Integration of signals by neuron.

One of the options for their processing and integration (summation) is the transformation in synapses and summation of postsynaptic potentials on the membrane of the body and neuron processes. Perceived signals are converted at synapses into fluctuations in the potential difference of the postsynaptic membrane (postsynaptic potentials). Depending on the type of synapse, the received signal can be converted into a small (0.5-1.0 mV) depolarizing change in the potential difference (EPSP - synapses in the diagram are shown as light circles) or hyperpolarizing (TPSP - synapses in the diagram are shown as black circles). Many signals can simultaneously arrive at different points of the neuron, some of which are transformed into EPSP, and others - into EPSP.

These fluctuations in the potential difference propagate with the help of local circular currents along the neuron membrane in the direction of the axonal hillock in the form of depolarization waves (in the diagram white) and hyperpolarization (in the black diagram), superimposed on each other (in the diagram, the gray areas). With this superposition, the amplitudes of the waves of one direction are summed up, and the amplitudes of the opposite ones are reduced (smoothed). This algebraic summation of the potential difference across the membrane is called spatial summation (fig. 4 and 5). The result of this summation can be either depolarization of the membrane of the axonal hillock and generation of a nerve impulse (cases 1 and 2 in Fig. 4), or its hyperpolarization and prevention of the emergence of a nerve impulse (cases 3 and 4 in Fig. 4).

In order to shift the potential difference of the membrane of the axonal hillock (about 30 mV) to E k, it must be depolarized by 10-20 mV. This will lead to the opening of the voltage-gated sodium channels available in it and the generation of a nerve impulse. Since when one AP arrives and transforms it into EPSP, membrane depolarization can reach up to 1 mV, and its propagation to the axon hillock is attenuated, then the generation of a nerve impulse requires the simultaneous arrival of 40-80 nerve impulses from other neurons to the neuron through excitatory synapses and summation the same amount of EPSP.

Figure: 5. Spatial and temporal summation of EPSP by neuron; a - BPSP to a single stimulus; and - EPSP for multiple stimulation from different afferents; c - EPSP for frequent stimulation through a single nerve fiber

If at this time a certain number of nerve impulses arrive at the neuron through inhibitory synapses, then its activation and generation of a response nerve impulse will be possible with a simultaneous increase in the flow of signals through the excitatory synapses. Under conditions when signals arriving through inhibitory synapses cause hyperpolarization of the neuron membrane, equal to or greater than the depolarization caused by signals arriving through excitatory synapses, depolarization of the axon hillock membrane will be impossible, the neuron will not generate nerve impulses and will become inactive.

The neuron also carries out time summation signals EPSP and TPSP arriving to it almost simultaneously (see Fig. 5). The changes in the potential difference in the parasynaptic regions caused by them can also be algebraically summed up, which is called temporary summation.

Thus, each nerve impulse generated by a neuron, as well as the period of silence of a neuron, contains information received from many other nerve cells. Usually, the higher the frequency of signals coming to a neuron from other cells, the more often it generates response nerve impulses, which it sends along the axon to other nerve or effector cells.

Due to the fact that there are sodium channels (albeit in a small number) in the membrane of the body of the neuron and even its dendrites, the action potential that has arisen on the membrane of the axonal hillock can spread to the body and some part of the dendrites of the neuron. The significance of this phenomenon is not clear enough, but it is assumed that the spreading action potential momentarily smooths out all local currents on the membrane, nullifies the potentials and contributes to a more efficient perception of new information by the neuron.

Molecular receptors are involved in the transformation and integration of signals coming to the neuron. At the same time, their stimulation by signaling molecules can lead through changes in the state of ion channels initiated (by G-proteins, second messengers), transformation of received signals into fluctuations in the potential difference of the neuron membrane, summation and formation of a neuron response in the form of generation of a nerve impulse or its inhibition.

Transformation of signals by metabotropic molecular receptors of a neuron is accompanied by its response in the form of triggering a cascade of intracellular transformations. The response of the neuron in this case can be an acceleration of the general metabolism, an increase in the formation of ATP, without which it is impossible to increase its functional activity. Using these mechanisms, the neuron integrates the received signals to improve the efficiency of its own activity.

Intracellular transformations in a neuron, initiated by the received signals, often lead to an increase in the synthesis of protein molecules that perform the functions of receptors, ion channels, and carriers in the neuron. By increasing their number, the neuron adapts to the nature of the incoming signals, increasing sensitivity to more significant ones and weakening - to less significant ones.

A neuron receiving a number of signals can be accompanied by the expression or repression of some genes, for example, the neuromodulators of peptide nature controlling the synthesis. Since they are delivered to the axonal terminals of a neuron and are used in them to enhance or weaken the action of its neurotransmitters on other neurons, the neuron, in response to the signals it receives, may, depending on the information received, exert a stronger or weaker effect on other nerve cells it controls. Given that the modulating effect of neuropeptides can last for a long time, the effect of a neuron on other nerve cells can also last for a long time.

Thus, due to the ability to integrate various signals, the neuron can subtly respond to them. wide range response reactions that allow you to effectively adapt to the nature of incoming signals and use them to regulate the functions of other cells.

Neural circuits

The neurons of the central nervous system interact with each other, forming various synapses at the point of contact. The resulting neural foams multiply the functionality of the nervous system. The most common neural circuits include: local, hierarchical, convergent and divergent neural circuits with one input (Fig. 6).

Local neural circuits formed by two or a large number neurons. In this case, one of the neurons (1) will give its axonal collateral to the neuron (2), forming an axosomatic synapse on its body, and the second will form a synapse with an axon on the body of the first neuron. Local neural networks can act as traps in which nerve impulses can circulate for a long time in a circle formed by several neurons.

The possibility of long-term circulation of a wave of excitation (nerve impulse) that once emerged due to transmission to a circular structure was experimentally shown by Professor I.A. Vetokhin in experiments on the nerve ring of a jellyfish.

Circular circulation of nerve impulses along local neural circuits performs the function of transformation of the rhythm of excitations, provides the possibility of prolonged excitation after the cessation of the receipt of signals to them, participates in the mechanisms of storing incoming information.

Local circuits can also perform a braking function. An example of it is recurrent inhibition, which is implemented in the simplest local neural circuit of the spinal cord, formed by the a-motoneuron and the Renshaw cell.

Fig. 6. The simplest neural circuits of the central nervous system. Description in text

In this case, the excitation that arose in the motor neuron spreads along the branch of the axon, activates the Renshaw cell, which inhibits the a-motor neuron.

Convergent chains are formed by several neurons, on one of which (usually efferent) the axons of a number of other cells converge or converge. Such circuits are widespread in the central nervous system. For example, the axons of many neurons of the sensory fields of the cortex converge on the pyramidal neurons of the primary motor cortex. Axons of thousands of sensory and intercalary neurons of various levels of the central nervous system converge on motor neurons of the ventral horns of the spinal cord. Convergent circuits play an important role in the integration of signals by efferent neurons and in the coordination of physiological processes.

Single Entry Divergent Chains are formed by a neuron with a branching axon, each of the branches of which forms a synapse with another nerve cell. These circuits perform the function of simultaneously transmitting signals from one neuron to many other neurons. This is achieved through strong branching (the formation of several thousand branches) of the axon. Such neurons are often found in nuclei. reticular formation brain stem. They provide a rapid increase in the excitability of numerous parts of the brain and the mobilization of its functional reserves.