Nerve cells definition. The human nervous system. The structure of the nervous system

The human body is made up of trillions of cells, and the brain alone contains approximately 100 billion neurons of all shapes and sizes. The question arises, how is a nerve cell arranged, and how does it differ from other cells in the body?

The structure of the human nerve cell

Like most other cells in the human body, nerve cells have nuclei. But compared to the rest, they are unique in that they have long, thread-like branches through which nerve impulses are transmitted.

The cells of the nervous system are similar to others, as they are also surrounded by a cell membrane, have nuclei containing genes, cytoplasm, mitochondria and other organelles. They are involved in fundamental cellular processes such as protein synthesis and energy production.

Neurons and nerve impulses

It consists of a bundle of nerve cells. A nerve cell that transmits certain information is called a neuron. The data that neurons carry is called nerve impulses. Like electrical impulses, they carry information at an incredible speed. Fast signal transmission is provided by axons of neurons covered with a special myelin sheath.

This sheath coats the axon like the plastic coating on electrical wires and allows nerve impulses to travel faster. What is a neuron? It has a special shape that allows you to transmit a signal from one cell to another. A neuron consists of three main parts: a cell body, many dendrites, and one axon.

Types of neurons

Neurons are usually classified based on the role they play in the body. There are two main types of neurons - sensory and motor. Sensory neurons conduct nerve impulses from the sense organs and internal organs to Motor neurons, on the contrary, carry nerve impulses from the central nervous system to organs, glands and muscles.

The cells of the nervous system are arranged in such a way that both types of neurons work together. Sensory neurons carry information about the internal and external environment. This data is used to send signals through motor neurons to tell the body how to respond to the information received.

Synapse

The place where the axon of one neuron meets the dendrites of another is called a synapse. Neurons communicate with each other through an electrochemical process. In this case, chemicals called neurotransmitters enter into the reaction.

cell body

The device of a nerve cell assumes the presence of a nucleus and other organelles in the cell body. The dendrites and axons connected to the cell body resemble the rays emanating from the sun. Dendrites receive impulses from other nerve cells. Axons carry nerve impulses to other cells.

One neuron can have thousands of dendrites, so it can communicate with thousands of other cells. The axon is covered with a myelin sheath, a fatty layer that insulates it and allows it to transmit a signal much faster.

Mitochondria

Answering the question of how a nerve cell is arranged, it is important to note the element responsible for the supply of metabolic energy, which can then be easily utilized. Mitochondria play a key role in this process. These organelles have their own outer and inner membrane.

The main source of energy for the nervous system is glucose. Mitochondria contain the enzymes needed to convert glucose into high-energy compounds, mainly adenosine triphosphate (ATP) molecules, which can then be transported to other areas of the body that need their energy.

Core

The complex process of protein synthesis begins in the nucleus of the cell. The nucleus of a neuron contains genetic information, which is stored as encoded strings of deoxyribonucleic acid (DNA). Each contains for all cells in the body.

It is in the nucleus that the process of building protein molecules begins, by writing the corresponding part of the DNA code on complementary ribonucleic acid (RNA) molecules. Released from the nucleus into the intercellular fluid, they start the process of protein synthesis, in which the so-called nucleoli also take part. This is a separate structure within the nucleus responsible for building molecular complexes called ribosomes that are involved in protein synthesis.

Do you know how a nerve cell works?

Neurons are the most tenacious and longest cells in the body! Some of them remain in the human body throughout life. Other cells die and are replaced by new ones, but many neurons cannot be replaced. With age, they become less and less. Hence the expression that nerve cells are not restored. However, research data from the late 20th century prove the opposite. In one area of the brain, the hippocampus, new neurons can grow even in adults.

Neurons can be quite large, several meters long (corticospinal and afferent). In 1898, renowned nervous system specialist Camillo Golgi reported his discovery of a ribbon-like apparatus specializing in neurons in the cerebellum. This device now bears the name of its creator and is known as the "Golgi apparatus".

From the way the nerve cell is arranged, its definition follows as the main structural and functional element of the nervous system, the study of the simple principles of which can serve as the key to solving many problems. This mainly concerns the autonomic nervous system, which includes hundreds of millions of interconnected cells.

The human nervous system is a stimulator of the muscular system, which we talked about in. As we already know, muscles are needed to move parts of the body in space, and we even studied specifically which muscles are designed for which work. But what powers the muscles? What and how makes them work? This will be discussed in this article, from which you will draw the necessary theoretical minimum for mastering the topic indicated in the title of the article.

First of all, it is worth saying that the nervous system is designed to transmit information and commands to our body. The main functions of the human nervous system are the perception of changes within the body and the space surrounding it, the interpretation of these changes and the response to them in the form of a certain form (including muscle contraction).

Nervous system- a set of different, interacting nervous structures, which, along with the endocrine system, provides coordinated regulation of the work of most of the body's systems, as well as a response to changes in the conditions of the external and internal environment. This system combines sensitization, motor activity and the correct functioning of such systems as endocrine, immune and not only.

The structure of the nervous system

Excitability, irritability and conductivity are characterized as functions of time, that is, it is a process that occurs from irritation to the appearance of an organ response. The propagation of a nerve impulse in the nerve fiber occurs due to the transition of local foci of excitation to neighboring inactive areas of the nerve fiber. The human nervous system has the property of transforming and generating the energies of the external and internal environment and transforming them into a nervous process.

The structure of the human nervous system: 1- brachial plexus; 2- musculocutaneous nerve; 3- radial nerve; 4- median nerve; 5- ilio-hypogastric nerve; 6- femoral-genital nerve; 7- locking nerve; 8- ulnar nerve; 9- common peroneal nerve; 10 - deep peroneal nerve; 11- superficial nerve; 12- brain; 13- cerebellum; 14- spinal cord; 15- intercostal nerves; 16 - hypochondrium nerve; 17- lumbar plexus; 18 - sacral plexus; 19- femoral nerve; 20 - sexual nerve; 21- sciatic nerve; 22 - muscular branches of the femoral nerves; 23 - saphenous nerve; 24- tibial nerve

The nervous system functions as a whole with the sense organs and is controlled by the brain. The largest part of the latter is called the cerebral hemispheres (in the occipital region of the skull there are two smaller hemispheres of the cerebellum). The brain is connected to the spinal cord. The right and left cerebral hemispheres are interconnected by a compact bundle of nerve fibers called the corpus callosum.

Spinal cord- the main nerve trunk of the body - passes through the canal formed by the openings of the vertebrae, and stretches from the brain to the sacral spine. From each side of the spinal cord, nerves depart symmetrically to different parts of the body. Touch in general terms is provided by certain nerve fibers, the innumerable endings of which are located in the skin.

Classification of the nervous system

The so-called types of the human nervous system can be represented as follows. The whole integral system is conditionally formed: the central nervous system - CNS, which includes the brain and spinal cord, and the peripheral nervous system - PNS, which includes numerous nerves extending from the brain and spinal cord. The skin, joints, ligaments, muscles, internal organs and sensory organs send input signals to the CNS via PNS neurons. At the same time, outgoing signals from the central NS, the peripheral NS sends to the muscles. As a visual material, below, in a logically structured way, the entire human nervous system (diagram) is presented.

central nervous system- the basis of the human nervous system, which consists of neurons and their processes. The main and characteristic function of the central nervous system is the implementation of reflective reactions of various degrees of complexity, which are called reflexes. The lower and middle sections of the central nervous system - the spinal cord, medulla oblongata, midbrain, diencephalon and cerebellum - control the activity of individual organs and systems of the body, implement communication and interaction between them, ensure the integrity of the body and its correct functioning. The highest department of the central nervous system - the cerebral cortex and the nearest subcortical formations - for the most part controls the communication and interaction of the body as an integral structure with the outside world.

Peripheral nervous system- is a conditionally allocated part of the nervous system, which is located outside the brain and spinal cord. Includes nerves and plexuses of the autonomic nervous system, connecting the central nervous system with the organs of the body. Unlike the CNS, the PNS is not protected by bones and can be subject to mechanical damage. In turn, the peripheral nervous system itself is divided into somatic and autonomic.

somatic nervous system- part of the human nervous system, which is a complex of sensory and motor nerve fibers responsible for the excitation of muscles, including skin and joints. She also manages the coordination of body movements, and the receipt and transmission of external stimuli. This system performs actions that a person controls consciously.
autonomic nervous system divided into sympathetic and parasympathetic. The sympathetic nervous system governs the response to danger or stress and, among other things, can cause an increase in heart rate, an increase in blood pressure, and excitation of the senses by increasing the level of adrenaline in the blood. The parasympathetic nervous system, in turn, controls the state of rest, and regulates pupillary contraction, slowing of the heart rate, dilation of blood vessels, and stimulation of the digestive and genitourinary systems.

Above you can see a logically structured diagram, which shows the parts of the human nervous system, in the order corresponding to the above material.

The structure and functions of neurons

All movements and exercises are controlled by the nervous system. The main structural and functional unit of the nervous system (both central and peripheral) is the neuron. Neurons are excitable cells that are capable of generating and transmitting electrical impulses (action potentials).

The structure of the nerve cell: 1- cell body; 2- dendrites; 3- cell nucleus; 4- myelin sheath; 5- axon; 6- end of the axon; 7- synaptic thickening

The functional unit of the neuromuscular system is the motor unit, which consists of a motor neuron and the muscle fibers innervated by it. Actually, the work of the human nervous system on the example of the process of muscle innervation occurs as follows.

The cell membrane of the nerve and muscle fiber is polarized, that is, there is a potential difference across it. Inside the cell contains a high concentration of potassium ions (K), and outside - sodium ions (Na). At rest, the potential difference between the inner and outer side of the cell membrane does not lead to the appearance of an electric charge. This defined value is the resting potential. Due to changes in the external environment of the cell, the potential on its membrane constantly fluctuates, and if it rises, and the cell reaches its electrical threshold of excitation, there is a sharp change in the electrical charge of the membrane, and it begins to conduct an action potential along the axon to the innervated muscle. By the way, in large muscle groups, one motor nerve can innervate up to 2-3 thousand muscle fibers.

In the diagram below, you can see an example of how a nerve impulse travels from the moment a stimulus occurs to receiving a response to it in each individual system.

Nerves are connected to each other through synapses, and to muscles through neuromuscular junctions. Synapse- this is the place of contact between two nerve cells, and - the process of transmitting an electrical impulse from a nerve to a muscle.

synaptic connection: 1- neural impulse; 2- receiving neuron; 3- axon branch; 4- synaptic plaque; 5- synaptic cleft; 6 - neurotransmitter molecules; 7- cell receptors; 8 - dendrite of the receiving neuron; 9- synaptic vesicles

Neuromuscular contact: 1 - neuron; 2- nerve fiber; 3- neuromuscular contact; 4- motor neuron; 5- muscle; 6- myofibrils

Thus, as we have already said, the process of physical activity in general and muscle contraction in particular is completely controlled by the nervous system.

Conclusion

Today we learned about the purpose, structure and classification of the human nervous system, as well as how it is related to its motor activity and how it affects the work of the whole organism as a whole. Since the nervous system is involved in the regulation of the activity of all organs and systems of the human body, including, and possibly, first of all, the cardiovascular system, in the next article from the series on the systems of the human body, we will move on to its consideration.

nervous tissue- the main structural element of the nervous system. IN composition of nervous tissue contains highly specialized nerve cells neurons, And neuroglial cells performing supporting, secretory and protective functions.

Neuron is the main structural and functional unit of the 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 performance of the functions of a neuron is facilitated by the synthesis in its axoplasm of substances-transmitters - neurotransmitters: acetylcholine, catecholamines, etc.

The number of brain neurons approaches 10 11 . One neuron can have up to 10,000 synapses. If these elements are considered information storage cells, then we can conclude that the nervous system can store 10 19 units. information, i.e. capable of containing almost all the knowledge accumulated by mankind. Therefore, the notion that the human brain remembers everything that happens in the body and when it communicates with the environment is quite reasonable. However, the brain cannot extract from all the information that is stored in it.

Certain types of neural organization are characteristic of various brain structures. Neurons that regulate a single function form the so-called groups, ensembles, columns, nuclei.

Neurons differ in structure and function.

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

According to functional properties allocate afferent(or centripetal) neurons that carry excitation from receptors in, efferent, motor, motor neurons(or centrifugal), transmitting excitation 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 process extending from the cell body is divided into two branches in a T-shape, 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 (Fig. 1). 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. They can also be bipolar, such as retinal neurons that have a short branching dendrite and a long axon. Motor neurons are located mainly in the anterior horns of the spinal cord.

Rice. 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. Virchow and named by him neuroglia, which means "nerve glue". Neuroglia 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 CNS of mammals reaches 140 billion. With age, the number of neurons in the human brain decreases, and the number of glial cells increases.

It has been established that neuroglia is related to the metabolism in the nervous tissue. Some neuroglia cells secrete substances that affect the state of excitability of neurons. It is noted that the secretion of these cells changes in various mental states. Long-term trace processes in the CNS are associated with the functional state of neuroglia.

Types of glial cells

According to the nature of the structure of glial cells and their location in the CNS, they distinguish:

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

Glial cells perform supporting and protective functions for neurons. They are included in the structure. astrocytes are the most numerous glial cells, filling the spaces between neurons and covering. They prevent the spread of neurotransmitters diffusing from the synaptic cleft into the CNS. Astrocytes have receptors for neurotransmitters, the activation of which can cause fluctuations in the membrane potential difference and changes in the metabolism of astrocytes.

Astrocytes tightly surround the capillaries of the blood vessels of the brain, located between them and neurons. On this basis, it is suggested that astrocytes play an important role in the metabolism of neurons, by regulating 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 during high neuronal activity. Gap junction channels are formed in the areas of astrocytes' tight fit, through which astrocytes can exchange various small ions and, in particular, K+ ions. This increases the ability of them to absorb K+ ions. Uncontrolled accumulation of K+ ions in the interneuronal space would lead to an increase in the excitability of neurons. Thus, astrocytes, absorbing an excess of K+ ions from the interstitial fluid, prevent an increase in the excitability of neurons 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 are involved 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 brain dysfunction.

Neurons and astrocytes are separated by intercellular gaps of 15–20 µm, 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 nervous tissue and brain vessels, nervous tissue and brain membranes in the process of growth and development of nervous tissue.

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

microglial cells make up 5-20% of the total number of glial cells and are scattered throughout the CNS. It has been established that the antigens of their surface are identical to the antigens 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 when the nervous tissue is damaged, the number of phagocytic cells increases due to blood macrophages and activation of the phagocytic properties of microglia. They remove dead neurons, glial cells and their structural elements, phagocytize foreign particles.

Schwann cells form the myelin sheath of peripheral nerve fibers outside the CNS. The membrane of this cell repeatedly wraps around, and the thickness of the resulting myelin sheath can exceed the diameter of the nerve fiber. The length of the myelinated sections of the nerve fiber is 1-3 mm. In the intervals between them (the intercepts of Ranvier), 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 at the Ranvier interception membrane, which provides a higher speed of nerve impulse conduction in myelinated nerve fibers compared to unmyelinated ones.

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

Structure and functions of neurons

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

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

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

Rice. 2. Structure of a neuron

body of the nerve cell

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

The body of the neuron contains the neuroplasm and the nucleus separated from it by membranes, the rough and smooth endoplasmic reticulum, the Golgi apparatus, and 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 body of the neuron, 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 necessary for the synthesis of neurotransmitters, are delivered to the axon 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 collapse. If the body of the neuron is preserved, but the process is damaged, then its slow recovery (regeneration) and the restoration of the innervation of denervated muscles or organs occur.

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 axon.

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 maintain the asymmetry of ion concentrations on both sides of the membrane. Consequently, the neuron is in constant readiness not only to perceive various signals, but also to respond to them - the generation of nerve impulses and their use to control the functions of other cells.

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

Dendrites of a nerve cell

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). On the dendrites of a neuron there are thousands of synapses formed by the axons or dendrites of other neurons.

Rice. 3. Synaptic contacts of the interneuron. The arrows on the left show the flow 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 dendritic membrane 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 elevations, or outgrowths (1-2 microns), 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 region of spines, secondary messengers of intracellular signal transduction were found, as well as ribosomes, on which protein is synthesized in response to synaptic signals. The exact role of the spines remains unknown, but it is clear that they increase the surface area of the dendritic tree for synapse formation. Spines are also neuron structures for receiving input signals and processing them. Dendrites and spines ensure the transmission of information from the periphery to the body of the neuron. The dendritic membrane is polarized in mowing 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 across the membrane in the form of local circular currents (electrotonically) that occur between the postsynaptic membranes and the areas of the dendrite membrane adjacent to them.

Local currents during their propagation along the dendrite membrane attenuate, but they turn out to be sufficient in magnitude to transmit to the membrane of the neuron body the signals received through the synaptic inputs to the dendrites. No voltage-gated sodium and potassium channels have yet been found in the dendritic membrane. It does not have excitability and the ability to generate action potentials. However, it is known that the action potential arising on the membrane of the axon 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 the dendritic tree and the number of synapses are reduced in some areas of the cerebral cortex of the elderly.

neuron axon

axon - a branch of a nerve cell that is not found in other cells. Unlike dendrites, the number of which is different for a neuron, the axon of all neurons is the same. Its length can reach up to 1.5 m. At the exit point of the axon from the body of the neuron, there is a thickening - the axon mound, covered with a plasma membrane, which is soon covered with myelin. The area of the axon hillock that is not covered by myelin is called the initial segment. The axons of neurons, up to their terminal branches, are covered with a myelin sheath, interrupted by interceptions of Ranvier - microscopic non-myelinated areas (about 1 micron).

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

Properties of the membrane covering the body and axon of a neuron, are different. This difference primarily concerns the permeability of the membrane for 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 axon membrane, especially in the area of Ranvier nodes, 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 the axon regions more distant from the cell body, the value of the transmembrane potential is about 70 mV. The low value of 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 the 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 axon hillock membrane to a critical level (E k), then the neuron will respond to signals from other nerve cells coming to it 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 lines from other neurons can prevent the generation of a nerve impulse.

Classification and types of neurons

Classification of neurons is carried out both according to morphological and functional features.

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

According to the nature of connections with other cells and the function performed, they distinguish touch, plug-in And motor neurons. Touch neurons are also called afferent neurons, and their processes are centripetal. 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 classified as motor, or efferent, their axons are called centrifugal.

Afferent (sensory) neurons perceive information with 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 pseudounipolar neurons, the axon and dendrite of which depart from the body of the neuron together and then separate. The dendrite follows the periphery to organs and tissues as part of sensory or mixed nerves, and the axon as part of the posterior roots enters the dorsal horns of the spinal cord or as part of the cranial nerves into the brain.

Insertion, or associative, neurons perform the functions of processing incoming information and, in particular, ensure 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 information received and transmitting efferent nerve impulses from the brain and spinal cord to the cells of the executive (effector) organs.

Integrative activity of a neuron

Each neuron receives a huge amount of signals through numerous synapses located on its dendrites and body, as well as through molecular receptors in plasma membranes, cytoplasm and nucleus. Many different types of neurotransmitters, neuromodulators, and other signaling molecules are used in signaling. Obviously, in order to form a response to the simultaneous receipt of multiple signals, the 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.

The perception and processing of signals arriving at the neuron is carried out with the participation of dendrites, the cell body, and the axon hillock of the neuron (Fig. 4).

Rice. 4. Integration of signals by a neuron.

One of the options for their processing and integration (summation) is the transformation in synapses and the summation of postsynaptic potentials on the membrane of the body and processes of the neuron. The perceived signals are converted in the 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 are shown in the diagram as light circles) or hyperpolarizing (TPSP - synapses are shown in the diagram as black circles). Many signals can simultaneously arrive at different points of the neuron, some of which are transformed into EPSPs, while others are transformed into IPSPs.

These oscillations of the potential difference propagate with the help of local circular currents along the neuron membrane in the direction of the axon hillock in the form of waves of depolarization (in the white diagram) and hyperpolarization (in the black diagram), overlapping each other (in the diagram, gray areas). With this superimposition of the amplitude of the waves of one direction, they are summed up, and the opposite ones are reduced (smoothed out). 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 axon hillock membrane and generation of a nerve impulse (cases 1 and 2 in Fig. 4), or its hyperpolarization and prevention of the occurrence of a nerve impulse (cases 3 and 4 in Fig. 4).

In order to shift the potential difference of the axon hillock membrane (about 30 mV) to Ek, it must be depolarized by 10-20 mV. This will lead to the opening of the voltage-gated sodium channels present in it and the generation of a nerve impulse. Since the depolarization of the membrane can reach up to 1 mV upon receipt of one AP and its transformation into an EPSP, and all propagation to the axon hillock occurs with attenuation, generation of a nerve impulse requires simultaneous delivery of 40-80 nerve impulses from other neurons to the neuron through excitatory synapses and summation the same amount of EPSP.

Rice. 5. Spatial and temporal summation of EPSP by a neuron; (a) EPSP to a single stimulus; and — EPSP to multiple stimulation from different afferents; c — EPSP for frequent stimulation through a single nerve fiber

If at this time a neuron receives a certain number of nerve impulses 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 excitatory synapses. Under conditions when signals coming through inhibitory synapses cause hyperpolarization of the neuron membrane equal to or greater than the depolarization caused by signals coming through excitatory synapses, depolarization of the axon colliculus membrane will be impossible, the neuron will not generate nerve impulses and will become inactive.

The neuron also performs time summation EPSP and IPTS signals coming to it almost simultaneously (see Fig. 5). The changes in the potential difference caused by them in the near-synaptic areas can also be algebraically summed up, which is called temporal 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 the neuron from other cells, the more frequently it generates response nerve impulses that are sent 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 arising on the membrane of the axon 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 propagating action potential momentarily smooths out all local currents present on the membrane, resets the potentials, and contributes to a more efficient perception of new information by the neuron.

Molecular receptors take part 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 mediators), transformation of perceived 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.

The transformation of signals by the metabotropic molecular receptors of the neuron is accompanied by its response in the form of a cascade of intracellular transformations. The response of the neuron in this case may be an acceleration of the overall 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 the more significant of them and weakening to the less significant ones.

The receipt by a neuron of a number of signals may be accompanied by the expression or repression of certain genes, for example, those controlling the synthesis of neuromodulators of a peptide nature. Since they are delivered to the axon terminals of the neuron and used in them to enhance or weaken the action of its neurotransmitters on other neurons, the neuron, in response to the signals it receives, can, depending on the information received, have a stronger or weaker effect on other nerve cells controlled by it. Considering that the modulating effect of neuropeptides can last for a long time, the influence of a neuron on other nerve cells can also last for a long time.

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

neural circuits

CNS neurons interact with each other, forming various synapses at the point of contact. The resulting neural foams greatly increase 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 more 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 one will form an axonome synapse on the body of the first neuron. Local neural networks can act as traps in which nerve impulses are able to circulate for a long time in a circle formed by several neurons.

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

Circular circulation of nerve impulses along local neural circuits performs the function of transforming the rhythm of excitations, provides the possibility of prolonged excitation after the cessation of signals coming to them, and 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 realized in the simplest local neural circuit of the spinal cord, formed by the a-motoneuron and the Renshaw cell.

Rice. 6. The simplest neural circuits of the CNS. Description in text

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

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 widely distributed in the CNS. For example, the axons of many neurons in the sensory fields of the cortex converge on the pyramidal neurons of the primary motor cortex. The axons of thousands of sensory and intercalary neurons of various levels of the CNS converge on the 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.

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

The main function of the nervous system is the transmission of information using electrical stimuli. For this you need:

1. Exchange of chemicals with the environment - membrane-long information processes.

2. Fast signaling - special areas on the membrane - synapses

3. The mechanism of rapid signal exchange between cells - special chemicals - mediators secreted by some cells and perceived by others in synapses

4. The cell responds to changes in synapses located on short processes - dendrites using slow changes in electrical potentials

5. The cell transmits signals over long distances using fast electrical signals along long processes - axons

axon- one neuron, has an extended structure, conducts fast electrical impulses from the cell body

Dendrites- can be many, branching, short, conducts slow gradual electrical impulses to the cell body

Nerve cell, or neuron, consists of a body and processes of two types. Body The neuron is represented by the nucleus and the cytoplasm surrounding it. It is the metabolic center of the nerve cell; when it is destroyed, she dies. The bodies of neurons are located mainly in the brain and spinal cord, i.e. in the central nervous system (CNS), where their clusters form gray matter of the brain. Clusters of nerve cell bodies outside the CNS form ganglia, or ganglia.

Short, tree-like processes extending from the body of a neuron are called dendrites. They perform the functions of perceiving irritation and transmitting excitation to the body of the neuron.

The most powerful and longest (up to 1 m) non-branching process is called an axon, or nerve fiber. Its function is to conduct excitation from the body of the nerve cell to the end of the axon. It is covered with a special white lipid sheath (myelin), which plays the role of protecting, nourishing and isolating nerve fibers from each other. Accumulations of axons in the CNS form the white matter of the brain. Hundreds and thousands of nerve fibers that go beyond the CNS, with the help of connective tissue, are combined into bundles - nerves that give numerous branches to all organs.

Lateral branches depart from the ends of axons, ending in extensions - axopal endings, or terminals. This is the zone of contact with other nerve, muscle or glandular marks. It is called a synapse, the function of which is the transmission of excitation. One neuron can connect to hundreds of other cells through its synapses.

There are three types of neurons according to their functions. Sensitive (centripetal) neurons perceive irritation from receptors that are excited under the influence of stimuli from the external environment or from the human body itself, and in the form of a nerve impulse transmit excitation from the periphery to the central nervous system. Motor (centrifugal) neurons send a nerve signal from the central nervous system to muscles, glands, t i.e. to the periphery. Nerve cells that perceive excitation from other neurons and transmit it also to nerve cells are interneurons, or interneurons. They are located in the CNS. Nerves, which include both sensory and motor fibers, are called mixed.

Anya: Neurons, or nerve cells, are the building blocks of the brain. Although they have the same genes, the same general structure and the same biochemical apparatus as other cells, they also have unique features that make the function of the brain quite different from that of, say, the liver. It is believed that the human brain consists of 10 to 10 neurons: about the same number as the stars in our galaxy. No two neurons are identical in appearance. Despite this, their forms usually fit into a small number of categories, and most neurons have certain structural features that make it possible to distinguish three regions of the cell: the cell body, dendrites, and axon.

The cell body - soma, contains the nucleus and the biochemical apparatus for the synthesis of enzymes and various molecules necessary for the life of the cell. Typically, the body is approximately spherical or pyramidal in shape, ranging in size from 5 to 150 microns in diameter. Dendrites and axons are processes extending from the body of a neuron. Dendrites are thin tubular outgrowths that branch many times, forming, as it were, a crown of a tree around the body of a neuron (dendron tree). Nerve impulses travel along the dendrites to the body of the neuron. Unlike numerous dendrites, the axon is single and differs from dendrites both in structure and in the properties of its outer membrane. The length of the axon can reach one meter, it practically does not branch, forming processes only at the end of the fiber, its name comes from the word axis (ass-axis). Along the axon, the nerve impulse leaves the cell body and is transmitted to other nerve cells or executive organs - muscles and glands. All axons are enclosed in a sheath of Schwann cells (a type of glial cell). In some cases, Schwann cells simply wrap a thin layer around the axon. In many cases, the Schwann cell coils around the axon, forming several dense layers of insulation called myelin. The myelin sheath is interrupted approximately every millimeter along the length of the axon by narrow gaps - the so-called nodes of Ranvier. In axons with this type of sheath, the propagation of a nerve impulse occurs by jumping from node to node, where the extracellular fluid is in direct contact with the cell membrane. Such conduction of a nerve impulse is called saltotropic. The evolutionary meaning of the myelin sheath, apparently, is to save the metabolic energy of the neuron. Generally, myelinated nerve fibers conduct nerve impulses faster than unmyelinated ones.

According to the number of processes, neurons are divided into unipolar, bipolar and multipolar.

According to the structure of the cell body, neurons are divided into stellate, pyramidal, granular, oval, etc.

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.

Overview

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, grasp, 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 is apparently synthesized in the body of the neuron, transported to the growth cone in the form of vesicles, and incorporated here into the plasma membrane by exocytosis, thus lengthening the outgrowth 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.