The structure of nerve cells. 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 the 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.

Nervous tissue is a collection of interconnected nerve cells (neurons, neurocytes) and auxiliary elements (neuroglia), which regulates the activity of all organs and systems of living organisms. This is the main element of the nervous system, which is divided into central (includes the brain and spinal cord) and peripheral (consisting of nerve nodes, trunks, endings).

The main functions of the nervous tissue

Perception of irritation;
the formation of a nerve impulse;
rapid delivery of excitation to the central nervous system;
data storage;
production of mediators (biologically active substances);
adaptation of the organism to changes in the external environment.

properties of nervous tissue

Regeneration- occurs very slowly and is possible only in the presence of an intact perikaryon. Restoration of the lost shoots goes by germination.
Braking- prevents the occurrence of arousal or weakens it
Irritability- response to the influence of the external environment due to the presence of receptors.
Excitability- generation of an impulse when the threshold value of irritation is reached. There is a lower threshold of excitability, at which the smallest influence on the cell causes excitation. The upper threshold is the amount of external influence that causes pain.

The structure and morphological characteristics of nerve tissues

The main structural unit is neuron. It has a body - the perikaryon (in which the nucleus, organelles and cytoplasm are located) and several processes. It is the processes that are the hallmark of the cells of this tissue and serve to transfer excitation. Their length ranges from micrometers to 1.5 m. The bodies of neurons are also of different sizes: from 5 microns in the cerebellum to 120 microns in the cerebral cortex.

Until recently, it was believed that neurocytes are not capable of division. It is now known that the formation of new neurons is possible, although only in two places - this is the subventricular zone of the brain and the hippocampus. The lifespan of neurons is equal to the lifespan of an individual. Every person at birth has about trillion neurocytes and in the process of life loses 10 million cells every year.

offshoots There are two types - dendrites and axons.

The structure of the axon. It starts from the body of the neuron as an axon mound, does not branch out throughout, and only at the end is divided into branches. An axon is a long process of a neurocyte that carries out the transmission of excitation from the perikaryon.

The structure of the dendrite. At the base of the cell body, it has a cone-shaped extension, and then it is divided into many branches (this is the reason for its name, "dendron" from ancient Greek - a tree). The dendrite is a short process and is necessary for the translation of the impulse to the soma.

According to the number of processes, neurocytes are divided into:

unipolar (there is only one process, the axon);
bipolar (both axon and dendrite are present);
pseudo-unipolar (one process departs from some cells at the beginning, but then it divides into two and is essentially bipolar);
multipolar (have many dendrites, and among them there will be only one axon).

Multipolar neurons prevail in the human body, bipolar neurons are found only in the retina of the eye, in the spinal nodes - pseudo-unipolar. Monopolar neurons are not found at all in the human body; they are characteristic only of poorly differentiated nervous tissue.

neuroglia

Neuroglia is a collection of cells that surrounds neurons (macrogliocytes and microgliocytes). About 40% of the CNS is accounted for by glial cells, they create conditions for the production of excitation and its further transmission, perform supporting, trophic, and protective functions.

Macroglia:

Ependymocytes- are formed from glioblasts of the neural tube, line the canal of the spinal cord.

Astrocytes- stellate, small in size with numerous processes that form the blood-brain barrier and are part of the gray matter of the GM.

Oligodendrocytes- the main representatives of neuroglia, surround the perikaryon along with its processes, performing the following functions: trophic, isolation, regeneration.

neurolemocytes- Schwann cells, their task is the formation of myelin, electrical insulation.

microglia - consists of cells with 2-3 branches that are capable of phagocytosis. Provides protection against foreign bodies, damage, as well as removal of products of apoptosis of nerve cells.

Nerve fibers- these are processes (axons or dendrites) covered with a sheath. They are divided into myelinated and unmyelinated. Myelinated in diameter from 1 to 20 microns. It is important that myelin is absent at the junction of the sheath from the perikaryon to the process and in the area of axonal ramifications. Unmyelinated fibers are found in the autonomic nervous system, their diameter is 1-4 microns, the impulse travels at a speed of 1-2 m/s, which is much slower than myelinated ones, they have a transmission speed of 5-120 m/s.

Neurons are subdivided according to functionality:

Afferent- that is, sensitive, accept irritation and are able to generate an impulse;
associative- perform the function of impulse translation between neurocytes;
efferent- complete the transfer of the impulse, performing a motor, motor, secretory function.

Together they form reflex arc, which ensures the movement of the impulse in only one direction: from sensory fibers to motor ones. One individual neuron is capable of multidirectional transmission of excitation, and only as part of a reflex arc does a unidirectional impulse flow occur. This is due to the presence of a synapse in the reflex arc - an interneuronal contact.

Synapse consists of two parts: presynaptic and postsynaptic, between them there is a gap. The presynaptic part is the end of the axon that brought the impulse from the cell, it contains mediators, it is they that contribute to the further transmission of excitation to the postsynaptic membrane. The most common neurotransmitters are: dopamine, norepinephrine, gamma-aminobutyric acid, glycine, for which there are specific receptors on the surface of the postsynaptic membrane.

Chemical composition of nervous tissue

Water is contained in a significant amount in the cerebral cortex, less in white matter and nerve fibers.

Protein substances represented by globulins, albumins, neuroglobulins. Neurokeratin is found in the white matter of the brain and axon processes. Many proteins in the nervous system belong to mediators: amylase, maltase, phosphatase, etc.

The chemical composition of the nervous tissue also includes carbohydrates are glucose, pentose, glycogen.

Among fat phospholipids, cholesterol, cerebrosides were found (it is known that newborns do not have cerebrosides, their number gradually increases during development).

trace elements in all structures of the nervous tissue are distributed evenly: Mg, K, Cu, Fe, Na. Their importance is very great for the normal functioning of a living organism. So magnesium is involved in the regulation of the nervous tissue, phosphorus is important for productive mental activity, potassium ensures the transmission of nerve impulses.

nervous tissue controls all processes in the body.

Nervous tissue is made up of neurons(nerve cells) and neuroglia(intercellular substance). Nerve cells have different shapes. The nerve cell is equipped with tree-like processes - dendrites, which transmit irritations from receptors to the cell body, and a long process - an axon, which ends on the effector cell. Sometimes the axon is not covered by myelin sheath.

Nerve cells are capable of under the influence of irritation come to a state arousal, generate impulses and transmit their. These properties determine the specific function of the nervous system. Neuroglia is organically connected with nerve cells and performs trophic, secretory, protective and support functions.

Nerve cells - neurons, or neurocytes, are process cells. The dimensions of the body of a neuron vary considerably (from 3-4 to 130 microns). The shape of nerve cells is also very different. The processes of nerve cells conduct a nerve impulse from one part of the human body to another, the length of the processes is from several microns to 1.0-1.5 m.

The structure of a neuron. 1 - cell body; 2 - core; 3 - dendrites; 4 - neurite (axon); 5 - branched ending of the neurite; 6 - neurolemma; 7 - myelin; 8 - axial cylinder; 9 - interceptions of Ranvier; 10 - muscle

There are two types of processes of the nerve cell. The processes of the first type conduct impulses from the body of the nerve cell to other cells or tissues of the working organs, they are called neurites, or axons. A nerve cell always has only one axon, which ends with a terminal apparatus on another neuron or in a muscle, gland. The processes of the second type are called dendrites, they branch like a tree. Their number in different neurons is different. These processes conduct nerve impulses to the body of the nerve cell. The dendrites of sensitive neurons have special perceptive apparatuses at their peripheral end - sensitive nerve endings, or receptors.

Classification of neurons by function:

perceiving (sensitive, sensory, receptor). They serve to perceive signals from the external and internal environment and transmit them to the central nervous system;
contact (intermediate, intercalary, interneurons). Provide processing, storage and transmission of information to motor neurons. Most of them are in the central nervous system;
motor (efferent). Control signals are formed and transmitted to peripheral neurons and executive organs.

Types of neurons by the number of processes:

unipolar - having one process;
pseudo-unipolar - one process departs from the body, which then divides into 2 branches;
bipolar - two processes, one dendrite, the other axon;
multipolar - have one axon and many dendrites.

Neurons(nerve cells). A - multipolar neuron; B - pseudounipolar neuron; B - bipolar neuron; 1 - axon; 2 - dendrite

Sheathed axons are called nerve fibers. Distinguish:

continuous- covered with a continuous membrane, are part of the autonomic nervous system;
pulpy- covered with a complex, discontinuous sheath, impulses can pass from one fiber to other tissues. This phenomenon is called irradiation.

Nerve endings. A - motor ending on the muscle fiber: 1 - nerve fiber; 2 - muscle fiber; B - sensitive endings in the epithelium: 1 - nerve endings; 2 - epithelial cells

Sensory nerve endings receptors) are formed by the terminal branches of the dendrites of sensory neurons.

exteroreceptors perceive irritation from the external environment;
interoreceptors perceive irritation from internal organs;
proprioreceptors perceiving irritations from the inner ear and articular bags.

According to their biological significance, receptors are divided into: food, genital, defensive.

According to the nature of the response, receptors are divided into: motor- located in the muscles; secretory- in the glands; vasomotor- in the blood vessels.

Effector- an executive link of nervous processes. Effectors are of two types - motor and secretory. Motor (motor) nerve endings are terminal branches of neurites of motor cells in muscle tissue and are called neuromuscular endings. Secretory endings in the glands form neuroglandular endings. These types of nerve endings represent a neuro-tissue synapse.

Communication between nerve cells is carried out with the help of synapses. They are formed by terminal branches of the neurite of one cell on the body, dendrites or axons of another. In the synapse, the nerve impulse travels in only one direction (from the neurite to the body or dendrites of another cell). In different parts of the nervous system, they are arranged differently.

Nerve cell Not to be confused with neutron.

Pyramidal cells of neurons in the mouse cerebral cortex

Neuron(nerve cell) is the structural and functional unit of the nervous system. This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure. There are over one hundred billion neurons in the human body.

Review

The complexity and diversity of the nervous system depends on 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

cell body

The neuron consists of a body with a diameter of 3 to 100 microns, containing a nucleus (with a large number of nuclear pores) and other organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), and processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). 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

Diagram of the structure of a neuron

Synapse

Synapse- the place of contact between two neurons or between a neuron and an effector cell receiving a signal. It serves to transmit a nerve impulse 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.

Classification

Structural classification

Based on the number and arrangement of deindrites 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 near the spinal cord in the intervertebral ganglia, which do not have anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of axonless neurons is poorly understood.

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

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

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

Pseudo-unipolar neurons- are unique in their kind. One sharp point leaves 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).

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 of the sense organs 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 - non-ultimatum.

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

Morphological classification

Nerve cells are stellate and spindle-shaped, pyramidal, granular, pear-shaped, etc.

Development and growth of a neuron

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

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

Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axon transport in a mature neuron. Since the average rate of advance of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs at the far end of the neuron process during the growth of the neuron process. New membrane material is added, apparently, at the end. The growth cone is an area of rapid exocytosis and endocytosis, as evidenced by the many vesicles found here. Small membrane vesicles are transported along the process of the neuron from the cell body to the growth cone with a stream of fast axon transport. Membrane material, apparently, is synthesized in the body of the neuron, transferred to the growth cone in the form of vesicles, and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell.

The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons settle and find a permanent place for themselves.

Types of neurons

Neurons come in several shapes and sizes and are classified according to their morphology and function. For example, the anatomist Camillo Golgi divided neurons into two groups. To the first group, he attributed neurons with long axons, which transmit signals over long distances. To the second group, he attributed neurons with short axons, which could be confused with dendrites.

Neurons are classified according to their structure into the following groups:

Unipolar. The axon and dendrites emerge from the same appendage.
Bipolar. The axon and a single dendrite are located on opposite sides of the soma.
Multipolar. At least two dendrites are located separately from the axon.
Golgi type I. The neuron has a long axon.
Golgi type II. Neurons with axons located locally.
Anaxon neurons. When the axon is indistinguishable from the dendrites.
basket cages- interneurons that form densely woven endings throughout the soma of target cells. Present in the cerebral cortex and cerebellum.
Betz cells. They are large motor neurons.
Lugaro cells- interneurons of the cerebellum.
Medium spiky neurons. Present in the striatum.
Purkinje cells. They are large multipolar neurons of the cerebellum of the Golgi type I.
pyramidal cells. Neurons with a triangular soma of the Golgi II type.
Renshaw Cells. Neurons connected at both ends to alpha motor neurons.
Unipolar racemose cells. Interneurons that have unique dendritic endings in the form of a brush.
Cells of the anterior horn. They are motor neurons located in the spinal cord.
Spindle cages. Interneurons connecting distant regions of the brain.
Afferent neurons. Neurons that transmit signals from tissues and organs to the central nervous system.
Efferent neurons. Neurons that transmit signals from the central nervous system to effector cells.
interneurons that connect neurons in specific areas of the central nervous system.

Action of neurons

All neurons are electrically excitable and maintain voltage across their membranes via metabolically conductive ion pumps coupled with ion channels that are embedded in the membrane to generate ion differentials such as sodium, chloride, calcium, and potassium. Voltage changes in the cross-membrane lead to a change in the functions of voltage-dependent ionic feces. When the voltage changes at a sufficiently high level, the electrochemical impulse causes the generation of an active potential, which quickly moves along the cells of the axon, activating synaptic connections with other cells.

Most nerve cells are the basic type. A certain stimulus causes an electrical discharge in the cell, a discharge similar to that of a capacitor. This produces an electrical impulse of about 50-70 millivolts, which is called the active potential. An electrical impulse propagates along the fiber, along the axons. The pulse propagation speed depends on the fiber, it is about tens of meters per second on average, which is noticeably lower than the propagation speed of electricity, which is equal to the speed of light. As soon as the impulse reaches the axon bundle, it is transmitted to neighboring nerve cells under the action of a chemical mediator.

A neuron acts on other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect of a postsynaptic neuron is determined not by the presynaptic neuron or neurotransmitter, but by the type of receptor that is activated. The neurotransmitter is like a key, and the receptor is a lock. In this case, one key can be used to open "locks" of various types. Receptors, in turn, are classified into excitatory (increasing the rate of transmission), inhibitory (slowing down the rate of transmission) and modulating (causing long-term effects).

Communication between neurons is carried out through synapses, in this place is the end of the axon (axon terminal). Neurons such as Purkinje cells in the cerebellum can have over a thousand dendritic junctions, communicating with tens of thousands of other neurons. Other neurons (large neuronal cells in the supraoptic nucleus) have only one or two dendrites, each receiving thousands of synapses. Synapses can be either excitatory or inhibitory. Some neurons communicate with each other through electrical synapses, which are direct electrical connections between cells.

In a chemical synapse, when the action potential reaches the axon, a voltage opens in the calcium channel, which allows calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to penetrate the membrane, releasing the contents into the synaptic cleft. There is a process of diffusion of mediators through the synaptic cleft, which in turn activate receptors on the postsynaptic neuron. In addition, highly cytosolic calcium in the axon terminal induces mitochondrial calcium uptake, which in turn activates mitochondrial energy metabolism to produce ATP, which maintains continuous neurotransmission.