Nervous. The human nervous system: its structure and features

Nervous system consists of winding networks of nerve cells that make up various interconnected structures and control all the activities of the body, both desired and conscious actions, and reflexes and automatic actions; nervous system allows us to interact with the outside world, and is also responsible for mental activity.

The nervous system consists made up of various interconnected structures that together constitute an anatomical and physiological unit. consists of organs located inside the skull (brain, cerebellum, brain stem) and spine (spinal cord); is responsible for interpreting the state and various needs of the body based on the information received, in order to then generate commands designed to produce appropriate responses.

consists of many nerves that go to the brain (cerebral pairs) and the spinal cord (vertebral nerves); acts as a transmitter of sensory stimuli to the brain and commands from the brain to the organs responsible for their execution. The autonomic nervous system controls the functions of numerous organs and tissues through antagonistic effects: the sympathetic system is activated during anxiety, and the parasympathetic system is activated during rest.

Central nervous system
Includes the spinal cord and brain structures.

The human nervous system is similar in structure to the nervous system of higher mammals, but differs in the significant development of the brain. The main function of the nervous system is to control the vital functions of the entire organism.

Neuron

All organs of the nervous system are built from nerve cells called neurons. A neuron is capable of receiving and transmitting information in the form of a nerve impulse.

Rice. 1. Structure of a neuron.

The body of a neuron has processes with which it communicates with other cells. The short processes are called dendrites, the long ones are called axons.

The structure of the human nervous system

The main organ of the nervous system is the brain. Connected to it is the spinal cord, which looks like a cord about 45 cm long. Together, the spinal cord and brain make up the central nervous system (CNS).

Rice. 2. Scheme of the structure of the nervous system.

The nerves that arise from the central nervous system make up the peripheral part of the nervous system. It consists of nerves and ganglia.

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Nerves are formed from axons, the length of which can exceed 1 m.

Nerve endings contact each organ and transmit information about their condition to the central nervous system.

There is also a functional division of the nervous system into somatic and autonomic (autonomic).

The part of the nervous system that innervates the striated muscles is called somatic. Her work is associated with the conscious efforts of a person.

The autonomic nervous system (ANS) regulates:

circulation;
digestion;
selection;
breath;
metabolism;
smooth muscle function.

Thanks to the work of the autonomic nervous system, many processes of normal life occur that we do not consciously regulate and usually do not notice.

The importance of the functional division of the nervous system in ensuring the normal functioning of the finely tuned mechanisms of the internal organs, independent of our consciousness.

The highest organ of the ANS is the hypothalamus, located in the intermediate part of the brain.

The VNS is divided into 2 subsystems:

sympathetic;
parasympathetic.

Sympathetic nerves activate organs and control them in situations that require action and increased attention.

Parasympathetic slows down the functioning of organs and turns on during rest and relaxation.

For example, sympathetic nerves dilate the pupil and stimulate the secretion of saliva. Parasympathetic, on the contrary, constrict the pupil and slow down salivation.

Reflex

This is the body's response to irritation from external or internal environment.

The main form of activity of the nervous system is a reflex (from the English reflection - reflection).

An example of a reflex is withdrawing a hand from a hot object. The nerve ending senses high temperature and transmits a signal about it to the central nervous system. A response impulse arises in the central nervous system, going to the muscles of the arm.

Rice. 3. Reflex arc diagram.

The sequence: sensory nerve - CNS - motor nerve is called a reflex arc.

Brain

The brain is distinguished by the strong development of the cerebral cortex, in which the centers of higher education are located. nervous activity.

The characteristics of the human brain sharply distinguished it from the animal world and allowed it to create a rich material and spiritual culture.

What have we learned?

The structure and functions of the human nervous system are similar to those of mammals, but differ in the development of the cerebral cortex with the centers of consciousness, thinking, memory, and speech. The autonomic nervous system controls the body without the participation of consciousness. The somatic nervous system controls body movement. The principle of activity of the nervous system is reflex.

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The nervous system is the center of nerve communications and the body's most important regulatory system: it organizes and coordinates vital actions. But it has only two main functions: stimulating muscles for movement and regulating the functioning of the body, as well as the endocrine system.

The nervous system is divided into the central nervous system and the peripheral nervous system.

From a functional point of view, the nervous system can be divided into somatic (controlling voluntary actions) and autonomic or autonomic (coordinating involuntary actions) systems.

Central nervous system

Includes the spinal cord and brain. Cognitive and emotional functions person. From here all movements are controlled and the weight of feeling is developed.

Brain

In an adult, the brain is one of the heaviest organs in the body, weighing approximately 1300 g.

It is the center of interaction of the nervous system, and its main function is to transmit and respond to received nerve impulses. In its various areas it acts as a mediator of respiratory processes, solving specific problems and hunger.

The brain is divided structurally and functionally into several main parts:

Spinal cord

It is located in the spinal canal and is surrounded by meninges that protect it from injury. In an adult, the length of the spinal cord reaches 42-45 cm and extends from the elongated brain (or the inner part of the brain stem) to the second lumbar vertebra and has a different diameter in different parts of the spine.

31 pairs of peripheral spinal nerves depart from the spinal cord, which connect it to the entire body. Its most important function is to connect various parts of the body to the brain.

Both the brain and spinal cord are protected by three layers of connective tissue. Between the most superficial and middle layers there is a cavity where fluid circulates, which, in addition to protection, also nourishes and cleanses the nerve tissue.

Peripheral nervous system

Consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. It constitutes an intricate network that forms nervous tissue that is not part of the central nervous system and is represented mainly by peripheral nerves responsible for muscles and internal organs.

Cranial nerves

12 pairs of cranial nerves arise from the brain and pass through the openings of the skull.

All cranial nerves are found in the head and neck, with the exception of the tenth nerve (vagus), which also involves various structures of the chest and stomach.

Spinal nerves

Each of the 31 pairs of nerves originate in the dorsal M03IC and then pass through the intervertebral foramina. Their names are associated with the place where they originate: 8 cervical, 12 thoracic, 5 lumbar, 5 cruciate and 1 coccygeal. After passing through the intervertebral foramen, each branch is divided into 2 branches: the anterior, large one, which stretches into the distance to cover the muscles and skin on the front and sides and the skin of the extremities, and the posterior, smaller one, which covers the muscles and skin of the back. The thoracic spinal nerves also communicate with the sympathetic part of the autonomic nervous system. At the top of the neck, the roots of these nerves are very short and located horizontally.

NERVOUS SYSTEM
a complex network of structures that permeates the entire body and ensures self-regulation of its vital functions due to the ability to respond to external and internal influences (stimuli). The main functions of the nervous system are receiving, storing and processing information from the external and internal environment, regulating and coordinating the activities of all organs and organ systems. In humans, like in all mammals, the nervous system includes three main components: 1) nerve cells (neurons); 2) glial cells associated with them, in particular neuroglial cells, as well as cells forming neurilemma; 3) connective tissue. Neurons provide the conduction of nerve impulses; neuroglia performs supporting, protective and trophic functions both in the brain and in the spinal cord, and the neurilemma, consisting mainly of specialized, so-called. Schwann cells, participates in the formation of peripheral nerve fiber sheaths; Connective tissue supports and binds together the various parts of the nervous system. The human nervous system is divided in different ways. Anatomically, it consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system includes the brain and spinal cord, and the PNS, which provides communication between the central nervous system and various parts of the body, includes the cranial and spinal nerves, as well as nerve ganglia and nerve plexuses lying outside the spinal cord and brain.

Neuron. The structural and functional unit of the nervous system is the nerve cell - neuron. It is estimated that there are more than 100 billion neurons in the human nervous system. A typical neuron consists of a body (i.e., the nuclear part) and processes, one usually non-branching process, an axon, and several branching ones - dendrites. The axon carries impulses from the cell body to muscles, glands or other neurons, while the dendrites carry them into the cell body. A neuron, like other cells, has a nucleus and a number of tiny structures - organelles (see also CELL). These include the endoplasmic reticulum, ribosomes, Nissl bodies (tigroid), mitochondria, Golgi complex, lysosomes, filaments (neurofilaments and microtubules).

Nerve impulse. If the stimulation of a neuron exceeds a certain threshold value, then a series of chemical and electrical changes occur at the point of stimulation that spread throughout the entire neuron. The transmitted electrical changes are called nerve impulses. Unlike a simple electrical discharge, which due to the resistance of the neuron will gradually weaken and will be able to cover only a short distance, a much slower “running” nerve impulse is constantly restored (regenerated) in the process of propagation. The concentrations of ions (electrically charged atoms) - mainly sodium and potassium, as well as organic substances - outside and inside the neuron are not the same, so the nerve cell at rest is negatively charged from the inside and positively charged from the outside; As a result, a potential difference appears on the cell membrane (the so-called “resting potential” is approximately -70 millivolts). Any change that reduces the negative charge within the cell and thereby the potential difference across the membrane is called depolarization. The plasma membrane surrounding the neuron is a complex formation consisting of lipids (fats), proteins and carbohydrates. It is practically impenetrable to ions. But some of the protein molecules in the membrane form channels through which certain ions can pass. However, these channels, called ion channels, are not constantly open, but, like gates, can open and close. When a neuron is stimulated, some of the sodium (Na+) channels open at the point of stimulation, allowing sodium ions to enter the cell. The influx of these positively charged ions reduces the negative charge of the inner surface of the membrane in the channel area, which leads to depolarization, which is accompanied by a sharp change in voltage and discharge - the so-called. "action potential", i.e. nerve impulse. The sodium channels then close. In many neurons, depolarization also causes potassium (K+) channels to open, causing potassium ions to leave the cell. The loss of these positively charged ions again increases the negative charge on the inner surface of the membrane. The potassium channels then close. Other membrane proteins also begin to work - the so-called. potassium-sodium pumps that move Na+ out of the cell and K+ into the cell, which, along with the activity of potassium channels, restores the original electrochemical state (resting potential) at the point of stimulation. Electrochemical changes at the point of stimulation cause depolarization at an adjacent point on the membrane, triggering the same cycle of changes in it. This process is constantly repeated, and at each new point where depolarization occurs, an impulse of the same magnitude is born as at the previous point. Thus, along with the renewed electrochemical cycle, the nerve impulse spreads along the neuron from point to point. Nerves, nerve fibers and ganglia. A nerve is a bundle of fibers, each of which functions independently of the others. The fibers in a nerve are organized into groups surrounded by specialized connective tissue, in which vessels pass that supply nerve fibers with nutrients and oxygen and remove carbon dioxide and decay products. The nerve fibers along which impulses travel from peripheral receptors to the central nervous system (afferent) are called sensitive or sensory. Fibers that transmit impulses from the central nervous system to muscles or glands (efferent) are called motor or motor. Most nerves are mixed and consist of both sensory and motor fibers. A ganglion (nerve ganglion) is a collection of neuron cell bodies in the peripheral nervous system. The axonal fibers in the PNS are surrounded by the neurilemma, a sheath of Schwann cells that are located along the axon, like beads on a string. A significant number of these axons are covered with an additional sheath of myelin (a protein-lipid complex); they are called myelinated (pulpy). Fibers surrounded by neurilemma cells, but not covered with a myelin sheath, are called unmyelinated (unmyelinated). Myelinated fibers are found only in vertebrates. The myelin sheath is formed from plasma membrane Schwann cells, which winds onto the axon like a skein of tape, forming layer after layer. The section of the axon where two adjacent Schwann cells touch each other is called the node of Ranvier. In the central nervous system, the myelin sheath of nerve fibers is formed by a special type of glial cells - oligodendroglia. Each of these cells forms the myelin sheath of several axons at once. Unmyelinated fibers in the CNS lack a sheath of any special cells. The myelin sheath speeds up the conduction of nerve impulses that “jump” from one node of Ranvier to another, using this sheath as a connecting electrical cable. The speed of impulse conduction increases with the thickening of the myelin sheath and ranges from 2 m/s (along unmyelinated fibers) to 120 m/s (along fibers especially rich in myelin). For comparison: propagation speed electric current over metal wires - from 300 to 3000 km/s.
Synapse. Each neuron has specialized connections to muscles, glands, or other neurons. The area of functional contact between two neurons is called a synapse. Interneuron synapses are formed between different parts of two nerve cells: between an axon and a dendrite, between an axon and a cell body, between a dendrite and a dendrite, between an axon and an axon. A neuron that sends an impulse to a synapse is called presynaptic; the neuron receiving the impulse is postsynaptic. The synaptic space has the shape of a cleft. A nerve impulse propagating along the membrane of a presynaptic neuron reaches the synapse and stimulates the release of a special substance - a neurotransmitter - into a narrow synaptic cleft. Neurotransmitter molecules diffuse across the gap and bind to receptors on the membrane of the postsynaptic neuron. If a neurotransmitter stimulates a postsynaptic neuron, its action is called excitatory; if it suppresses, it is called inhibitory. The result of the summation of hundreds and thousands of excitatory and inhibitory impulses simultaneously flowing to a neuron is the main factor determining whether this postsynaptic neuron will generate a nerve impulse in at the moment. In a number of animals (for example, the lobster), a particularly close connection is established between the neurons of certain nerves with the formation of either an unusually narrow synapse, the so-called. gap junction, or, if the neurons are in direct contact with each other, tight junction. Nerve impulses pass through these connections not with the participation of a neurotransmitter, but directly, through electrical transmission. Mammals, including humans, also have a few tight junctions of neurons.
Regeneration. By the time a person is born, all his neurons and most of the interneuron connections have already been formed, and in the future only a few new neurons are formed. When a neuron dies, it is not replaced by a new one. However, the remaining ones can take over the functions of the lost cell, forming new processes that form synapses with those neurons, muscles or glands with which the lost neuron was connected. Cut or damaged PNS neuron fibers surrounded by the neurilemma can regenerate if the cell body remains intact. Below the site of transection, the neurilemma is preserved as a tubular structure, and that part of the axon that remains connected to the cell body grows along this tube until it reaches the nerve ending. In this way, the function of the damaged neuron is restored. Axons in the central nervous system that are not surrounded by a neurilemma are apparently unable to re-grow to the site of their previous termination. However, many neurons of the central nervous system can produce new short processes - branches of axons and dendrites that form new synapses.
CENTRAL NERVOUS SYSTEM

The central nervous system consists of the brain and spinal cord and their protective membranes. The outermost is the dura mater, under it is the arachnoid (arachnoid), and then the pia mater, fused with the surface of the brain. Between the pia mater and the arachnoid membrane is the subarachnoid space, which contains cerebrospinal fluid, in which both the brain and spinal cord literally float. The action of the buoyant force of the fluid leads to the fact that, for example, the adult brain, which has an average mass of 1500 g, actually weighs 50-100 g inside the skull. The meninges and cerebrospinal fluid also play the role of shock absorbers, softening all kinds of shocks and shocks that tests the body and which could lead to damage to the nervous system. The central nervous system is made up of gray and white matter. Gray matter is composed of cell bodies, dendrites, and unmyelinated axons, organized into complexes that include countless synapses and serve as information processing centers for many functions of the nervous system. White matter consists of myelinated and unmyelinated axons that act as conductors transmitting impulses from one center to another. The gray and white matter also contains glial cells. CNS neurons form many circuits that perform two main functions: they provide reflex activity, as well as complex information processing in higher brain centers. These higher centers, such as the visual cortex (visual cortex), receive incoming information, process it, and transmit a response signal along the axons. The result of the activity of the nervous system is one or another activity, which is based on the contraction or relaxation of muscles or the secretion or cessation of secretion of glands. It is with the work of muscles and glands that any way of our self-expression is connected. Incoming sensory information is processed through a sequence of centers connected by long axons that form specific pathways, for example pain, visual, auditory. Sensitive (ascending) pathways go in an ascending direction to the centers of the brain. Motor (descending) tracts connect the brain with motor neurons of the cranial and spinal nerves. The pathways are usually organized in such a way that information (for example, pain or tactile) from the right side of the body enters the left side of the brain and vice versa. This rule also applies to the descending motor pathways: the right half of the brain controls the movements of the left half of the body, and the left half controls the right. From this general rule however, there are a few exceptions. The brain consists of three main structures: the cerebral hemispheres, the cerebellum and the brainstem. The cerebral hemispheres - the largest part of the brain - contain higher nerve centers that form the basis of consciousness, intelligence, personality, speech, and understanding. In each of the cerebral hemispheres, the following formations are distinguished: underlying isolated accumulations (nuclei) of gray matter, which contain many important centers; a large mass of white matter located above them; covering the outside of the hemispheres is a thick layer of gray matter with numerous convolutions that makes up the cerebral cortex. The cerebellum also consists of an underlying gray matter, an intermediate mass of white matter, and an outer thick layer of gray matter that forms many convolutions. The cerebellum primarily provides coordination of movements. The brainstem is formed by a mass of gray and white matter that is not divided into layers. The trunk is closely connected with the cerebral hemispheres, the cerebellum and the spinal cord and contains numerous centers of sensory and motor pathways. The first two pairs of cranial nerves arise from the cerebral hemispheres, while the remaining ten pairs arise from the trunk. The trunk regulates vital functions such as breathing and blood circulation.
See also HUMAN BRAIN.
Spinal cord. Located inside the spinal column and protected by its bone tissue, the spinal cord has a cylindrical shape and is covered with three membranes. In a cross section, the gray matter is shaped like the letter H or a butterfly. Gray matter is surrounded by white matter. Sensitive fibers of the spinal nerves end in the dorsal (posterior) parts of the gray matter - the dorsal horns (at the ends of the H, facing the back). The bodies of motor neurons of the spinal nerves are located in the ventral (anterior) parts of the gray matter - the anterior horns (at the ends of the H, distant from the back). In the white matter there are ascending sensory pathways ending in the gray matter of the spinal cord, and descending motor pathways coming from the gray matter. In addition, many fibers in the white matter connect different parts of the gray matter of the spinal cord.
PERIPHERAL NERVOUS SYSTEM
The PNS provides two-way communication between the central parts of the nervous system and the organs and systems of the body. Anatomically, the PNS is represented by the cranial (cranial) and spinal nerves, as well as the relatively autonomous enteric nervous system, located in the intestinal wall. All cranial nerves (12 pairs) are divided into motor, sensory or mixed. Motor nerves begin in the motor nuclei of the trunk, formed by the bodies of the motor neurons themselves, and sensory nerves are formed from the fibers of those neurons whose bodies lie in ganglia outside the brain. 31 pairs of spinal nerves depart from the spinal cord: 8 pairs of cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. They are designated according to the position of the vertebrae adjacent to the intervertebral foramina from which these nerves emerge. Each spinal nerve has an anterior and a posterior root, which fuse to form the nerve itself. The posterior root contains sensory fibers; it is closely connected with the spinal ganglion (dorsal root ganglion), consisting of the cell bodies of neurons, the axons of which form these fibers. The anterior root consists of motor fibers formed by neurons whose cell bodies lie in the spinal cord.
AUTONOMIC NERVOUS SYSTEM
The autonomic, or autonomic, nervous system regulates the activity of involuntary muscles, the heart muscle, and various glands. Its structures are located both in the central nervous system and in the peripheral. The activity of the autonomic nervous system is aimed at maintaining homeostasis, i.e. a relatively stable state of the body's internal environment, such as a constant body temperature or blood pressure that meets the body's needs. Signals from the central nervous system enter the working (effector) organs through pairs of sequentially connected neurons. The bodies of neurons of the first level are located in the CNS, and their axons end in the autonomic ganglia, which lie outside the CNS, and here they form synapses with the bodies of neurons of the second level, the axons of which are in direct contact with the effector organs. The first neurons are called preganglionic, the second - postganglionic. In the part of the autonomic nervous system called the sympathetic nervous system, the cell bodies of preganglionic neurons are located in the gray matter of the thoracic (thoracic) and lumbar (lumbar) spinal cord. Therefore, the sympathetic system is also called the thoracolumbar system. The axons of its preganglionic neurons terminate and form synapses with postganglionic neurons in ganglia located in a chain along the spine. Axons of postganglionic neurons contact effector organs. The endings of postganglionic fibers secrete norepinephrine (a substance close to adrenaline) as a neurotransmitter, and therefore the sympathetic system is also defined as adrenergic. The sympathetic system is complemented by the parasympathetic nervous system. The bodies of its preganglinar neurons are located in the brainstem (intracranial, i.e. inside the skull) and the sacral (sacral) part of the spinal cord. Therefore, the parasympathetic system is also called the craniosacral system. The axons of preganglionic parasympathetic neurons terminate and form synapses with postganglionic neurons in ganglia located near the working organs. The endings of postganglionic parasympathetic fibers release the neurotransmitter acetylcholine, on the basis of which the parasympathetic system is also called cholinergic. As a rule, the sympathetic system stimulates those processes that are aimed at mobilizing the body’s forces in extreme situations or under stress. The parasympathetic system contributes to the accumulation or restoration of the body’s energy resources. The reactions of the sympathetic system are accompanied by the consumption of energy resources, an increase in the frequency and strength of heart contractions, an increase in blood pressure and blood sugar, as well as an increase in blood flow to the skeletal muscles by reducing its flow to the internal organs and skin. All of these changes are characteristic of the "fear, flight or fight" response. The parasympathetic system, on the contrary, reduces the frequency and strength of heart contractions, lowers blood pressure, and stimulates the digestive system. The sympathetic and parasympathetic systems act in a coordinated manner and cannot be viewed as antagonistic. They jointly support the functioning of internal organs and tissues at a level corresponding to the intensity of stress and emotional state person. Both systems function continuously, but their activity levels fluctuate depending on the situation.
REFLEXES
When an adequate stimulus acts on the receptor of a sensory neuron, a volley of impulses appears in it, triggering a response action called a reflex act (reflex). Reflexes underlie most of the vital functions of our body. The reflex act is carried out by the so-called. reflex arc; This term refers to the path of transmission of nerve impulses from the point of initial stimulation on the body to the organ that performs the response action. The reflex arc that causes skeletal muscle contraction consists of at least two neurons: a sensory neuron, whose body is located in the ganglion, and the axon forms a synapse with neurons in the spinal cord or brain stem, and a motor (lower, or peripheral, motor neuron), whose body is located in the gray matter, and the axon ends at the motor end plate on skeletal muscle fibers. The reflex arc between the sensory and motor neurons may also include a third, intermediate, neuron located in the gray matter. The arcs of many reflexes contain two or more interneurons. Reflex actions are carried out involuntarily, many of them are not realized. The knee jerk reflex, for example, is triggered by tapping the quadriceps tendon at the knee. This is a two-neuron reflex, its reflex arc consists of muscle spindles (muscle receptors), a sensory neuron, a peripheral motor neuron and a muscle. Another example is the reflexive withdrawal of the hand from a hot object: the arc of this reflex includes a sensory neuron, one or more interneurons in the gray matter of the spinal cord, a peripheral motor neuron, and a muscle. Many reflex acts have a much more complex mechanism. The so-called intersegmental reflexes are made up of combinations of simpler reflexes, in the implementation of which many segments of the spinal cord take part. Thanks to such reflexes, for example, coordination of movements of the arms and legs when walking is ensured. Complex reflexes that occur in the brain include movements associated with maintaining balance. Visceral reflexes, i.e. reflex reactions of internal organs are mediated by the autonomic nervous system; they ensure bladder emptying and many processes in the digestive system.
See also REFLEX.
DISEASES OF THE NERVOUS SYSTEM
Damages to the nervous system occur due to organic diseases or injuries of the brain and spinal cord, meninges, and peripheral nerves. Diagnosis and treatment of diseases and injuries of the nervous system are the subject of a special branch of medicine - neurology. Psychiatry and clinical psychology deal primarily with mental disorders. The scope of these medical disciplines often overlap. See selected diseases of the nervous system: ALZHEIMER'S DISEASE;
STROKE ;
MENINGITIS;
NEURITIS;
PARALYSIS;
PARKINSON'S DISEASE;
POLIOMYELITIS;
MULTIPLE SCLEROSIS;
TETANUS;
CEREBRAL PALSY;
HOREA;
ENCEPHALITIS;
EPILEPSY.
See also
COMPARATIVE ANATOMY;
HUMAN ANATOMY.
LITERATURE
Bloom F., Leiserson A., Hofstadter L. Brain, mind and behavior. M., 1988 Human Physiology, ed. R. Schmidt, G. Tevs, vol. 1. M., 1996

Collier's Encyclopedia. - Open Society. 2000 .

The structure and functions of the human nervous system are so complex that a separate section of anatomy called neuroanatomy is devoted to their study. The central nervous system is responsible for everything, for human life itself - and this is not an exaggeration. If there is a deviation in the functional activity of one of the departments, the integrity of the system is violated, and human health is at risk.

The nervous system is a collection of anatomically and functionally interconnected nerve cells with their processes. There are central and peripheral nervous systems. The central nervous system includes the brain and spinal cord, the peripheral nervous system includes the cranial and spinal nerves and their associated roots, spinal nodes and plexuses.

The main function of the nervous system is to regulate the vital functions of the body, maintain a constant internal environment, metabolic processes, and communicate with the outside world.

The nervous system consists of nerve cells, nerve fibers and neuroglial cells.

You will learn more about the structure and functions of the nervous system from this article.

Neuron as a structural and functional unit of the human nervous system

A nerve cell - neuron - is a structural and functional unit of the nervous system. A neuron is a cell that can perceive irritation, become excited, produce nerve impulses and transmit them to other cells.

That is, a neuron of the nervous system performs two functions:

Processes the information received by it and transmits a nerve impulse
Maintains its vital functions

A neuron as a structural unit of the nervous system consists of a body and processes - short, branching ones (dendrites) and one long one (axon), which can give rise to numerous branches. The point of contact between neurons is called a synapse. Synapses can be between an axon and a nerve cell body, an axon and a dendrite, two axons, and less commonly, between two dendrites. At synapses, impulses are transmitted bioelectrically or through chemically active mediator substances (acetylcholine, norepinephrine, dopamine, serotonin, etc.). Numerous neuropeptides (enkephalins, endorphins, etc.) also participate in synaptic transmission.

Transportation of biologically active substances along the axon from the neuron body in the central nervous system to the synapse and back (axonal transport) ensures the supply and renewal of mediators, as well as the formation of new processes - axons and dendrites. Thus, two interconnected processes are constantly going on in the brain - the emergence of new processes and synapses and the partial disintegration of existing ones. And this underlies learning, adaptation, as well as restoration and compensation of impaired functions.

The cell membrane (cell membrane) is a thin lipoprotein plate penetrated by channels through which K, Na, Ca, C1 ions are selectively released. Functions of the cell membrane of the human nervous system - creation electric charge cells, due to which excitation and impulse arise.

Neuroglia is a connective tissue supporting structure of the nervous system (stroma) that performs a protective function.

The interweaving of axons, dendrites and processes of glial cells creates a picture of the neuropil.

A nerve fiber in the structure of the nervous system is a process of a nerve cell (axial cylinder), covered to a greater or lesser extent with myelin and surrounded by a Schwann membrane, which performs protective and trophic functions. In myelinated fibers, the impulse moves at speeds of up to 100 m/sec.

The accumulation of neuron cell bodies in the human nervous system forms the gray matter of the brain, and their processes form the white matter. A collection of neurons located outside the central nervous system is called a ganglion. A nerve is a trunk of united nerve fibers. Depending on the function, motor, sensory, autonomic and mixed nerves are distinguished.

Speaking about the structure of the human nervous system, the set of neurons that regulate any function is called the nerve center. A complex of physiological mechanisms associated with the performance of any specific function, is called a functional system.

It includes cortical and subcortical nerve centers, pathways, peripheral nerves, and executive organs.

The functional activity of the nervous system is based on a reflex. A reflex is the body's response to stimulation. The reflex is carried out through a chain of neurons (at least two), called a reflex arc. The neuron that perceives irritation is the afferent part of the arc; the neuron that carries out the response is the efferent part. But the reflex act does not end with a one-time response from the working organ. Exists feedback, affecting muscle tone, is a self-regulatory ring in the form of a gamma loop.

The reflex activity of the nervous system ensures that the body perceives any changes in the external world.

The ability to perceive external phenomena is called reception. Sensitivity is the ability to sense stimuli perceived by the nervous system. Formations of the central and peripheral nervous system that perform the perception and analysis of information about phenomena both inside the body and in environment, are called analyzers. There are visual, auditory, gustatory, olfactory, sensitive and motor analyzers. Each analyzer consists of a peripheral (receptor) section, a conductive part and a cortical section, in which the analysis and synthesis of perceived stimuli occurs.

Since the central sections of various analyzers are located in the cerebral cortex, all information coming from the external and internal environment is concentrated in it, which is the basis for mental higher nervous activity. Analysis of the information received by the cortex is recognition, gnosis. The functions of the cerebral cortex also include the development of action plans (programs) and their implementation - praxis.

The following describes how the spinal cord of the human nervous system works.

Human central nervous system: how the spinal cord works (with photo)

The spinal cord, as part of the central nervous system, is a cylindrical cord 41-45 cm long, located in the spinal canal from the first cervical vertebra to the second lumbar. It has two thickenings - cervical and lumbosacral, providing innervation to the limbs. The lumbosacral thickening passes into the medullary cone, ending in a filament-like continuation - the terminal filament, reaching the end of the spinal canal. The spinal cord performs conductor and reflex functions.

The spinal cord of the nervous system has a segmental structure. A segment is a section of the spinal cord with two pairs of spinal roots. In total, the spinal cord has 31-32 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1-2 coccygeal (vestigial). The anterior and posterior horns of the spinal cord, anterior and posterior spinal roots, spinal ganglia and spinal nerves make up the segmental apparatus of the spinal cord. As the spine develops, it becomes longer than the spinal cord, so the roots become longer and form a cauda equina.

In a section through the spinal cord of the human nervous system, gray and white matter can be seen. The gray matter consists of cells, has the shape of the letter “H” with the anterior - motor horns, the posterior - sensitive and the lateral - vegetative. The central canal of the spinal cord runs through the center of the gray matter. The spinal cord is divided into left and right halves, connected by the white and gray interconnections, through the median fissure (in front) and the median sulcus (at the back).

The gray matter is surrounded by nerve fibers - conductors, forming the white matter, in which anterior, lateral and posterior columns are distinguished. The front pillars are located between the front horns, the rear ones - between the rear ones, the lateral ones - between the front and rear horns of each side.

These photos show the structure of the spinal cord of the human nervous system:

Spinal nerves as part of the nervous system

Spinal nerves as part of the human nervous system are formed by the fusion of the anterior (motor) and posterior (sensory) roots of the spinal cord and exit the spinal canal through the intervertebral foramina. Each pair of these nerves innervates a specific area of the body - a metamer.

Leaving the spinal canal, the spinal nerves of the nervous system are divided into four branches:

Front, innervating the skin and muscles of the limbs and the anterior surface of the body;
Rear, innervating the skin and muscles of the posterior surface of the body;
Meningeal, heading to the dura mater of the spinal cord;
Connecting, next to the sympathetic nodes.

Anterior branches The spinal nerves form plexuses: cervical, brachial, lumbar, sacral and coccygeal.

Cervical plexus formed by the anterior branches of the cervical nerves C:-C4; innervates the skin of the back of the head, the lateral surface of the face, the supra-, subclavian and superior scapular regions, and the diaphragm.

Brachial plexus formed by the anterior branches of C4-T1; innervates the skin and muscles of the upper limb.

Anterior branches T2-T11, without forming a plexus, together with the posterior branches provide innervation to the skin and muscles of the chest, back and abdomen.

Lumbosacral plexus is a combination of the lumbar and sacral.

Lumbar plexus formed by the anterior branches of T12–L 4; innervates the skin and muscles of the lower abdomen, the anterior and lateral surface of the thigh.

Sacral plexus formed by the anterior branches of the L5-S4 nerves; innervates the skin and muscles of the gluteal region, perineum, posterior thigh, lower leg and foot. The largest nerve in the body, the sciatic, departs from it.

Coccygeal plexus formed by the anterior branches of S5-C0C2; innervates the perineum.

The next section of the article is devoted to the structure and functions of the main parts of the brain.

Human nervous system: structure and functions of the main parts of the brain

The brain, which is part of the nervous system, is located in the cranium, covered with meninges, between which cerebrospinal fluid (CSF) circulates. The brain is connected to the spinal cord through the foramen magnum. The weight of the adult human brain is on average 1300-1500 g. The function of the human brain is to regulate all processes occurring in the body.

The brain as part of the nervous system consists of the following sections: two hemispheres, the cerebellum and the brainstem.

The brainstem consists of the medulla oblongata, pons, and cerebral peduncles ( midbrain), as well as the base and tire.

The medulla oblongata is a continuation of the spinal cord. Conditional border The medulla oblongata and spinal cord serve as the intersection of the pyramidal tracts. The medulla oblongata contains vital centers that regulate breathing, blood circulation, and swallowing; it contains all the motor and sensory pathways connecting the spinal cord and brain.

The structure of the bridge of the nervous system of the brain includes the nuclei of the V, VI, VII and VIII pairs of cranial nerves, sensory pathways in the medial lemniscus, fibers of the auditory tract in the form of the lateral lemniscus, etc.

The cerebral peduncles are part of the midbrain; they connect the pons to the hemispheres and include ascending and descending pathways. The roof of the midbrain has a plate on which the quadrigemina is located. The primary subcortical center of vision is located in the superior colliculi, and the primary subcortical hearing center is located in the inferior colliculi. Thanks to the mounds, the body’s indicative and protective reactions occur under the influence of visual and auditory stimuli. Under the roof of the midbrain is the midbrain aqueduct, which connects the third and fourth ventricles of the cerebral hemispheres.

The diencephalon consists of the thalamus (optic thalamus), epithalamus, metathalamus and hypothalamus. The cavity of the diencephalon is the third ventricle. The thalamus is a collection of nerve cells located on both sides of the third ventricle. The thalamus is one of the subcortical centers of vision and the center of afferent impulses from throughout the body, sent to the cerebral cortex. In the thalamus, sensations are formed and impulses are transmitted to the extra-pyramidal system.

The metathalamus, as part of the brain of the human nervous system, also contains one of the subcortical centers of vision and the subcortical center of hearing (medial and lateral geniculate body).

The epithalamus includes the pineal gland, which is an endocrine gland that regulates the function of the adrenal cortex and the development of sexual characteristics.

The hypothalamus consists of the gray tubercle, infundibulum, medullary appendage (neurohypophysis) and paired mastoid bodies. The hypothalamus contains accumulations of gray matter in the form of nuclei, which are centers of the autonomic nervous system that regulate all types of metabolism, respiration, blood circulation, the activity of internal organs and endocrine glands. The hypothalamus maintains a constant internal environment in the body (homeostasis) and, thanks to connections with the limbic system, participates in the formation of emotions, providing their vegetative coloring.

Along the entire length of the brain stem, a phylogenetically ancient formation of gray matter is located and occupies a central position in the form of a dense network of nerve cells with many processes - the reticular formation. Branches from all types of sensory systems are directed to the reticular formation, so any irritation coming from the periphery is transmitted along ascending pathways to the cerebral cortex, activating its activity. Thus, the reticular formation is involved in the implementation of normal biological rhythms of wakefulness and sleep, and is an ascending, activating system of the brain - an “energy generator”.

Together with the limbic structures, the reticular formation ensures normal cortical-subcortical relationships and behavioral reactions. It is also involved in the regulation muscle tone, and its descending pathways provide reflex activity of the spinal cord.

The cerebellum is located under the occipital lobes of the brain and is separated from them by the dura mater - the cerebellar tentorium. It is divided into a central part - the cerebellar vermis and lateral sections - the hemispheres. In the depths of the white matter of the cerebellar hemispheres there are the dentate nucleus and smaller nuclei - cortical and spherical. The roof nucleus is located in the middle part of the cerebellum. The cerebellar nuclei are involved in the coordination of movements and balance, as well as in the regulation of muscle tone. Three pairs of legs connect the cerebellum with all parts of the brain stem, providing its connection with the extrapyramidal system, cerebral cortex and spinal cord.

The structure and main functions of the cerebral hemispheres

The structure of the cerebrum includes two hemispheres connected to each other by the large white commissure - the corpus callosum, consisting of fibers connecting the lobes of the brain of the same name. The surface of each hemisphere is covered with a cortex consisting of cells and divided by many grooves. The areas of the cortex located between the grooves are called gyri. The deepest grooves divide each hemisphere into lobes: frontal, parietal, occipital and temporal. The central (Rolandic) sulcus separates the parietal lobe from the frontal lobe; in front of it is the precentral gyrus. Horizontal grooves divide the frontal lobe into superior, middle and inferior gyri.

Behind the central sulcus in the structure of the cerebral hemispheres is the postcentral gyrus. The parietal lobe is divided by the transverse intraparietal sulcus into the superior and inferior parietal lobes.

The deep lateral (Sylvian) fissure separates the temporal lobe from the frontal and parietal lobes. On the lateral surface of the temporal lobe, the superior, middle and inferior temporal gyri are located longitudinally. On the inner surface of the temporal lobe is a gyrus called the hippocampus.

On the inner surface of the hemispheres, the parieto-occipital sulcus separates the parietal lobe from the occipital lobe, and the calcarine sulcus divides the occipital lobe into two gyri - the precuneus and the cuneus.

On the medial surface of the hemispheres above the corpus callosum, the cingulate gyrus is located in an arcuate manner, passing into the parahippocampal gyrus.

The cerebral cortex is the youngest part of the central nervous system in evolutionary terms, consisting of neurons. It is most developed in humans. The cortex is a layer of gray matter 1.3-4 mm thick, covering the white matter of the hemispheres, consisting of axons, dendrites of nerve cells and neuroglia.

The cortex plays a very important role in the regulation of vital processes in the body, the implementation of behavioral acts and mental activity.

The function of the frontal lobe cortex is to organize movements, speech motor skills, complex forms of behavior and thinking. The center of voluntary movements is located in the precentral gyrus, and the pyramidal tract begins from here.

The parietal lobe contains the centers of the analyzer of general sensitivity, gnosis, praxis, writing, and counting.

The functions of the temporal lobe of the cerebrum are the perception and processing of auditory, taste and olfactory sensations, analysis and synthesis of speech sounds, and memory mechanisms. The basal sections of the cerebral hemispheres are connected with the higher autonomic centers.

The occipital lobe contains the cortical centers of vision.

Not all functions of the cerebral hemispheres are represented symmetrically in the cortex. For example, speech, reading and writing are functionally associated with the left hemisphere for most people.

The right hemisphere provides orientation in time, place, and is associated with the emotional sphere.

The axons and dendrites of the nerve cells of the cortex constitute pathways that connect various parts of the cortex, the cortex and other parts of the brain and spinal cord. The pathways form the corona radiata, consisting of fan-shaped diverging fibers, and the internal capsule, located between the basal (subcortical) nuclei.

The subcortical nuclei (caudate, lenticular, amygdala, fence) are located deep in the white matter around the ventricles of the brain. Morphologically and functionally, the caudate nucleus and putamen are combined into the striatum (striatum). The globus pallidus, red nucleus, substantia nigra and reticular formation of the midbrain are combined into the pallidum (pallidum). The striatum and pallidum form a very important functional system - striopallidal or extrapyramidal. The extrapyramidal system ensures the preparation of various muscle groups to perform integral movements, also provides facial, auxiliary and friendly movements, gestures, automated motor acts (grimaces, whistling, etc.).

A special role is played by the most ancient in evolutionary terms sections of the cerebral cortex, located on the inner surface of the hemispheres - the cingulate and parahippocampal gyri. Together with the amygdala, olfactory bulb and olfactory tract, they form the limbic system, which is closely connected with the reticular formation of the brain stem and constitutes a single functional system - the limbic-reticular complex (LRK). Speaking about the structure and functions of the cerebrum, it should be noted that the limbic-reticular complex is involved in the formation of instinctive and emotional reactions (food, sexual, defensive instincts, anger, rage, pleasure) of human behavior. The LRC also takes part in the regulation of the tone of the cerebral cortex, the processes of sleep, wakefulness, and adaptation.

See how the large brain of the human nervous system works in these photos:

12 pairs of cranial nerves of the nervous system and their functions (with video)

At the base of the brain, 12 pairs of cranial nerves emerge from the medulla. Based on their function, they are divided into sensory, motor and mixed. Proximally, the cranial nerves are connected to the brainstem nuclei, subcortical nuclei, cerebral cortex and cerebellum. Distally, the cranial nerves are connected to various functional structures (eyes, ears, facial muscles, tongue, glands, etc.).

I pair - olfactory nerve ( n. olfactorius) . The receptors are located in the mucous membrane of the nasal concha, connected to the sensitive neurons of the olfactory bulb. Along the olfactory tract, signals enter the primary olfactory centers (nuclei of the olfactory triangle) and then to the internal parts of the temporal lobe (hippocampus), where the cortical centers of smell are located.

II pair - optic nerves ( n. opticus) . The receptors of this pair of cranial nerves are the cells of the retina, from the ganglion layer of which the nerves themselves begin. Passing at the base of the frontal lobes in front of the sella turcica, the optic nerves partially cross, forming a chiasm, and are directed as part of the visual tracts to the subcortical visual centers, and from them to the occipital lobes.

III pair - oculomotor nerves ( n. oculomotorius) . They contain motor and parasympathetic fibers that innervate the muscles that elevate the upper eyelids, constrict the pupil, and the muscles of the eyeball, with the exception of the superior oblique and abductor muscles.

IV pair - trochlear nerves ( n. trochlearis) . This pair of cranial nerves innervates the superior oblique muscles of the eyes.

V pair - trigeminal nerves ( n. trigeminus) . They are mixed nerves. Sensitive neurons of the trigeminal (Gasserian) ganglion form three large branches: the ophthalmic, maxillary and mandibular nerves, which emerge from the cranial cavity and innervate the frontoparietal part of the scalp, facial skin, eyeballs, mucous membranes of the nasal cavities, mouth, anterior two-thirds of the tongue, teeth, dura mater. The central processes of the cells of the Gasserian ganglion go deep into the brain stem and connect with second sensory neurons, forming a chain of nuclei. Signals from the brainstem nuclei pass through the thalamus to the postcentral gyrus (fourth neuron) of the opposite hemisphere. Peripheral innervation corresponds to the branches of the nerve, segmental innervation has the form of ring zones. Motor fibers of the trigeminal nerve regulate the functioning of the masticatory muscles.

VI pair - abducens nerves ( n. abducens) . Innervates the abductor muscles of the eye.

VII pair - facial nerves ( n. facialis) . Innervates the facial muscles. When leaving the pons, the intermediate nerve joins the facial nerve, providing taste innervation to the anterior two-thirds of the tongue, parasympathetic innervation of the submandibular and sublingual glands, and lacrimal glands.

VIII pair - cochleovestibular (auditory, vestibulocochlear) nerve ( n. vestibulo-cochlearis) . This pair of cranial nerves ensures the function of hearing and balance, and have extensive connections with the structures of the extrapyramidal system, cerebellum, spinal cord, and cortex.

IX pair - glossopharyngeal nerves ( n. glossopharyngeus).

They function in close connection with the X-pair - the vagus nerves ( n. vagus) . These nerves have a number of common nuclei in the medulla oblongata that perform sensory, motor and secretory functions. They innervate the soft palate, pharynx, upper esophagus, parotid salivary gland, posterior third of the tongue. The vagus nerve provides parasympathetic innervation of all internal organs to the level of the pelvis.

XI pair - accessory nerves ( n. accessorius) . Innervates the sternocleidomastoid and trapezius muscles.

XII pair - hypoglossal nerves ( n. hypoglossus) . Innervates the muscles of the tongue.

Autonomic division of the human nervous system: structure and main functions

Autonomic nervous system (ANS)- This is part of the nervous system that ensures the vital functions of the body. It innervates the heart, blood vessels, internal organs, and also carries out tissue trophism and ensures the constancy of the internal environment of the body. In the autonomic part of the nervous system, there are sympathetic and parasympathetic parts. They interact as antagonists and synergists. Thus, the sympathetic nervous system dilates the pupil, increases the frequency of heart contractions, constricts blood vessels, increases blood pressure, reduces the secretion of glands, slows down the peristalsis of the stomach and intestines, and contracts the sphincters. Parasympathetic, on the contrary, constricts the pupil, slows the heartbeat, dilates blood vessels, lowers blood pressure, increases the secretion of glands and intestinal motility, and relaxes the sphincters.

The sympathetic autonomic nervous system carries out trophic function, enhances oxidative processes, nutrient consumption, respiratory and cardiovascular activity, and changes permeability cell membrane. The role of the parasympathetic system is protective. In a state of rest, the vital activity of the body is ensured by the parasympathetic system, and during stress - by the sympathetic system.

In the structure of the autonomic nervous system, segmental and suprasegmental sections are distinguished.

The segmental part of the ANS is represented by sympathetic and parasympathetic formations at the spinal and brain stem level.

The centers of the human sympathetic autonomic nervous system are located in the lateral columns of the spinal cord at the level of C8-L3. Sympathetic fibers exit the spinal cord with the anterior roots, are interrupted in the nodes of the paired sympathetic trunk, which is located on the anterior surface of the spinal column and consists of 20-25 pairs of nodes, containing sympathetic cells. Fibers depart from the nodes of the sympathetic trunk, forming sympathetic plexuses and nerves that are directed to organs and vessels.

The centers of the parasympathetic nervous system are located in the brain stem and in the sacral segments S2-S4 of the spinal cord. The processes of cells of the parasympathetic nuclei of the brain stem as part of the oculomotor, facial, glossopharyngeal and vagus nerves provide innervation of the glands and smooth muscles of all internal organs, with the exception of the pelvic organs. The fibers of the cells of the parasympathetic nuclei of the sacral segments form the pelvic splanchnic nerves going to the bladder, rectum, and genitals.

Both sympathetic and parasympathetic fibers are interrupted in the peripheral autonomic ganglia located near the innervated organs or in their walls.

The fibers of the autonomic nervous system form a number of plexuses: solar, pericardial, mesenteric, pelvic, which innervate the internal organs and regulate their function.

The higher suprasegmental division of the autonomic nervous system includes the nuclei of the hypothalamus, the limbic-reticular complex, the basal structures of the temporal lobe and some parts of the associative zone of the cerebral cortex. The role of these formations is to integrate basic mental and somatic functions.

In a state of rest, the vital activity of the body is ensured by the parasympathetic system, and during stress - by the sympathetic system.

The centers of the sympathetic nervous system are located in the lateral columns of the spinal cord at the level of C8-L3; sympathetic fibers leave the spinal cord with the anterior roots and are interrupted at the nodes of the paired sympathetic trunk.

Here you can watch the video “The Human Nervous System” to better understand how it works:

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