Human nervous system: classification, organs and functions

The human body is a multi-stage structure, each organ and system of which is closely interconnected with each other and with the environment. And so that this connection is not interrupted even for a split second, a nervous system is provided - a complex network that permeates the entire human body and is responsible for self-regulation and the ability to adequately respond to external and internal stimuli. Thanks to the well-coordinated work of the nervous system, a person can adapt to the factors of the external world: any, even minor, change in the environment causes nerve cells to transmit hundreds of impulses at an incredibly high speed so that the body can instantly adapt to new conditions. Internal self-regulation works in a similar way, in which the activity of cells is coordinated in accordance with current needs.

The functions of the nervous system affect the most important processes of life, without which the normal existence of the body is unthinkable. These include:

• regulation of the work of internal organs in accordance with external and internal impulses;
• coordination of all units of the body, from the smallest cells to organ systems;
• harmonious interaction between humans and the environment;
• the basis of higher psychophysiological processes characteristic of humans.

How does this complex mechanism work? What cells, tissues and organs make up the human nervous system and what is each of its sections responsible for? A brief excursion into the basic anatomy and physiology of the human body will help you find answers to these questions.

Stages of nervous system development

In evolution, the nervous system has undergone several stages of development, which became turning points in the qualitative organization of its activities.
These stages differ in the number and types of neuronal formations, synapses, signs of their functional specialization, and in the formation of groups of neurons interconnected by common functions. There are three main stages of the structural organization of the nervous system: diffuse, nodular, tubular. The diffuse nervous system is the most ancient, found in coelenterates (hydra). Such a nervous system is characterized by a multiplicity of connections between neighboring elements, which allows excitation to freely spread throughout the nervous network in all directions.

This type of nervous system provides wide interchangeability and thereby greater reliability of functioning, but these reactions are imprecise and vague.

The nodal type of nervous system is typical for worms, mollusks, and crustaceans.

It is characterized by the fact that the connections of nerve cells are organized in a certain way, excitation passes along strictly defined paths. This organization of the nervous system turns out to be more vulnerable. Damage to one node causes dysfunction of the entire organism as a whole, but its qualities are faster and more accurate.

The tubular nervous system is characteristic of chordates; it includes features of the diffuse and nodular types. The nervous system of higher animals took all the best: high reliability of the diffuse type, accuracy, locality, speed of organization of nodal type reactions.

The leading role of the nervous system

At the first stage of the development of the world of living beings, interaction between the simplest organisms was carried out through the aquatic environment of the primitive ocean, into which the chemical substances released by them entered. The first oldest form of interaction between the cells of a multicellular organism is chemical interaction through metabolic products entering the body fluids. Such metabolic products, or metabolites, are the breakdown products of proteins, carbon dioxide, etc. This is the humoral transmission of influences, the humoral mechanism of correlation, or connections between organs.

The humoral connection is characterized by the following features:

• lack of an exact address to which a chemical substance entering the blood or other body fluids is sent;
• the chemical spreads slowly;
• the chemical acts in minute quantities and is usually quickly broken down or eliminated from the body.

Humoral connections are common to both the animal and plant worlds. At a certain stage of development of the animal world, in connection with the appearance of the nervous system, a new, nervous form of connections and regulation is formed, which qualitatively distinguishes the animal world from the plant world. The higher the development of an animal’s organism, the greater the role played by the interaction of organs through the nervous system, which is designated as reflex. In higher living organisms, the nervous system regulates humoral connections. Unlike the humoral connection, the nervous connection has a precise direction to a specific organ and even a group of cells; communication is carried out hundreds of times faster than the speed of distribution of chemicals. The transition from a humoral connection to a nervous connection was not accompanied by the destruction of the humoral connection between the cells of the body, but by the subordination of nervous connections and the emergence of neurohumoral connections.

At the next stage of development of living beings, special organs appear - glands, in which hormones are produced, formed from food substances entering the body. The main function of the nervous system is both to regulate the activity of individual organs among themselves, and in the interaction of the body as a whole with its external environment. Any impact of the external environment on the body appears, first of all, on receptors (sensory organs) and is carried out through changes caused by the external environment and the nervous system. As the nervous system develops, its highest department—the cerebral hemispheres—becomes “the manager and distributor of all the activities of the body.”

Structure of the nervous system

The nervous system is formed by nervous tissue, which consists of a huge number of neurons - a nerve cell with processes.

The nervous system is conventionally divided into central and peripheral.

The central nervous system includes the brain and spinal cord, and the peripheral nervous system includes the nerves that arise from them.

The brain and spinal cord are a collection of neurons. In a cross section of the brain, white and gray matter are distinguished. Gray matter consists of nerve cells, and white matter consists of nerve fibers, which are processes of nerve cells. In different parts of the central nervous system, the location of white and gray matter is different. In the spinal cord, gray matter is located inside, and white matter is outside, but in the brain (cerebral hemispheres, cerebellum), on the contrary, gray matter is outside, white matter is inside. In various parts of the brain there are separate clusters of nerve cells (gray matter) located inside the white matter - the nuclei. Clusters of nerve cells are also located outside the central nervous system. They are called nodes and belong to the peripheral nervous system.

Structure of the central nervous system

The CNS consists of the brain and spinal cord, each of which has white and gray matter. White matter consists of pathways, myelinated and unmyelinated axons. Myelin is white, which gives the corresponding shade to the tissue. Gray matter consists of neuron cell bodies. It can be located in the nervous system in the form of a tube (spinal cord); nuclei, or ganglia (clusters of neuron bodies in the thickness of the white matter), as well as the cortex (gray matter on the surface of the white matter).

The spinal cord is located in the spinal canal and its mass is 40 g. On its lateral surface, the dorsal roots, carrying afferent (sensitive, to the brain) information, enter from the back, and the anterior roots, carrying efferent (motor, from the brain) information, exit from the front. The section of the spinal cord corresponding to each pair of roots is called a segment. The segments are named according to where the roots exit the spine. The spinal cord has 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments. In general, the number of spinal cord segments corresponds to the number of vertebrae. Exceptions are the cervical region, where there are 8 segments per 7 vertebrae; and coccygeal, where there is 1 segment for 3-4 vertebrae (Fig. 1).

Rice. 1. Structure and location of spinal cord segments.

In a cross section of the spinal cord, there is gray matter in the center surrounded by white matter. The gray matter has the shape of a butterfly, in the center of which is the spinal foramen, filled with cerebrospinal fluid (CSF). The butterfly consists of approximately 13 million neurons and has anterior and posterior horns (Fig. 2b, 3). The middle horns are also well defined in the middle sections of the spinal cord. Sensitive (sensory) information to interneurons (interneurons) enters the dorsal horns along the dorsal root. The anterior horns contain motoneurons (motor neurons) that send motor information to the muscles; it is their axons that form the anterior root. The middle horns contain neurons of the central sections of the ANS.

The spinal cord works on a reflex principle. A reflex is a stereotypical response of the body to any (external or internal) influence. The simplest reflex is monosynaptic. To implement it, two neurons are enough. An example of such a reflex is the knee reflex. When the receptor is irritated, the impulse is transmitted along the dendrite to the body of the neuron located in the nerve ganglion near the spinal cord. The axon of this neuron enters the spinal cord through the dorsal roots and forms a synapse with the motor neuron in the anterior horn. The axon of the motor neuron exits through the anterior roots and goes to the effector organ, where it changes the activity of the organ itself (Fig. 50a). The polysynaptic reflex includes an additional link in the form of one or more interneurons between the ganglion and motor neurons. Interneurons can additionally process information, compare it with other stimuli and the internal state of the body, making a decision about how to respond to the stimulus.

Rice. 2. Reflex arc (a) and histological section (b) of the spinal cord.

Rice. 3. Diagram of the structure of a section of the spinal cord.

The white matter of the spinal cord includes pathways. It is divided by the butterfly into anterior, posterior and lateral funiculi (Fig. 3).

In the posterior cords there are ascending tracts through which information is transmitted from the PNS to the spinal cord and further to the brain. In the anterior horns of the spinal cord there are descending tracts through which information goes from the brain to the spinal cord, and from the latter to the PNS. In the lateral horns, the ascending tracts are located posteriorly, and the descending tracts are located anteriorly.

The brain is located in the skull and consists of 5 sections. Its average mass is 1.5 kg and it contains up to 100 billion neurons. There are 12 pairs of cranial nerves (cranial nerves) that arise from the brain.

The medulla oblongata is the junction of the spinal cord and the brain. Its length is approximately 25 mm. In the lower part of the medulla oblongata one can still distinguish a butterfly; in the upper parts the bodies of neurons are collected into nuclei. The IX-XII pairs of the cranial nerves (Fig. 5) depart from the medulla oblongata and the corresponding nuclei lie in it. These nerves control movement and sensation in the throat, tongue, and neck. The medulla oblongata contains the largest center of the parasympathetic nervous system, which, through the X nerve (vagus, vagus nerve), controls the activity of all internal organs. The medulla oblongata contains centers for regulating breathing and vital reflexes, such as sneezing and coughing. Here is the olive kernel, which is responsible for balance. All tracts from the spinal cord to the brain pass through the medulla oblongata.

The hindbrain consists of the pons and cerebellum. The pons serves as a continuation of the medulla oblongata. It contains a lot of white matter that connects the cerebellum to the rest of the brain. This white substance forms a ridge on the underside of the bridge, making it easy to distinguish. The pons, together with the medulla oblongata, form the bottom of the 4th ventricle of the brain (a continuation and expansion of the spinal canal). V-VIII ChMN depart from the bridge. Here lie the auditory and vestibular nuclei, nuclei that innervate sensitivity and facial muscles (including facial muscles). The pons contains the locus coeruleus, which is responsible for regulating sleep.

Rice. 4. Main parts of the brain.

Rice. 5. Cranial nerves. I-olfactory, II-visual, III-oculomotor, IV-trochlear, V-trigeminal, VI-abducens, VII-facial, VIII-vestibular-cochlear, IX-glossopharyngeal, X-vagus, XI-accessory, XII-hyoid.

The cerebellum is well developed in humans due to upright posture and fine motor skills of the hands. This part of the brain is responsible for maintaining posture, balance, motor learning, and some motor reflexes. The cerebellum has a cortical structure. The cerebellar cortex consists of three layers and is divided into two hemispheres by the vermis. Under the cortex there is white matter, among which there are 3 pairs of cerebellar nuclei. To carry out its functions, it receives information from the vestibular apparatus, olive and other parts of the human motor system.

Rice. 6. External structure (a) and histological section (b) of the cerebellar cortex.

The midbrain consists of the cerebral peduncles and the roof (Fig. 7). The aqueduct of Sylvius passes through the center of the spinal cord and connects the third and fourth ventricles. The third and fourth pairs of cranial nerves depart from the midbrain. These nerves control the movements of the eyeballs. The third nerve contains parasympathetic fibers that control pupil width. The midbrain contains elements of the motor system: the red nucleus and the substantia nigra. On the roof of the brain is the quadrigeminal region. The colliculus receives visual information, and the inferior colliculus receives auditory information. This is necessary for the implementation of the orientation reflex.

Rice. 7. Appearance (a) and section (b) of the midbrain.

The medulla oblongata, pons, and midbrain together form the brainstem. The reticular formation runs through the entire trunk, regulating the overall level of brain activity.

The diencephalon consists of the thalamus, hypothalamus, pituitary gland and pineal gland. The third ventricle of the brain is located here. It serves as the origin of the second cranial nerve. The pituitary gland is the gland through which the nervous system controls the humoral one. The pineal gland is also a gland that regulates circadian rhythms. The thalamus filters information entering the cortex and removes unimportant repetitive sensory stimuli (heartbeat, gastrointestinal function, nose in the field of view, touch of clothing, etc.) In addition, the thalamus contains nuclei of the limbic system (forms mood), motor and associative kernels. The hypothalamus controls the activity of the pituitary gland and also regulates the internal state of the body. It contains the centers of hunger, thirst, sexual behavior, pleasure, displeasure, etc. Thus, the main function of the hypothalamus is to maintain homeostasis of the entire organism.

The telencephalon (forebrain) consists of the cerebral cortex and basal ganglia (nuclei). The first and second ventricles of the brain are located symmetrically under the cortex. Its area is about 220 cm2, it forms grooves and convolutions (Fig. 8). It consists of 6 layers. The hemispheres are connected to each other by the corpus callosum - a ridge of white matter. The cerebral cortex processes sensory information, the formation of voluntary movements, memory and higher nervous activity. The first cranial nerve approaches the olfactory bulbs. The basal ganglia are nuclei of gray matter located in the thickness of the white matter. They play an important role in voluntary movements, motor learning and the formation of emotions.

Rice. 8. Structure (a) and histological sections (b, c) of the cerebral cortex.

Reflex activity of the nervous system

The main form of activity of the nervous system is the reflex. Reflex is the body’s reaction to changes in the internal or external environment, carried out with the participation of the central nervous system in response to irritation of receptors.

With any irritation, excitation from the receptors is transmitted along centripetal nerve fibers to the central nervous system, from where, through the interneuron along centrifugal fibers, it goes to the periphery to one or another organ, the activity of which changes. This entire path through the central nervous system to the working organ, called the reflex arc, is usually formed by three neurons: sensory, intercalary and motor. A reflex is a complex act in which a significantly larger number of neurons take part. Excitation, entering the central nervous system, spreads to many parts of the spinal cord and reaches the brain. As a result of the interaction of many neurons, the body responds to irritation.

Spinal cord

The spinal cord is a cord about 45 cm long, 1 cm in diameter, located in the spinal canal, covered with three meninges: dura, arachnoid and soft (vascular).

The spinal cord is located in the spinal canal and is a cord that at the top passes into the medulla oblongata and at the bottom ends at the level of the second lumbar vertebra. The spinal cord consists of gray matter containing nerve cells and white matter consisting of nerve fibers. Gray matter is located inside the spinal cord and is surrounded on all sides by white matter.

In a cross section, the gray matter resembles the letter H. It distinguishes the anterior and posterior horns, as well as the connecting crossbar, in the center of which there is a narrow canal of the spinal cord containing cerebrospinal fluid. In the thoracic region there are lateral horns. They contain the bodies of neurons that innervate internal organs. The white matter of the spinal cord is formed by nerve processes. Short processes connect sections of the spinal cord, and long ones make up the conductive apparatus of bilateral connections with the brain.

The spinal cord has two thickenings - cervical and lumbar, from which nerves extend to the upper and lower extremities. 31 pairs of spinal nerves arise from the spinal cord. Each nerve begins from the spinal cord with two roots - anterior and posterior. The dorsal roots are sensitive and consist of processes of centripetal neurons. Their bodies are located in the spinal ganglia. The anterior roots - motor - are processes of centrifugal neurons located in the gray matter of the spinal cord. As a result of the fusion of the anterior and posterior roots, a mixed spinal nerve is formed. The spinal cord contains centers that regulate the simplest reflex acts. The main functions of the spinal cord are reflex activity and conduction of excitation.

The human spinal cord contains reflex centers for the muscles of the upper and lower extremities, sweating and urination. The function of excitation is that impulses from the brain to all areas of the body and back pass through the spinal cord. Centrifugal impulses from organs (skin, muscles) are transmitted through ascending pathways to the brain. Along descending pathways, centrifugal impulses are transmitted from the brain to the spinal cord, then to the periphery, to the organs. When the pathways are damaged, there is a loss of sensitivity in various parts of the body, a violation of voluntary muscle contractions and the ability to move.

A man in cross-section. Nervous system. Spinal cord and spinal nerves

The nervous system plays the main role in regulating the activities of all organs and systems of the body, combining them into a single whole and communicating the body with the environment. The nervous system includes the brain and spinal cord, as well as nerves, ganglia, plexuses, etc. All these formations are predominantly built from nervous tissue, which is capable of being excited under the influence of irritation from the environment internal or external to the body and conducting excitation in the form of a nerve impulse to various nerve centers for analysis, and then transmitting the generated one to the executive organs to obtain a response from the body in the form movement (movement in space) or changes in the function of internal organs. Irritation is perceived by the nervous system through the sensory organs (eye, ear, organs of smell and taste) and special sensitive nerve endings - receptors located in the skin, internal organs, blood vessels, skeletal muscles and joints.

The nervous system is usually divided into central and peripheral. The central nervous system includes the brain and spinal cord. The peripheral nervous system is formed by nerves that arise from the spinal cord and brain, which are called spinal and cranial, respectively. The peripheral nervous system communicates the brain and spinal cord with all organs of the human body (Fig. 1).

The anatomical and functional unit of the nervous system is the nerve cell - neuron (Fig. 2). The number of neurons reaches 1012. Neurons have processes with which they connect with each other and with innervated formations (muscle fibers, blood vessels, glands). The processes of a nerve cell are functionally unequal: some of them conduct stimulation to the neuron body - these are dendrites, and only one process - the axon - from the nerve cell body to other neurons or organs.

The functioning of the nervous system is based on reflex activity. A reflex is the body’s response to external or internal stimulation with the participation of the nervous system. The reflex pathway in the body is a chain of sequentially interconnected neurons that transmit irritation from the receptor to the spinal cord or brain, and from there to the working organ (muscle, gland). This is called a reflex arc (Figure 3).

Each neuron in the reflex arc performs its own function. Among the neurons, three types can be distinguished: the one that perceives irritation - the sensitive (afferent) neuron, which transmits irritation to the working organ - the motor (efferent) neuron, which connects the sensory and motor neurons - the intercalary (associative neuron). In this case, excitation is always carried out in one direction: from the sensitive to the motor neuron. The processes of neurons are surrounded by membranes and combined into bundles, which form nerves. The membranes isolate the processes of different neurons from each other and contribute to the conduction of excitation. The sheathed processes of nerve cells are called nerve fibers. The number of nerve fibers in different nerves ranges from 102 to 105. Most nerves contain processes of both sensory and motor neurons. Interneurons are predominantly located in the spinal cord and brain, their processes form the pathways of the central nervous system.

The spinal cord is located in the spinal canal along the length from the first cervical to the second lumbar vertebra (see Fig. 1). Externally, the spinal cord resembles a cylindrical cord. 31 pairs of spinal nerves depart from the spinal cord, which leave the spinal canal through the corresponding intervertebral foramina and branch symmetrically in the right and left halves of the body. The spinal cord is divided into cervical, thoracic, lumbar, sacral and coccygeal sections, respectively; among the spinal nerves, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1-3 coccygeal nerves are considered. The section of the spinal cord corresponding to a pair (right and left) of the spinal nerves is called a spinal cord segment.

Each spinal nerve is formed as a result of the fusion of the anterior and posterior roots extending from the spinal cord (see Fig. 3, 4). On the dorsal root there is a thickening - the spinal ganglion, where the bodies of sensory neurons are located. The processes of sensory neurons carry excitation from the receptors to the spinal cord. The anterior roots of the spinal nerves are formed by processes of motor neurons, which transmit commands from the central nervous system to skeletal muscles and internal organs.

In connection with the development of the limbs, the areas of the spinal cord that innervate the limbs have received the greatest development. Therefore, there are thickenings in the cervical and lumbar parts of the spinal cord. In the area of ​​thickenings of the spinal cord, the roots of the spinal nerves contain the largest number of nerve fibers and have the greatest thickness.

Internally, the spinal cord consists of gray matter - a collection of cell bodies of neurons - and white matter formed by the processes of neurons. In a cross-section of the spinal cord, the gray matter appears as centrally located paired anterior, posterior and lateral horns (the latter are present only in the thoracic spinal cord), surrounded by white matter (see Fig. 4). In the thickness of the gray matter (the brain, throughout its entire length there is a narrow central canal filled with cerebrospinal fluid.

In the gray matter of the spinal cord, nuclei are distinguished, which are clusters of nerve cells that perform a specific function. The nuclei of the dorsal horns of the spinal cord are sensitive; they transmit nerve impulses from sensory neurons to intercalary neurons. The nuclei of the anterior horns - motor - are represented by the bodies of motor neurons that innervate the muscles of the trunk and limbs. The nuclei of the lateral horns take part in the innervation of internal organs.

The white matter of the spinal cord contains paired anterior, posterior and lateral cords. They are a collection of nerve cell processes that connect various parts of the spinal cord and brain. These are the so-called pathways of the central nervous system.

At the level of the spinal cord, reflex arcs close, providing the simplest reflex reactions, such as tendon reflexes (for example, the knee reflex), flexion reflexes when irritating pain receptors in the skin, muscles and internal organs. An example of a simple spinal reflex is the withdrawal of a hand when it touches a hot object. The reflex activity of the spinal cord is associated with maintaining posture, maintaining a stable body position when turning and tilting the head, alternating flexion and extension of paired limbs when walking, running, etc. In addition, the spinal cord plays an important role in regulating the activity of internal organs, in particular the intestines, bladder, and blood vessels.

Rice. 5 - Posterior coda zones, innervated by spinal nerves. C - cervical nerves D - thoracic nerves L - lumbar nerves S - sacral nerves.

The activity of the spinal cord is controlled by the nerve centers of the brain. Therefore, damage to the spinal cord disrupts the activity of those parts of it that are located below the site of injury, and this is primarily due to the interruption of connections with the brain. For example, if the spinal cord is damaged, urination and defecation may be impaired. With unilateral damage to the spinal cord as a result of injury or disease, muscle paralysis, pain and muscle sensitivity disorders, and vascular disorders develop on the affected side. On the opposite side, voluntary movements are preserved, but pain and temperature sensitivity disappears. This nature of skin sensitivity disorders is explained by the crossover of the conductive pathways, that is, the transition of nerve fibers from one half of the brain to the opposite side. Restoration of reflex activity occurs very slowly, and begins with the simplest reflexes.

spinal nerves,

as already indicated, 31 pairs depart from the spinal cord and innervate the trunk and limbs.

Upon exiting the intervertebral foramen, each spinal nerve divides into branches; two of them are long - anterior and posterior, they are directed to the skin and muscles of the trunk and limbs.

The posterior branches of the spinal nerves are distributed evenly in all parts of the body. Each of the posterior branches is divided into smaller branches that innervate the deep back muscles located along the spine, as well as the skin of the back of the head, neck, back, lower back, and sacral region.

The anterior branches remain evenly distributed only in the thoracic region, where they form the intercostal nerves. The latter, in the amount of 12 pairs, pass in the intercostal spaces along with the vessels. The six lower nerves, reaching the anterior end of the intercostal spaces, continue to the anterior wall of the abdomen. These nerves innervate the intercostal muscles, abdominal muscles, as well as the skin of the chest and abdomen.

In other parts of the body, the anterior branches of the spinal nerves, connecting with each other, form the cervical, brachial, lumbar and sacral plexuses (see Fig. 1). The loss of the uniform course of most of the anterior branches of the spinal nerves is associated with the development of complex muscles of the limbs.

Cervical plexus

formed by the anterior rami of the four superior cervical spinal nerves and lies on the deep muscles of the neck lateral to the transverse processes of the vertebrae. Cutaneous, muscle nerves and the phrenic nerve depart from the plexus. Cutaneous nerves innervate the skin of the lateral parts of the head, auricle, neck and upper chest. Muscle nerves travel to the neck muscles. The phrenic nerve enters the chest cavity and reaches the diaphragm. The innervation of the diaphragm from the cervical plexus is explained by the development of this muscle in the prenatal period in the neck area.

Rice. 6 - Anterior cutaneous zones innervated by spinal nerves. C - cervical nerves D - thoracic nerves L - lumbar nerves S - sacral nerves.

Brachial plexus

formed by the anterior branches of the four lower cervical spinal nerves and a branch from the first thoracic nerve. It is located behind the collarbone and in the armpit. Short and long nerves extend from the brachial plexus. Short nerves emerge from the plexus above the collarbone and innervate the muscles of the shoulder girdle. The largest of them, the axillary nerve, gives branches to the deltoid muscle, shoulder joint and skin of the shoulder. The long nerves of the brachial plexus innervate the muscles, joints and skin of the free upper limb. Among them are the median, ulnar and radial nerves.

Lumbar plexus

formed by the anterior branches of the three upper lumbar spinal nerves, as well as branches from the twelfth thoracic and fourth lumbar nerves. The lumbar plexus lies deep within the psoas muscle. The nerves of this plexus innervate the skin and muscles of the lower abdominal wall, as well as the external genitalia, skin and muscles of the thigh. The femoral nerve is the largest nerve of the lumbar plexus. It enters the thigh under the inguinal ligament and innervates the muscles of the front of the thigh (quadriceps femoris and sartorius), the skin above them, as well as the skin of the inner surface of the leg and foot. The obturator nerve leaves the pelvic cavity on the inner side of the thigh, where it innervates the adductor muscles and skin located here. The femoral and obturator nerves give branches to the hip joint.

Sacral plexus

formed by the anterior branches of the fourth and fifth lumbar, all sacral and coccygeal spinal nerves. The sacral plexus is located in the small pelvis, from the cavity of which the plexus nerves exit through the greater sciatic foramen. Short nerves branch in the pelvic muscles (gluteal muscles, etc.), in the skin and muscles of the perineum and in the external genitalia. Long nerves travel to the back of the thigh. The sciatic nerve - the largest in the human body - innervates the muscles of the back of the thigh, and in the region of the popliteal fossa it is divided into two branches that innervate the knee joint, muscles, skin and joints of the lower leg and foot.

The branches of the spinal nerves also contain autonomic nerve fibers that innervate the vessels and glands of the skin, regulate metabolism in the skeletal muscles, and also go to the internal organs.

From a practical point of view, you should know that each dorsal root of the spinal nerve is related to the innervation of that segment of the skin that is associated with it during development. In the same way, each anterior root innervates those muscles that developed along with it. Therefore, all skin and muscles can be divided into a number of successive radicular zones, or belts, which have innervation from a specific spinal nerve (see Fig. 5, 6). That is why, when the posterior root of a spinal nerve becomes inflamed, girdle pain appears, exactly corresponding to the given radicular girdle of the skin.

Most nerves in the human body are mixed, meaning they contain both sensory and motor nerve fibers. That is why, when nerves are damaged, sensory disorders are almost always combined with motor disorders. However, the zones of innervation of neighboring nerves partially overlap, so complete loss of sensitivity of the skin area, as a rule, does not occur. Changes in the spine with age (for example, with osteochondrosis) and various injuries can affect the condition of the spinal cord and the nerves extending from it. Over the years, intervertebral discs lose elasticity and become flattened. At a certain point, often due to unsuccessful loading movement, nerve roots or nerves are injured in the narrowed intervertebral space. If they are affected, a diagnosis of “radiculitis” is made (Latin radix - root, spine, suffix “it” - indicates the inflammatory nature of the disease).

Source: Quality of Life. Prevention. No. 6, 2003

Evolution of the vertebrate brain

The formation of the central nervous system in the form of a neural tube first appears in chordates. In lower chordates, the neural tube is preserved throughout life; in higher chordates, vertebrates, a neural plate is formed on the dorsal side during the embryonic stage, which is immersed under the skin and folded into a tube. In the embryonic stage of development, the neural tube forms three swellings in the anterior part - three brain vesicles, from which parts of the brain develop: the anterior vesicle gives rise to the forebrain and diencephalon, the middle vesicle turns into the midbrain, the posterior vesicle forms the cerebellum and medulla oblongata. These five brain regions are characteristic of all vertebrates.

Lower vertebrates - fish and amphibians - are characterized by a predominance of the midbrain over other parts. In amphibians, the forebrain somewhat enlarges and a thin layer of nerve cells is formed in the roof of the hemispheres - the primary medullary vault, the ancient cortex. In reptiles, the forebrain increases significantly due to accumulations of nerve cells. Most of the roof of the hemispheres is occupied by the ancient cortex. For the first time in reptiles, the rudiment of a new cortex appears. The hemispheres of the forebrain creep onto other parts, as a result of which a bend is formed in the region of the diencephalon. Beginning with ancient reptiles, the cerebral hemispheres became the largest part of the brain.

The structure of the brain of birds and reptiles has much in common. On the roof of the brain is the primary cortex, the midbrain is well developed. However, in birds, compared to reptiles, the total brain mass and the relative size of the forebrain increase. The cerebellum is large and has a folded structure. In mammals, the forebrain reaches its greatest size and complexity. Most of the brain matter is made up of the neocortex, which serves as the center of higher nervous activity. The intermediate and middle parts of the brain in mammals are small. The expanding hemispheres of the forebrain cover them and crush them under themselves. Some mammals have a smooth brain without grooves or convolutions, but most mammals have grooves and convolutions in the cerebral cortex. The appearance of grooves and convolutions occurs due to the growth of the brain with limited dimensions of the skull. Further growth of the cortex leads to the appearance of folding in the form of grooves and convolutions.

The brain is the “administrator” of the nervous system and human psyche

Previously, it was believed that a person’s “soul” was contained in his heart. With the development of science, humanity gradually began to study what made us human, the crown of the animal world - the brain.

Our mental system is formed through the interaction of the cerebral cortex and the underlying sections (dietum, midbrain, brainstem). Each area is responsible for one or another function. But the most interesting thing is that when one section of neurons fails, its work can be partially replaced by another, which is called neuroplasticity.

The frontal lobe is involved in the formation of emotions, memory, speech and behavior. Evolutionarily, this part is the most developed in Homo sapiens, since it began to develop with the transition of primates to upright walking and the activation of fine motor skills of the upper limbs. Therefore, the frontal lobe is responsible for many functions. To understand what effect the frontal lobe has on a person’s mental state, we should mention the so-called frontal syndrome, which is observed in people with organic brain damage due to external influences, vascular, oncological and other pathologies. They experience disinhibition in behavior, self-control disappears, and a tendency toward brutal, antisocial behavior, as well as outbursts of aggressiveness, appears. In addition to behavior and emotions, memory is impaired, a person cannot concentrate on any task, and the function of cognition of the outside world suffers. In severe cases, the core of the personality is lost - we stop seeing the person as he was before.

There is a part of the brain called the hypothalamus, which is responsible for regulating autonomic functions. It communicates with the endocrine system of the body and is directly connected with the hormone regulator gland – the pituitary gland. The latter secretes special substances that give a signal to the pituitary gland to release hormones: gonadotropic (affecting the gonads), thyroid-tropic (thyroid), adrenocorticotropic (adrenal glands), somatotropic (tissue growth) and prolactin (breast).

Brain

If the spinal cord in all vertebrates is developed more or less equally, then the brain differs significantly in size and complexity of structure in different animals. The forebrain undergoes particularly dramatic changes during evolution. In lower vertebrates, the forebrain is poorly developed. In fish, it is represented by the olfactory lobes and nuclei of gray matter in the thickness of the brain. The intensive development of the forebrain is associated with the emergence of animals onto land. It differentiates into the diencephalon and two symmetrical hemispheres, which are called the telencephalon. Gray matter on the surface of the forebrain (cortex) first appears in reptiles, developing further in birds and especially in mammals. Truly large forebrain hemispheres become only in birds and mammals. In the latter, they cover almost all other parts of the brain.

The brain is located in the cranial cavity. It includes the brainstem and telencephalon (cerebral cortex).

The brainstem consists of the medulla oblongata, pons, midbrain and diencephalon.

The medulla oblongata is a direct continuation of the spinal cord and, expanding, passes into the hindbrain. It basically retains the shape and structure of the spinal cord. In the thickness of the medulla oblongata there are accumulations of gray matter - the nuclei of the cranial nerves. The posterior pons includes the cerebellum and the pons. The cerebellum is located above the medulla oblongata and has a complex structure. On the surface of the cerebellar hemispheres, gray matter forms the cortex, and inside the cerebellum - its nuclei. Like the spinal medulla oblongata, it performs two functions: reflex and conductive. However, the reflexes of the medulla oblongata are more complex. This is reflected in its importance in the regulation of cardiac activity, the condition of blood vessels, respiration, and sweating. The centers of all these functions are located in the medulla oblongata. Here are the centers for chewing, sucking, swallowing, saliva and gastric juice. Despite its small size (2.5–3 cm), the medulla oblongata is a vital part of the central nervous system. Damage to it can cause death due to cessation of breathing and heart activity. The conductor function of the medulla oblongata and the pons is to transmit impulses from the spinal cord to the brain and back.

In the midbrain there are primary (subcortical) centers of vision and hearing, which carry out reflexive orienting reactions to light and sound stimuli. These reactions are expressed in various movements of the torso, head and eyes towards the stimuli. The midbrain consists of the cerebral peduncles and quadrigeminalis. The midbrain regulates and distributes the tone (tension) of skeletal muscles.

The diencephalon consists of two sections - the thalamus and hypothalamus, each of which consists of a large number of nuclei of the visual thalamus and subthalamic region. Through the visual thalamus, centripetal impulses are transmitted to the cerebral cortex from all receptors of the body. Not a single centripetal impulse, no matter where it comes from, can pass to the cortex, bypassing the visual hillocks. Thus, through the diencephalon, all receptors communicate with the cerebral cortex. In the subtubercular region there are centers that influence metabolism, thermoregulation and endocrine glands.

The cerebellum is located behind the medulla oblongata. It consists of gray and white matter. However, unlike the spinal cord and brainstem, the gray matter - the cortex - is located on the surface of the cerebellum, and the white matter is located inside, under the cortex. The cerebellum coordinates movements, makes them clear and smooth, plays an important role in maintaining the balance of the body in space, and also influences muscle tone. When the cerebellum is damaged, a person experiences a decrease in muscle tone, movement disorders and changes in gait, speech slows down, etc. However, after some time, movement and muscle tone are restored due to the fact that the intact parts of the central nervous system take over the functions of the cerebellum.

The cerebral hemispheres are the largest and most developed part of the brain. In humans, they form the bulk of the brain and are covered with cortex over their entire surface. Gray matter covers the outside of the hemispheres and forms the cerebral cortex. The human cerebral cortex has a thickness of 2 to 4 mm and is composed of 6–8 layers formed by 14–16 billion cells, different in shape, size and functions. Under the cortex is a white substance. It consists of nerve fibers connecting the cortex with the lower parts of the central nervous system and the individual lobes of the hemispheres with each other.

The cerebral cortex has convolutions separated by grooves, which significantly increase its surface. The three deepest grooves divide the hemispheres into lobes. In each hemisphere there are four lobes: frontal, parietal, temporal, occipital. The excitation of different receptors enters the corresponding receptive areas of the cortex, called zones, and from here they are transmitted to a specific organ, prompting it to action. The following zones are distinguished in the cortex. The auditory zone is located in the temporal lobe and receives impulses from auditory receptors.

The visual zone lies in the occipital region. Impulses from the eye receptors arrive here.

The olfactory zone is located on the inner surface of the temporal lobe and is connected to the receptors of the nasal cavity.

The sensory-motor zone is located in the frontal and parietal lobes. This zone contains the main centers of movement of the legs, torso, arms, neck, tongue and lips. This is also where the center of speech lies.

The cerebral hemispheres are the highest division of the central nervous system, controlling the functioning of all organs in mammals. The importance of the cerebral hemispheres in humans also lies in the fact that they represent the material basis of mental activity. I.P. Pavlov showed that mental activity is based on physiological processes occurring in the cerebral cortex. Thinking is associated with the activity of the entire cerebral cortex, and not just with the function of its individual areas.

Structure and types of the nervous system: structural classification

To simplify the structure of the nervous system, in medicine there are several classification options depending on the structure and functions performed. Thus, anatomically, the human nervous system can be divided into 2 broad groups:

• central (CNS), formed by the brain and spinal cord;
• peripheral (PNS), represented by nerve ganglia, endings and nerves themselves.

The basis of this classification is extremely simple: the central nervous system is a kind of connecting link in which the analysis of the incoming impulse and further regulation of the activity of organs and systems is carried out. And the PNS serves to transport the received signal from the receptors to the CNS and the subsequent activator, but from the CNS to the cells and tissues that will perform a specific action.

central nervous system

The central nervous system is a key component of the nervous system, because it is here that the main reflexes are formed. It consists of the spinal cord and brain, each of which is reliably protected from external influences by bone structures. Such thoughtful protection is necessary, since each part of the central nervous system performs vital functions, without which it is impossible to maintain health.

Spinal cord

This structure is contained within the spinal column. It is responsible for the simplest reflexes and involuntary reactions of the body to stimuli.

In addition, spinal cord neurons coordinate the activity of muscle tissue, which regulates defense mechanisms. For example, upon feeling an extremely hot temperature, a person involuntarily withdraws his palm, thereby protecting himself from a thermal burn. This is a typical reaction controlled by the spinal cord.

Brain

The human brain consists of several sections, each of which performs a number of physiological and psychological functions:

1. The medulla oblongata is responsible for the vital functions of the body - digestion, respiration, blood movement through the vessels, etc. In addition, the nucleus of the vagus nerve is located here, which regulates the autonomic balance and psycho-emotional reaction. If the nucleus of the vagus nerve sends active impulses, a person’s vitality decreases, he becomes apathetic, melancholic and depressed. If the activity of impulses emanating from the core decreases, the psychological perception of the world changes to a more active and positive one.
2. The cerebellum regulates the precision and coordination of movements.
3. The midbrain is the main coordinator of muscle reflexes and tone. In addition, neurons regulated by this part of the central nervous system contribute to the adaptation of the sensory organs to external stimuli (for example, pupil accommodation at dusk).
4. The diencephalon is formed by the thalamus and hypothalamus. The thalamus is the most important organ that analyzes incoming information. The hypothalamus regulates the emotional background and metabolic processes; there are centers responsible for the sensation of hunger, thirst, fatigue, thermoregulation, and sexual activity. Thanks to this, not only physiological processes are coordinated, but also many human habits, for example, the tendency to overeat, the perception of cold, etc.
5. Cerebral cortex. The cerebral cortex is a key link in mental functions, including consciousness, speech, perception of information and its subsequent comprehension. The frontal lobe regulates motor activity, the parietal lobe is responsible for bodily sensations, the temporal lobe controls hearing, speech and other higher functions, and the occipital lobe contains the centers of visual perception.

Peripheral nervous system

The PNS provides interconnection between organs, tissues, cells and the central nervous system. Structurally, it is represented by the following morphofunctional units:

1. Nerve fibers, which, depending on the functions performed, are motor, sensory and mixed. Motor nerves transmit information from the central nervous system to muscle fibers, sensitive nerves, on the contrary, help to perceive information received through the senses and transmit it to the central nervous system, and mixed nerves are involved to one degree or another in both processes.
2. Nerve endings, which are also motor and sensory. Their function is no different from fiber structures with the only nuance - the nerve endings begin or, conversely, end the chain of impulses from the organs to the central nervous system and back.
3. Nerve ganglia, or ganglia, are clusters of neurons outside the central nervous system. The spinal ganglia are responsible for transmitting information received from the external environment, and the autonomic ganglia are responsible for transmitting information about the state and activity of the internal organs and resources of the body.

In addition, all peripheral nerves are classified depending on their anatomical features. Based on this characteristic, there are 12 pairs of cranial nerves that coordinate the activity of the head and neck, and 31 pairs of spinal nerves that are responsible for the torso, upper and lower extremities, as well as internal organs located in the abdominal and thoracic cavities.

Cranial nerves originate from the brain. The basis of their activity is the perception of sensory impulses, as well as partial participation in respiratory, digestive and cardiac activity. The function of each pair of cranial nerves is presented in more detail in the table.

The activity of the spinal nerves is classified much more simply - each specific pair or complex of pairs is responsible for its assigned area of ​​the body with the same name:

• cervical - 8 pairs,
• infants - 12 pairs,
• lumbar and sacral - 5 pairs respectively,
• coccygeal - 1 pair.

Each representative of this group belongs to mixed nerves, formed by two roots: sensory and motor. That is why the spinal nerves can both perceive an irritating effect, transmitting an impulse along the chain, and intensify activity in response to a message from the central nervous system.

Cerebral cortex

The surface of the human cerebral cortex is about 1500 cm2, which is many times greater than the inner surface of the skull. This large surface of the cortex was formed due to the development of a large number of grooves and convolutions, as a result of which most of the cortex (about 70%) is concentrated in the grooves. The largest grooves of the cerebral hemispheres are the central one, which runs across both hemispheres, and the temporal one, which separates the temporal lobe from the rest. The cerebral cortex, despite its small thickness (1.5–3 mm), has a very complex structure. It has six main layers, which differ in the structure, shape and size of neurons and connections. The cortex contains the centers of all sensory (receptor) systems, representatives of all organs and parts of the body. In this regard, centripetal nerve impulses from all internal organs or parts of the body approach the cortex, and it can control their work. Through the cerebral cortex, conditioned reflexes are closed, through which the body constantly, throughout life, very accurately adapts to the changing conditions of existence, to the environment.