What are nerves in biology. What is the human nervous system: the structure and functions of a complex structure

Nerves depart in pairs (left and right) from the brain and spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves; the totality of nerves and their derivatives makes up the peripheral nervous system, which, depending on the characteristics of the structure, functioning and origin, is divided into two parts: the somatic nervous system, which innervates the skeletal muscles and the skin of the body, and the autonomic nervous system, which innervates the internal organs, glands, circulatory system, etc.

The development of the cranial and spinal nerves is associated with the metameric (segmental) laying of the muscles, the development of the internal organs and the skin of the body. In a human embryo (at the 3rd-4th week of development), respectively, each of the 31 segments of the body (somite) has a pair of spinal nerves innervating the muscles and skin, as well as internal organs formed from the material of this somite.
Each spinal N. is laid in the form of two roots: anterior, containing motor nerve fibers, and posterior, consisting of sensory nerve fibers. On the 2nd month of intrauterine development, the anterior and posterior roots merge and the spinal nerve trunk is formed.

In an embryo 10 mm long, the brachial plexus is already defined, which is an accumulation of nerve fibers from different segments of the spinal cord at the level of the cervical and upper thoracic regions. At the level of the proximal end of the developing shoulder, the brachial plexus divides into the anterior and posterior neural plates, which subsequently give rise to nerves that innervate the muscles and skin of the upper limb. The anlage of the lumbosacral plexus, from which the nerves innervating the muscles and skin of the lower limb are formed, is determined in an embryo 11 mm long. Other nerve plexuses are formed later, however, already in an embryo 15-20 mm long, all nerve trunks of the limbs and trunk correspond to the position of N. in a newborn. Subsequently, the features of N.'s development in ontogenesis are associated with the timing and degree of myelination of nerve fibers. Motor nerves are myelinated earlier, mixed and sensory nerves later.

The development of cranial nerves has a number of features associated primarily with the laying of the sense organs and gill arches with their muscles, as well as the reduction of myotomes (myoblastic components of somites) in the head region. In this regard, the cranial nerves lost their original segmental structure in the process of phylogenesis and became highly specialized .

Each nerve consists of nerve fibers of a different functional nature, "packed" with the help of connective tissue membranes into bundles and an integral nerve trunk; the latter has a fairly strict topographic and anatomical localization. Some nerves, especially the vagus, contain nerve cells scattered along the trunk, which can accumulate in the form of microganglia.

The composition of the spinal and most of the cranial nerves includes somatic and visceral sensory, as well as somatic and visceral motor nerve fibers. The motor nerve fibers of the spinal nerves are processes of motor neurons located in the anterior horns of the spinal cord and passing through the anterior roots. Together with them, motor visceral (preganglionic) nerve fibers pass in the anterior roots. Sensory somatic and visceral nerve fibers originate from neurons located in the spinal ganglia. The peripheral processes of these neurons as part of the nerve and its branches reach the innervated substrate, and the central processes as part of the posterior roots reach the spinal cord and terminate at its nuclei. In cranial nerves, nerve fibers of various functional nature originate from the corresponding nuclei of the brain stem and nerve ganglia.

Nerve fibers can have a length of several centimeters to 1 m, their diameter varies from 1 to 20 microns. The process of the nerve cell, or axial cylinder, is the central part of the nerve fiber; outside it is surrounded by a thin cytoplasmic membrane - neurilemma. In the cytoplasm of the nerve fiber there are many neurofilaments and neurotubules; electronograms reveal microbubbles and mitochondria. Along the nerve fibers (in the motor in the centrifugal, and in the sensitive in the centripetal directions) the neuroplasm flow is carried out: slow - at a speed of 1-3 mm per day, with which vesicles, lysosomes and some enzymes are transferred, and fast - at a speed of about 5 mm per day. 1 hour, with which the substances necessary for the synthesis of neurotransmitters are transferred. Outside of the neurolemma is the glial, or Schwann sheath, formed by neurolemmocytes (Schwann cells). This sheath is the most important component of the nerve fiber and is directly related to the conduction of the nerve impulse along it.

In part of the nerve fibers between the axial cylinder and the cytoplasm of neurolemmocytes, a layer of myelin (myelin sheath) of varying thickness is found - a membrane complex rich in phospholipids that acts as an electrical insulator and plays an important role in the conduction of a nerve impulse. Fibers containing a myelin sheath are called myelin, or pulpy; other fibers that do not have this sheath are called amyelinated, or non-myelinated. Non-fleshy fibers are thin, their diameter ranges from 1 to 4 microns. In non-fleshy fibers outside of the axial cylinder there is a thin layer of the glial membrane. formed by chains of neurolemmocytes oriented along the nerve fiber.

In the pulpy fibers, the myelin sheath is arranged in such a way that the areas of the nerve fiber covered with myelin alternate with narrow areas that are not covered with myelin, they are called nodes of Ranvier. Neighboring nodes of Ranvier are located at a distance of 0.3 to 1.5 mm. It is believed that such a structure of the myelin sheath provides the so-called saltatory (jump-like) conduction of a nerve impulse, when depolarization of the nerve fiber membrane occurs only in the Ranvier intercept zone, and the nerve impulse seems to “jump” from one intercept to another. As a result, the speed of nerve impulse conduction in a myelin fiber is approximately 50 times higher than in an unmyelinated one. The speed of nerve impulse conduction in myelin fibers is the higher, the thicker their myelin sheath. Therefore, the process of myelination of nerve fibers inside N. during the period of development plays an important role in achieving certain functional characteristics of the nerve.

The quantitative ratio of the pulpy fibers having different diameter and different thickness of a myelin cover considerably varies not only in different N., but also in the same nerve at different individuals. The number of nerve fibers in nerves is extremely variable.

Inside the nerve, nerve fibers are packed into bundles of different sizes and unequal lengths. Outside, the bundles are covered with relatively dense plates of connective tissue - perineurium, in the thickness of which there are perineural gaps necessary for lymph circulation. Inside the bundles, nerve fibers are surrounded by loose connective tissue - endoneurium. Outside, the nerve is covered with a connective tissue sheath - epineurium. The sheaths of the nerve contain blood and lymphatic vessels, as well as thin nerve trunks that innervate the sheaths. The nerve is sufficiently abundantly supplied with blood vessels that form a network in the epineurium and between the bundles; the capillary network is well developed in the endoneurium. The blood supply to the nerve is carried out from nearby arteries, which often form, together with the nerve, a neurovascular bundle.

The intratrunk beam structure of the nerve is variable. It is customary to distinguish small-fascicular nerves, usually having a small thickness and a small number of bundles, and multi-fascicular nerves, which are characterized by greater thickness, a large number of bundles and many interfascicular connections. Monofunctional cranial nerves have the simplest intratruncal structure, and spinal and cranial nerves, which are branchial in origin, have a more complex bundle architectonics. The plurisegmental nerves, which form as branches of the brachial, lumbosacral, and other nerve plexuses, have the most complex intratrunk structure. A characteristic feature of the intrastem organization of nerve fibers is the formation of large axial bundles traced over a considerable distance, which provide a redistribution of motor and sensory fibers between numerous muscle and skin branches extending from the nerves.

There are no uniform principles for the classification of nerves; therefore, a variety of signs are reflected in the nomenclature of nerves. Some nerves got their name depending on their topographic position (for example, ophthalmic, facial, etc.), others - according to the innervated organ (for example, lingual, upper laryngeal, etc.). N., innervating the skin, are called skin, while N., innervating muscles, are called muscle branches. Sometimes the branches of the branches are called nerves (for example, the upper gluteal nerve).

Depending on the nature of the nerve fibers that make up the nerves and their intratrunk architectonics, three groups of nerves are distinguished: monofunctional, which include some motor cranial nerves (III, IV, VI, XI and XII pairs); monosegmental - all spinal N. and those cranial N., which by their origin belong to the gills (V, VII, VIII, IX and X pairs); plurisegmental, resulting from the mixing of nerve fibers. originating from different segments of the spinal cord, and developing as branches of the nerve plexuses (cervical, brachial and lumbosacral).

All spinal nerves have a typical structure. Formed after the fusion of the anterior and posterior roots, the spinal nerve, upon exiting the spinal canal through the intervertebral foramen, immediately divides into anterior and posterior branches, each of which is mixed in the composition of nerve fibers. In addition, connecting branches to the sympathetic trunk and a sensitive meningeal branch to the meninges of the spinal cord depart from the spinal nerve. The posterior branches are directed posteriorly between the transverse processes of the vertebrae, penetrate into the back region, where they innervate the deep intrinsic muscles of the back, as well as the skin of the occipital region, the back of the neck, the back, and partially the gluteal region. The anterior branches of the spinal nerves innervate the rest of the muscles, the skin of the trunk and extremities. The easiest way they are arranged in the thoracic region, where the segmental structure of the body is well expressed. Here, the anterior branches run along the intercostal spaces and are called the intercostal nerves. Along the way, they give short muscle branches to the intercostal muscles and skin branches to the skin of the lateral and anterior surfaces of the body.

The anterior branches of the four upper cervical spinal nerves form the cervical plexus, from which the plurisegmental nerves innervating the skin and muscles in the neck are formed.

The anterior branches of the lower cervical and two upper thoracic spinal nerves form the brachial plexus. The brachial plexus entirely provides innervation to the muscles and skin of the upper limb. All branches of the brachial plexus in terms of the composition of nerve fibers are mixed plurisegmental nerves. The largest of them are: the median and musculocutaneous nerve, which innervate most of the flexor and pronator muscles on the shoulder and forearm, in the area of ​​the hand (a muscle group of the thumb, as well as the skin on the anterolateral surface of the forearm and hand); the ulnar nerve, which innervates those flexors of the hand and fingers that are located above the ulna, as well as the skin of the corresponding areas of the forearm and hand; the radial nerve, which innervates the skin of the posterior surface of the upper limb and the muscles that provide extension and supination in its joints.

The lumbar plexus is formed from the anterior branches of the 12 thoracic and 1-4 lumbar spinal nerves; it gives short and long branches that innervate the skin of the abdominal wall, thigh, lower leg and foot, as well as the muscles of the abdomen, pelvis and free lower limb. The largest branch is the femoral nerve, its cutaneous branches go to the anterior and inner surface of the thigh, as well as to the anterior surface of the lower leg and foot. Muscular branches innervate the quadriceps femoris, sartorius and pectus muscles.

Anterior branches of 4 (partial), 5 lumbar and 1-4 sacral spinal nerves. form the sacral plexus, which, together with the branches of the lumbar plexus, innervate the skin and muscles of the lower limb, so they are sometimes combined into one lumbosacral plexus. Among the short branches, the most important are the superior and inferior gluteal nerves and the pudendal nerve, which innervate the skin and muscles of the respective areas. The largest branch is the sciatic nerve. Its branches innervate the posterior thigh muscle group. In the region of the lower third of the thigh, it is divided into the tibial nerve (it innervates the lower leg muscles and the skin of its posterior surface, and on the foot - all the muscles located on its plantar surface and the skin of this surface) and the common peroneal N. (its deep and superficial branches on The lower legs innervate the peroneal muscles and extensor muscles of the foot and fingers, as well as the skin of the lateral surface of the lower leg, dorsal and lateral surfaces of the foot).

Segmental innervation of the skin reflects the genetic connections formed at the stage of embryonic development, when connections are established between neurotomes and the corresponding dermatomes. Since the laying of the limbs can occur with cranial and caudal displacement of the segments going to their construction, the formation of the brachial and lumbosacral plexus with cranial and caudal displacements is possible. In this regard, there are shifts in the projection of the spinal segments on the skin of the body, and the same-name involvement of the skin in different individuals may have different segmental innervation. Muscles also have segmental innervation. However, due to the significant displacement of the material of the myotomes used for the construction of certain muscles, as well as the polysegmental origin and polysegmental innervation of most muscles, we can only talk about the predominant participation of certain segments of the spinal cord in their innervation.

Pathology:

Neuropathy:

According to etiology, hereditary neuropathies are distinguished, which include all neural amyotrophies, as well as neuropathies with Friedreich's ataxia (see Ataxia), ataxia-telangiectasia, some hereditary metabolic diseases; metabolic (for example, in diabetes mellitus); toxic - in case of poisoning with salts of heavy metals, organophosphorus compounds, some drugs, etc.; neuropathy in systemic diseases (eg, porphyria, multiple myeloma, sarcoidosis, diffuse connective tissue diseases); ischemic (for example, with vasculitis). Tunnel neuropathies and injuries of the nerve trunks are especially distinguished.

Diagnosis of neuropathy involves the detection of characteristic clinical symptoms in the zone of innervation of the nerve. With mononeuropathy, the symptom complex consists of motor disorders with paralysis, atony and atrophy of denervated muscles, the absence of tendon reflexes, loss of skin sensitivity in the area of ​​innervation, vibrational and joint-muscular feeling, autonomic disorders in the form of impaired thermoregulation and sweating, trophic and vasomotor disorders in the zone of innervation.

With an isolated lesion of motor, sensory or autonomic nerve fibers in the zone of innervation, changes are observed associated with the predominant lesion of certain fibers. Mixed variants with the deployment of a full symptom complex are more often noted. Of great importance is an electromyographic study, recording denervation changes in the bioelectrical activity of denervated muscles and determining the speed of conduction along the motor and sensory fibers of the nerve. It is also important to determine changes in the parameters of the evoked potentials of the muscle and nerve in response to electrical stimulation. When a nerve is damaged, the speed of impulse conduction along it decreases, and most sharply during demyelination, to a lesser extent - with axonopathy and neuronopathy.

But with all variants, the amplitude of the evoked potentials of the muscle and the nerve itself decreases sharply. It is possible to study conduction in small segments of the nerve, which helps in the diagnosis of a conduction block, for example, in carpal tunnel syndrome or a closed injury of the nerve trunk. In polyneuropathies, sometimes a biopsy of the superficial cutaneous nerves is performed in order to study the nature of the damage to their fibers, vessels and nerves, endo- and perineural connective tissue. In the diagnosis of toxic neuropathy, biochemical analysis is of great importance in order to identify a toxic substance in biological fluids and hair. Differential diagnosis of hereditary neuropathy is carried out on the basis of the establishment of metabolic disorders, examination of relatives, as well as the presence of characteristic concomitant symptoms.

Along with common features, dysfunctions of individual nerves have characteristic features. So, with damage to the facial nerve, simultaneously with paralysis of the facial muscles on the same side, a number of concomitant symptoms are observed associated with the involvement in the pathological process of the adjacent lacrimal, salivary and gustatory nerves (lacrimation or dryness of the eye, taste disturbance in the anterior 2/3 of the tongue, salivation sublingual and submandibular salivary glands). Concomitant symptoms include pain behind the ear (involvement in the pathological process of a branch of the trigeminal nerve) and hyperacusis - increased hearing (paralysis of the stapedial muscle). Since these fibers depart from the trunk of the facial nerve at its different levels, according to the existing symptoms, an accurate topical diagnosis can be made.

The trigeminal nerve is mixed, its lesion is manifested by loss of sensation on the face or in the area corresponding to the location of its branch, as well as paralysis of the masticatory muscles, accompanied by deviation of the lower jaw when opening the mouth. More often, the pathology of the trigeminal nerve is manifested by neuralgia with excruciating pain in the orbit and forehead, upper or lower jaw.

The vagus nerve is also mixed, it provides parasympathetic innervation to the eye, salivary and lacrimal glands, as well as almost all organs located in the abdominal and chest cavities. When it is damaged, disorders occur due to the predominance of the tone of the sympathetic division of the autonomic nervous system. Bilateral shutdown of the vagus nerve leads to the death of the patient due to paralysis of the heart and respiratory muscles.

Damage to the radial nerve is accompanied by drooping of the hand with arms extended forward, impossibility of extension of the forearm and hand, abduction of the first finger, absence of ulnar extensor and carporadial reflexes, sensitivity disorder of I, II and partially III fingers of the hand (with the exception of the terminal phalanges). Damage to the ulnar nerve is characterized by atrophy of the muscles of the hand (interosseous, worm-like, eminence of the fifth finger and partially of the first finger), the hand takes the form of a “clawed paw”, when you try to squeeze it into a fist III, IV and V fingers remain unbent, anesthesia of the fifth and half of the fourth is noted fingers from the side of the palm, as well as V, IV and half of the III fingers on the back and medial part to the level of the wrist.

When the median nerve is damaged, atrophy of the muscles of the elevation of the thumb occurs with its installation in the same plane with the second finger (the so-called monkey hand), pronation and palmar flexion of the hand, flexion of 1-III fingers and extension of II and III are disturbed. Sensitivity is disturbed on the outer part of the palm and on the palmar half of the I-III and partially IV fingers. Due to the abundance of sympathetic fibers in the trunk of the median nerve, a kind of pain syndrome - causalgia, can be observed, especially with traumatic damage to the nerve.

Damage to the femoral nerve is accompanied by impaired flexion of the hip and extension of the lower leg, atrophy of the muscles of the anterior surface of the thigh, a sensory disorder on the lower 2/3 of the anterior surface of the thigh and the anterior inner surface of the lower leg, and the absence of a knee reflex. The patient cannot walk up stairs, run and jump.

Neuropathy of the sciatic nerve is characterized by atrophy and paralysis of the muscles of the back of the thigh, all muscles of the lower leg and foot. The patient cannot walk on heels and toes, the foot hangs down in the sitting position, there is no Achilles reflex. Sensitivity disorders extend to the foot, outer and back of the lower leg. As with damage to the median nerve, causalgia syndrome is possible.

Treatment is aimed at restoring conduction along the motor and sensory fibers of the affected nerve, trophism of denervated muscles, and functional activity of segmental motor neurons. A wide range of rehabilitation therapy is used: massage, exercise therapy, electrical stimulation and reflexology, drug treatment.

Injuries to the nerve (closed and open) lead to a complete interruption or partial disruption of conduction along the nerve trunk. Conduction disturbances along the nerves occur at the time of its damage. The degree of damage is determined by the symptoms of loss of movement functions, sensitivity and autonomic functions in the area of ​​innervation of the damaged nerve below the level of injury. In addition to the symptoms of prolapse, symptoms of irritation in the sensitive and vegetative sphere can be detected and even predominate.

There are anatomical breaks in the nerve trunk (complete or partial) and intrastem nerve damage. The main sign of a complete anatomical nerve break is a violation of the integrity of all fibers and membranes that make up its trunk. Intra-stem injuries (hematoma, foreign body, rupture of nerve bundles, etc.) are characterized by relatively severe widespread changes in nerve bundles and intra-stem connective tissue with little damage to the epineurium.

Diagnosis of nerve damage includes a thorough neurological and complex electrophysiological examination (classical electrodiagnostics, electromyography, evoked potentials from sensory and motor nerve fibers). To determine the nature and level of nerve damage, intraoperative electrical stimulation is performed, depending on the results of which the question of the nature of the necessary operation is decided (neurolysis, nerve suture.).

The use of an operating microscope, special microsurgical instruments, thin suture material, new suture technique and the use of interfascicular autotransplantation significantly expanded the possibilities of surgical interventions and increased the degree of recovery of motor and sensory function after them.

Indications for nerve suture are a complete anatomical rupture of the nerve trunk or nerve conduction disturbances in an irreversible pathological neural process. The main surgical technique is an epineural suture with precise alignment and fixation of transverse sections of the central and peripheral ends of the transected nerve trunk. Methods of perineural, interfascicular and mixed sutures have been developed, and for large defects, the method of interfascicular H autotransplantation. The effectiveness of these operations depends on the absence of nerve tension. at the suture site and accurate intraoperative identification of intraneural structures.

There are primary operations, in which the nerve suture is performed simultaneously with the primary surgical treatment of wounds, and delayed ones, which can be early (the first weeks after injury) and late (later than 3 months from the date of injury). The main conditions for the imposition of a primary suture are a satisfactory condition of the patient, a clean wound. nerve injury with a sharp object without crush foci.

Results of an operative measure at N.'s damage depend on duration of a disease, age of the patient, character. the degree of damage, its level, etc. In addition, electro- and physiotherapy, absorbable therapy are used, drugs that improve blood circulation are prescribed. Subsequently, sanatorium-resort and mud therapy are shown.

Nerve tumors:

Neurinoma (neurilemmoma, schwannoma) is a benign tumor associated with the Schwann sheath of nerves. It is localized in soft tissues along the peripheral nerve trunks, cranial nerves, less often in the walls of hollow internal organs. Neurofibroma develops from elements of the endo- and epinervium. It is localized in the depths of soft tissues along the nerves, in the subcutaneous tissue, in the roots of the spinal cord, in the mediastinum, and in the skin. Multiple, associated with the nerve trunks nodes of neurofibroma are characteristic of neurofibromatosis. In this disease, bilateral tumors of the II and VIII pairs of cranial nerves are often found.

Diagnosis in an outpatient setting is based on the localization of the tumor along the nerve trunks, symptoms of irritation or loss of sensory or motor function of the affected nerve, irradiation of pain and paresthesia along the branching of the nerve during its palpation, the presence, in addition to the tumor, on the skin of spots of the color "coffee with milk", segmental autonomic disorders in the zone of innervation of the affected autonomic nodes, etc. The treatment of benign tumors is surgical, which consists in excising or exfoliating the tumor. The prognosis for life with benign tumors of N. is favorable. The prognosis for recovery is doubtful in multiple neurofibromatosis and favorable in other forms of neoplasms. Prevention of amputation neuromas consists in the correct processing of the nerve during limb amputations.

Malignant tumors of the nerves are sarcomas, which are divided into neurogenic sarcoma (malignant neurilemmoma, malignant schwannoma), malignant neurofibroma, neuroblastoma (sympathogonioma, sympathetic neuroblastoma, embryonic sympathoma) and ganglioneuroblastoma (malignant ganglioneuroma, ganglion cell neuroblastoma). The clinical picture of these tumors depends on the location and histological features. Often the tumor is noticeable on examination. The skin over the tumor is shiny, stretched, tense. The tumor infiltrates the surrounding muscles, is mobile in the transverse direction and does not move in the longitudinal direction. It is usually associated with a nerve.

Neurogenic sarcoma is rare, more often in young men, can be encapsulated, sometimes represented by several nodes along the nerve. It spreads through the perineural and perivascular spaces. Malignant neurofibroma occurs more often as a result of malignancy of one of the neurofibroma nodes. Neuroblastoma develops in the retroperitoneal space, soft tissues of the extremities, mesentery, adrenal glands, lungs, and mediastinum. Sometimes it is multiple. It occurs mainly in childhood. It grows rapidly, metastasizes early to the lymph nodes, liver, bones. Bone metastases from neuroblastomas are often misdiagnosed as Ewing's sarcoma.

Ganglioneuroblastoma is a malignant variant of ganglioneuroma. It occurs more often in children and young people, in clinical manifestations it is similar to ganglioneuroma, but less dense and prone to sprouting into neighboring tissues. The most important role in the diagnosis is given to the puncture of the tumor, and in cases where there is a suspicion of neuroblastoma, to the study of the bone marrow. Treatment of neurogenic malignant tumors - combined, includes surgical, radiation and chemotherapy methods. The prognosis for recovery and life is uncertain.

Operations:

Neurotomy (dissection, intersection of the nerve) is used for the purpose of denervation in non-healing leg ulcers, tuberculous tongue ulcers, to relieve pain, spasticity in paralysis and reflex contractures, athetosis, and amputation neuromas. Selective fascicular neurotomy is performed in cerebral palsy, post-traumatic hemitonia, etc. Neurotomy is also used in reconstructive operations on peripheral nerves and the brachial plexus.

Neurectomy - excision of the nerve. A variant of this operation is neurexeresis - pulling out the nerve. The operation is performed for pain in the amputation stump, phantom pain caused by the presence of a neuroma, cicatricial processes in the stump, as well as to change muscle tone in Little's disease, post-traumatic hemitonia.

Neurotripsy - crushing a nerve in order to turn off its function; operation is rarely used. It is indicated for persistent pain syndromes (for example, with phantom pains) in cases where it is necessary to turn off the function of the nerve for a long time.