What does "semicircular canals" mean? Functions of the semicircular canals of the labyrinth. The importance of the semicircular canals in the development of vertigo

Structure and functions of semicircular canals

Receptors are secondary sensory cells: they do not have their own processes, but are innervated afferent fibers neurons vestibular ganglion included in vestibular nerve. They also end on receptor cells efferent nerve fibers. Afferent fibers transmit information about the level of excitation of a peripheral organ to central nervous system. Efferent fibers change the sensitivity of receptors, but the significance of this influence is still not entirely clear.

Since the cilia are immersed in a jelly-like mass (cupula), when the latter moves, they deflect (bend). The shift of the cilia bundle is an adequate stimulus for the receptor. When such a shift is directed towards the kinocilium, the corresponding afferent nerve fiber is activated, causing excitation of the receptors. If the beam moves in the opposite direction, the pulse frequency decreases. A shift in the perpendicular direction does not cause a change in activity. Information is transmitted from the receptor cell to the ending of the afferent nerve through receptor potential and an as yet unidentified mediator.

Structure and functions of the otolithic apparatus

On each side of the head there are two maculas (statolith organs) - the macula utriculus (round sac) and the macus sacculus (oval sac). Macules are constantly exposed to gravity. When head is in normal position, the macula of the utriculus is located almost horizontally, so that otolith membrane does not apply shear force to sensory epithelium. When the head tilts, the utriculus macula becomes angled and the heavy otolithic membrane slides onto short distance along the sensory epithelium, as a result of which the cilia bend and the receptor is stimulated. Depending on the direction of inclination, the frequency of impulses in afferent fibers increases or decreases. The sacculus macula is also stimulated. Thus, at any position of the head, each of the otolith membranes occupies a certain position relative to the sensory epithelium and the body receives information about the position of the head in space.

Otolith membrane, containing calcite crystals, has a significantly higher specific gravity (2.2) than endolymph(about 1), which fills the internal cavity sacculus And utriculus. If an organ is subjected to linear acceleration, the inertial force acting on the endolymph and the otolith membrane is different, since their density is different. Thus, the entire otolith apparatus glides very easily by inertia along the sensory epithelium. As a result, the cilia are deflected and the receptor receives adequate stimulus.

50. CENTRAL VESTIBULAR SYSTEM. VESTIBULAR REFLEXES

Innervation

The fibers of the afferent nerve go to the medulla oblongata, where there are the following vestibular nuclei: 1. Upper – Bekhterev’s nucleus; 2. Medial (middle) – Schwalbe nucleus; 3. Lateral (lateral) – Deiters nucleus; 4. Bottom – Roller core.

Afferent impulses coming from vestibular receptors to these nuclei do not give full information about the position of the body in space. Therefore, the vestibular nuclei receive additional information from the following structures: 1. Cervical receptors of the spinal cord provide information about the position of the head relative to the body; 2. The cerebellum regulates the balance of the body (loss of balance manifests itself in an unstable gait, a large range of movements, especially when walking (“cock’s” step) - cerebellar ataxia); 3. Nuclei of the oculomotor nerve, which regulate eye movements caused by vestibular activity; 4. The thalamus provides conscious orientation in space; 5. Hypothalamus, which is involved in the occurrence of kinetosis (sickness).

SEMICIRCULAR CANALS

SEMICIRCULAR CANALS, Part inner ear vertebrates and humans, involved in the regulation of balance and body position in space. In fish, terrestrial vertebrates, and humans, three vertebral cells develop, located in three mutually perpendicular planes. There are horizontal (in humans - lateral) and 2 vertical P. to.: anterior (in humans - upper) and posterior. In fossil agnathans and modern Cyclostome lampreys have only 2 vertical (anterior and posterior) vertebrae, while hagfishes have 1 posterior one. Membranous P. to., filled with liquid endolymph, are placed in bone cases; the space between the membranous and bone P. is filled with perilymph. Each P. to. ends with 2 sections - legs, one of which expands into an ampoule. In most vertebrates, 2 adjacent legs of vertical pedicles are combined into one common one; P. to. open into the vestibule with 5 holes. Each ampulla of P. to. contains groups of hair cells grouped into a crista, or comb. Above the crista there is a jelly-like cupula, which contains the hairs of sensory cells. The bases of the sensory cells are intertwined with fibers of the vestibular nerve, which transmit excitation to the brain. A change in the position of the animal's head or body, accompanied by movement of cupules, endolymph and displacement otoliths, suspended in the endolymph, causes irritation of the hairs and the sensory cells of the P. to., which leads to the appearance of electric. discharge transmitted along the nerve to the brain. From there, nerve impulses are sent to the muscles and other organs of the body responsible for regulating its position in space. See also articles Vestibular apparatus, Balance organs and lit. with them.

On the wall of the ampulla there is a crescent-shaped elevation, with its apex directed into the lumen of the ampulla, called the ampullar ridge. This ridge contains the receptor part of the vestibular nerve, consisting of sensory neuroepithelial hair cells and supporting cells that support them. The top of the ampullary ridge is covered with a translucent gelatinous mass - cupula ampullaris. Any angular movement of the head is accompanied by a displacement of the endolymph relative to the ampullar ridge. This causes the hairs of the sensory cells to deform and generate an action potential in the neuron. Thus, the function of the semicircular canals is to perceive angular movements of the head.

Research methods.

To study it, special coordination tests and tests with rotation are carried out: rotation in the Baranyi chair, Romberg test, finger-nose test, etc. Yarotsky's test is based on rotating the head in a circle, normally balance is maintained for 27.6 s; for athletes - 90 s. Orientation in space, as well as the stability of body balance, largely depend on the state of the vestibular analyzer. This is especially important in some complex types sports (acrobatics, trampoline, diving, figure skating, ski jumping, artistic gymnastics, etc.). In cases of dysfunction of the vestibular apparatus, nystagmus (involuntary rhythmic convulsive movements of the eyeball), missing the finger-nose test, and instability in simple and complicated Romberg poses are observed. With training, the function of the vestibular apparatus and its stability improve.

Somatosensory system. Receptor formations of the skin. Pacinian and Meissner corpuscles, Merkel discs, free nerve endings. Conductive and cortical sections of the somatic sensory system. Information processing in the cortex. Tactile sensitivity, methods of its research.

Somatosensory system- a set of sensory systems that provide coding of temperature, pain, and tactile stimuli.

The sensory system is a set of structures of the central nervous system:

Connected by nerve pathways with the receptor apparatus and with each other;

Designed for analyzing stimuli of the same nature with subsequent coding of an external signal.

In highly developed animals and humans, visual, auditory, vestibular, olfactory, gustatory, tactile and proprioceptive sensory systems are distinguished.

The somatosensory system includes the skin sensitivity system and the sensory system of the musculoskeletal system, in which the main role belongs to proprioception.

Skin reception

Cutaneous receptors are concentrated on a huge skin surface (1.4–2.1 m2). The skin contains many receptors that are sensitive to touch, pressure, vibration, heat and cold, as well as painful stimuli. They are very different in structure, localized at different depths of the skin and distributed unevenly over its surface. Most of them are found in the skin of the fingers, palms, soles, lips and genitals. In human skin with hair (90% of the entire skin surface), the main type of receptors are the free endings of nerve fibers running along small vessels, as well as more deeply localized branches of thin nerve fibers entwining the hair follicle. These endings make the hair highly sensitive to touch. Touch receptors are also tactile menisci (Merkel discs), formed in the lower part of the epidermis by contact of free nerve endings with modified epithelial structures. There are especially many of them in the skin of the fingers.

In hairless skin, many tactile corpuscles (Meissner's corpuscles) are found. They are localized in the papillary layer of the skin of the fingers and toes, palms, soles, lips, tongue, genitals and nipples. Other encapsulated nerve endings, but more deeply located, are the lamellar corpuscles, or Pacinian corpuscles (pressure and vibration receptors). They are also found in tendons, ligaments, and mesentery.

Mechanisms of excitation of skin receptors. A mechanical stimulus leads to deformation of the receptor membrane. As a result of this electrical resistance membrane decreases, i.e. its permeability to ions increases. An ionic current begins to flow through the receptor membrane, leading to the generation of a receptor potential. When the receptor potential reaches a critical level of depolarization, impulses are generated that propagate along the fiber to the central nervous system.

Adaptation of skin receptors. Based on the speed of adaptation during prolonged exposure to the stimulus, most skin receptors are divided into fast and slow adapting. The tactile receptors located in the hair follicles, as well as the lamellar bodies, adapt most quickly. Adaptation of skin mechanoreceptors leads to the fact that we stop feeling the constant pressure of clothing or get used to wearing contact lenses on the cornea of ​​​​the eyes.

Receptor function. The skin, being the peripheral part of the skin analyzer, is an extensive receptor field that perceives a number of sensations from the outside and transmits a number of sensations to the central nervous system.

There are the following types of skin sensitivity:

tactile (sense of touch and pressure);

· pain;

· temperature (feelings of cold and warmth).

The sense of touch (touch) occurs when light pressure is applied to the skin, when the skin surface comes into contact with surrounding objects, it makes it possible to judge their properties and navigate in the external environment. It is perceived by tactile bodies, the number of which varies in different areas of the skin. An additional receptor for touch is the nerve fibers that weave around the hair follicle (the so-called hair sensitivity). The feeling of deep pressure is perceived by the lamellar corpuscles.

Pain is perceived mainly by free nerve endings located in both the epidermis and dermis.

Temperature sense, the perception of heat and cold, has great value for reflex processes that regulate body temperature. It is assumed that thermal stimuli are perceived by Ruffini's corpuscles, and cold stimuli by Krause's end flasks. There are significantly more cold spots on the entire surface of the skin than heat spots.

Receptor function. - Special terminal nerve formations of the skin - receptors serve to perceive sensations: pain, itching, temperature, pressure. On average, per 1 square centimeter of skin there are up to 5000 sensitive endings, 200 pain points, 12 cold points, 2 heat points and 25 pressure points. Nerve receptors in the skin are distributed unevenly. They are especially numerous in the skin of the face, palms and fingers, and external genitalia. The nerves of the autonomic system, innervating glands, blood and lymphatic vessels, regulate physiological processes in the skin. Thus, the skin is a huge receptor field, millions of sensory nerve endings of which constantly carry out direct and feedback from the central nervous system. Each internal organ has its own representation on the field. By influencing certain areas and points of the skin, you can obtain a therapeutic effect. This is the basis for the use of individual physiotherapeutic procedures and acupuncture treatment.

skin receptor bodies

The part of the somatovisceral system that provides the sense of touch includes several types of skin mechanoreceptors, represented by free nerve endings or encapsulated, that is, enclosed in a capsule of connective tissue or modified epidermal cells (Fig. 17.4). Free nerve endings innervate the hair follicles of vellus hair, which covers most of the human body, as well as coarse hair growing on the head, in the armpits, on the pubis, and in men also on the face. The free nerve endings of the hair follicles are mechanoreceptors and are excited when the hair is displaced or twitched. Another variety of free nerve endings is found in the epidermis and in the papillary dermis, most of them are nociceptors or thermoreceptors, but some belong to mechanoreceptors, which specifically respond to weak near-threshold stimulation. It is assumed that when this type of receptor is irritated, tickling and itching sensations occur. Among the encapsulated endings there are Pacinian, Meissner, Ruffini corpuscles, Merkel discs, tactile Pincus-Iggo corpuscles, Krause flasks. Depending on the structure and shape of the capsule, the nerve endings are subject to the most severe effects either as a result of pressure from a stimulus acting perpendicularly, or as a result of lateral displacement of the capsule, which plays the role of a mechanical energy converter of external stimuli. Most encapsulated receptors are found in the hairless skin of the fingers and toes, palms and soles, face, lips, tongue, nipples and genitals, where they are distributed at varying densities and depths. Pacinian corpuscles are also found in tendons, ligaments and mesentery.

part of the inner ear (See Inner ear) of vertebrates and humans, involved in the regulation of balance and body position in space. In fish, terrestrial vertebrates, and humans, three vertebral cells develop, located in three mutually perpendicular planes. There are horizontal (in humans - lateral) and 2 vertical P. to.: anterior (in humans - upper) and posterior. Fossil jawless and modern cyclostomes - lampreys - have only 2 vertical (anterior and posterior) vertebrae, while hagfishes have 1 posterior one. Membranous P. to., filled with liquid endolymph, are placed in bone cases; the space between the membranous and bone P. is filled with perilymph. Each P. k. ends in 2 sections - legs, one of which expands into an ampoule. In most vertebrates, 2 adjacent legs of the vertical pedicles are combined into one common one; P. to. open into the vestibule with 5 holes. Each ampulla of P. to. contains groups of hair cells grouped into a crista, or comb. Above the crista there is a jelly-like cupula, which contains the hairs of sensory cells. The bases of the sensory cells are intertwined with fibers of the vestibular nerve, which transmit excitation to the brain. A change in the position of the head or body of an animal, accompanied by movement of cupules, endolymph and displacement of otoliths (See Otoliths) , suspended in the endolymph, causes irritation of the hairs and the sensory cells themselves, which leads to the appearance of an electrical discharge transmitted along the nerve to the brain. From there, nerve impulses are sent to the muscles and other organs of the body responsible for regulating its position in space. See also the articles Vestibular apparatus, Balance organs and literature related to them.

G. N. Simkin.

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"Semicircular canals" in books

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