vestibular sensory system. vestibular reflexes; clinical tests What is obtained from this interaction

Hearing thresholds, frequency range of perception of sounds

fluctuations eardrum, caused by sounds of different height, duration and volume, are perceived differently. Fluctuations up to 1000 Hz are transmitted without attenuation. At a frequency of more than 1000 Hz, the inertia of the sound-conducting apparatus of the middle ear becomes noticeable.

The auditory ossicles amplify sound vibrations transmitted to inner ear, about 60 times. They soften the force of high sound pressures. As soon as the pressure sound wave goes beyond 110-120 dB, the pressure of the stirrup on the round window of the inner ear changes.

threshold stimulus for the muscles of the auditory ossicles - a sound with a power of 40 dB.

The human ear perceives sound vibrations with a frequency of 16 to 20,000 Hz. It has the greatest excitability in the range of 1000-4000 Hz and below 16 Hz are ultra- and infrasonic. The reason that a person does not hear sounds with a frequency of more than 20,000 Hz is in the morphological features of the hearing organ, as well as in the possibilities of generating nerve impulses by the perceiving cells of the organ of Corti.

vestibular sensory system. Vestibular receptors and the mechanism of perception

Receptors of the vestibular system belong to mechanoreceptors. Those of them that are in the semicircular canals are excited mainly during the rotation of the body. The vestibules located in the sacs perceive mainly accelerations during rectilinear movements.

The semicircular canals are located in each ear in three planes, which makes it possible to perceive different movements. The semicircular canals have bony and membranous walls. Inside the membranous canals is a fluid - endolymph. One of the ends of each channel is expanded, it contains special cells, the hairs of which form brushes hanging down into the cavity of the channel. When the body rotates, these brushes move, which causes excitation of this part of the vestibular apparatus.

Excitation from sensitive cells of the vestibular apparatus is transmitted to the nuclei of the vestibular nerve, which is part of the 8th pair of cranial nerves.

Vestibular reflexes, vestibular stability

When the vestibular sensory system is irritated, a variety of motor and autonomic reflexes occur.. Motor reflexes are manifested in changes muscle tone which ensures that the normal posture of the body is maintained. Rotation of the body causes a change in the tone of the external muscles of the eye, which is accompanied by their special movements - nystgam. Irritation of vestibular receptors causes a number of vegetative and somatic reactions. There is an increase or slowdown in cardiac activity, a change in breathing, intestinal peristalsis increases, and pallor appears. Excitation of the nuclei of the vestibular nerve extends to the centers of vomiting, sweating, as well as to the nuclei oculomotor nerves. As a result, vegetative disorders appear: nausea, vomiting, increased sweating.

The level of functional stability of the vestibular sensory system measured by the magnitude of the motor and vegetative reactions that occur when it is irritated. The less pronounced these reflexes, the higher the functional stability. With low stability, even a few quick turns of the body around a vertical axis (for example, during a dance) cause discomfort, dizziness, loss of balance, blanching.

Significant irritation of the vestibular apparatus occurs when motion sickness on a ship or in an airplane (sea and air sickness).

Static and statokinetic reflexes. The balance is maintained reflexively, without the fundamental participation of consciousness in this. There are static and statokinetic reflexes. Vestibular receptors and somatosensory afferents, especially from cervical proprioceptors, are associated with both. Static reflexes provide an adequate relative position of the limbs, as well as a stable orientation of the body in space, i.e. postural reflexes. Vestibular afferentation comes in this case from the otolithic organs. A static reflex, easily observed in a cat due to the vertical shape of its pupil, is a compensatory rotation of the eyeball when turning the head around the long axis of the body (for example, with the left ear down). Pupils at the same time all the time retain a position very close to vertical. This reflex is also observed in humans. Statokinetic reflexes are reactions to motor stimuli that are themselves expressed in movements. They are caused by excitation of the receptors of the semicircular canals and otolithic organs; examples are the rotation of a cat's body in a fall, ensuring that it lands on all four legs, or the movement of a person regaining balance after he has stumbled.

One of the statokinetic reflexes is vestibular nystagmus. As mentioned above, the vestibular system causes various eye movements; nystagmus like them special form observed at the beginning of a rotation that is more intense than the usual short turns of the head. In this case, the eyes turn against the direction of rotation in order to keep the original image on the retina, however, not reaching their extreme possible position, they abruptly “jump” in the direction of rotation, and another section of space appears in the field of view. Then follows their slow return movement.

The slow phase of nystagmus is triggered by the vestibular system, and the fast "jumping" of the gaze is triggered by the prepontine part of the reticular formation.

When the body rotates around the vertical axis, practically only the horizontal semicircular canals are irritated, i.e., the deviation of their cupulae causes horizontal nystagmus. The direction of both its components (fast and slow) depends on the direction of rotation and, thus, on the direction of cupule deformation. If the body rotates around a horizontal axis (for example, passing through the ears or sagittally through the forehead), the vertical semicircular canals are stimulated and vertical, or rotational, nystagmus occurs. The direction of nystagmus is usually determined by its fast phase, i.e. with “right nystagmus”, the gaze “jumps” to the right.

With passive rotation of the body, two factors lead to the occurrence of nystagmus: stimulation of the vestibular apparatus and movement of the field of view relative to the person. Optokinetic (caused by visual afferentation) and vestibular nystagmus act synergistically.

Diagnostic value of nystagmus. Nystagmus is used in the clinic to test vestibular function. The subject sits in a special chair, which rotates at a constant speed for a long time, and then stops abruptly. The stop causes the cupula to deviate in the opposite direction to that in which it deviated at the beginning of the movement; the result is nystagmus. Its direction can be determined by registering the deformation of the cupula; it must be opposite to the direction of the previous movement. The recording of eye movements resembles that obtained in the case of optokinetic nystagmus. It's called a nystagmogram.

After testing for post-rotational nystagmus, it is important to eliminate the possibility of fixing the gaze at one point, since in oculomotor reactions, visual afferentation dominates over vestibular and, under certain conditions, can suppress nystagmus. Therefore, the subject is put on Frenzel glasses with highly convex lenses and a built-in light source. They make him "myopic" and unable to fix his gaze, while allowing the doctor to easily observe eye movements. Such glasses are also needed in the test for the presence of spontaneous nystagmus - the first, simplest and most important procedure for clinical trial vestibular function.

Another clinical way to trigger vestibular nystagmus is thermal stimulation of the horizontal semicircular canals. Its advantage is the ability to test each side of the body separately. The head of the seated subject is tilted back approximately 60° (in the person lying on his back, it is raised by 30°) so that the horizontal semicircular canal is in a strictly vertical direction. Then the external auditory meatus is washed with cold or warm water. The outer edge of the semicircular canal is very close to it, so it immediately cools or heats up. According to Barani's theory, the density of the endolymph decreases when heated; consequently, its heated part rises, creating a pressure difference on both sides of the cupula; the resulting deformity causes nystagmus. Based on its nature, this type of nystagmus is called caloric. When heated, it is directed to the place of thermal impact, when cooled - to reverse side. In people suffering from vestibular disorders, nystagmus differs from the normal qualitatively and quantitatively. The details of its testing are given in the paper. It should be noted that caloric nystagmus can occur in spacecraft under weightless conditions, when differences in endolymph density are insignificant. Consequently, at least one more mechanism, not yet known, is involved in its launch, for example, a direct thermal effect on the vestibular organ.

The function of the otolithic apparatus can be tested by observing oculomotor reactions during head tilts or reciprocating movements of the patient on a special platform.

Vestibular (labyrinth) and cervical postural reflexes were described by Magnus (Haltungsreflexe). Described - to put it mildly, the work for the 20s is absolutely grandiose.

There are problems not so much with its description, but with subsequent interpretations. First, it is generally accepted that Magnus described the neck reflex as asymmetric, and the labyrinth reflex as symmetrical with respect to the limbs. Below you can see that they are both equally asymmetrical but opposite.

Secondly, in textbooks you can often see something like this thought, with reverence attributed to Magnus (*)

It must be emphasized that impulses from the otolithic apparatus maintain a certain distribution of tone in the muscles of the body. Irritation of the otolith device and semicircular canals causes a corresponding reflex redistribution of tone between individual muscle groups ...

This statement is rather strange, if not illiterate. Such a "direct" work of the vestibular reflex could be useful for a mythical animal - a bun, but in humans and cats, the vestibular apparatus is located in the head, and it is on a flexible neck. However, it was precisely this concept, following Magnus, that was established throughout the 20th century - that the labyrinth and cervical postural reflexes "distribute" the tone between muscle groups.

cervical interaction

Coordinate transformation

Instead of the concept of "tone distribution" based on labyrinth sensations, and a separate "distribution" based on cervical sensations, this problem can be viewed differently.

The vestibular sensory stream would be very useful for postural control, but it reflects the movements of the head, not the center of mass of the body. For use in postural tasks in this flow, you need to take into account the movement of the neck, at least. In fact (the neck is more mobile than the body), must be subtracted from head movement (vestibular) neck movement (neck proprioception).

This subtraction is essentially a coordinate transformation - from the system associated with the head to the system of the body.

One can, of course, say that the reflex does not have to be so smart, that it is suppressed and directed by higher structures, and the task with such a complicated name should be solved somewhere there. But it turns out that such a transformation of coordinates is perfectly performed by the reflexes described by Magnus, interacting with each other at the level of the trunk(perhaps the cerebellum is involved). We are talking about the labyrinth position reflex and ASTR.

This has been successfully, and seemingly independently, demonstrated by the Scotsman Tristan DM Roberts, who reproduced Magnus' work in the 1970s, and the German Kornhuber. Both indicate that Magnus incorrectly described labyrinthine positional reflexes. They are exactly as asymmetric as ASTR, but opposite in sign. In fact, one can speak of asymmetric labyrinth tonic reflex - ALTR. And the very principle of coordinate transformation based on the interaction of neck and labyrinth reflexes was first described by von Holst and Mittelstaedt in their Das Reafferenzprinzip in 1950 (oddly enough, neither of them refer to them).

Moreover, there are almost direct observations of just such a work of neurons of the vestibular nuclei and the spinal cord. And there are practical observations (unpublished) that ALTR is observed in severe children in an explicit form.

Below I provide a translation of excerpts from the TDM Roberts article in Nature.

Asymmetric (!) Labyrinth reflex and Asymmetric Neck Tonic Reflex

a, Neck reflexes separately. The body is tilted, the head is straight, the paws are unbent from the side of the chin. b. Labyrinth reflexes separately. The head and body are rejected, the neck is straight - the lower legs are unbent. c. Head deflection separately. Paws are symmetrical do not unbend and do not bend, do not react to rotation at all (VM). d. Uneven support. the body is rejected, the paws are in a compensatory position, the head is free. e. Constant lateral acceleration. Paws asymmetrically correspond to the deviation of the body relative to the support vector. f. Constant lateral acceleration. Paws are symmetrical on adequately slanted support Figure from TDM Roberts article, see article for details

The success of maintaining an upright posture is usually attributed to reflexes initiated by labyrinth receptors in the inner ear. Traditional descriptions of the work of these reflections, however, do not explain the observed stability. According to Magnus, changing the position of the head changes the extensor tone of all four limbs of the animal in a symmetrical manner. In contrast, the tonic neck reflexes are described as asymmetrical in their response to the limbs, and the paws on the side where the jaw is rotated are straightened, while on the other side they are bent.

Accordingly, Roberts set out to re-investigate head tilt reflexes using cats decerebrated slightly above the intercollicular level to avoid excessive rigidity, using an apparatus that independently supports and rotates the cat's body, neck, and head (for a description, see Lindsay, TDM Roberts & Rosenberg 1976), including the terrifying ability to rotate the cervical vertebrae relative to the motionless torso and head.

Labyrinth reflexes in response to head tilt were found invariably asymmetrical and suitable for the stabilization function, in contrast to the symmetrical Magnus circuit.

They can be described by the principle "lower legs extend, upper legs bend"

When the neck is turned, "the paws on the side of the chin unbend", in full accordance with the scheme of Magnus and Klein.

However, the response to neck reflexes opposite responses to labyrinth reflexes with a similar turn of the neck. Acting simultaneously, these reflexes are summed up, and the interaction of these two sets of reflexes leads to to trunk stabilization independent of head rotation.

What comes out of this interaction?

Next, Roberts begins to write algebraic equations, but the principle of summing these reflexes (more precisely, subtracting - they are opposite, antagonistic in action) can be described more simply (for this I will use the picture from Kornhuber's work, they are, apparently, twin brothers):

1. With a stable position of the body, turning the head causes a labyrinth reaction (ALTR), which is completely compensated by ASTR - the total effect on the limbs is zero.
2. However, if the whole body leans along with the head, the labyrinth reaction (ALTR) will be greater than the ALTR, and the total reflex response will compensate for the deviation.
3. If the body "slips" out from under the stable head, then the ALTR will be greater than the labyrinth reaction (ALTR), and the total reflex response will again compensate for the deviation.

The overall effect is that

• the head can be rotated as you like (and it is necessary for vision tasks, for example)
• the overall reaction to the limbs is as if the vestibular "sensor" was in the trunk.

A task coordinate transformations solved successfully!

Who decides it? There is reason to believe that the process of "subtraction" is carried out by a certain subgroup of neurons in the vestibular nuclei. However, similar "subtracting" neurons were found in the interpositus nucleus of the cerebellum (by the same authors, see Luan & Gdowski) and in the cerebellar vermis (see Manzoni, Pompeano, Andre). Due to the direct connections between all these areas, it is difficult to say which of them is primary, despite the fact that Kornhuber claims that "subtraction" does not depend on the cerebellum. More accurate experiments by the Italians in 1998 show what depends.

The effect of both "bare reflex" and "reflex with coordinate transformation" seems to be observed as Short latency and Medium latency VSR in humans. See ibid. for the role of the cerebellum in these transformations.

I also note (see Manzoni, Pompeano, Andre) that for an upright person, not only the position of the neck is important, but also the mutual orientation of each of the segments of the axis. The overall picture is much more complicated than "ALTR minus ASHTR", but the principle of operation, apparently, is exactly this. See also below about lumbar reflexes.

Corollary discharge/reafferentation principle

It is no coincidence that the first mention of the described subtraction appears precisely in Das Reafferenzprinzip. With head movement (whether active or passive), the vestibular response is known, predictable sensory consequence, or Reafference which should be subtracted from the total sensory flow - then only Exafference, which will describe the movement of the body along with the head and neck.

That is, it does not matter how it is called - coordinate transformation or corollary discharge effect, it describes the same phenomenon in this case.

Why can ASTD manifest in infants?

The experiments described above are performed on decerebrated cats (and other animals), which makes the reflexes visible. The manifestation of ASTR is generally considered a sign of pathology, and in any case it is expected that it should disappear with age. However, even adult norm reflex circuits are quite present and active, although they require more subtle measurements (measuring EMG or proprioceptive reflexes) to detect them, or they come out in the form of movement / posture in situations of high stress, such as in sports.

The absence of visible reflexes in the norm in this case almost certainly means that the labyrinth and neck reflexes are so well synchronized with each other that they do not appear outwardly, compensating for each other. The coordinate transformation that they perform, however, seems to be too useful))

It can be assumed that the manifestation of ASTR is a consequence of immaturity or deviation in development. nervous system, when the already matured nerve circuit of the reflex does not yet receive the necessary adjustment from the cerebellum, or is it just a stage in this very adjustment, when the inconsistent action of ASTR and labyrinth reflexes creates unnecessary "motor noise". This noise should probably be detected in Inferior Olive and lead to cerebellar adjustment of reflex strength until they are fully coordinated. Or, the absence of noise and problems with it should lead to the success of the first motor tasks and the appearance of a reinforcement signal from the basal ganglia. One way or another, it can be assumed that the observation of ASTR in infants or patients with cerebral palsy is a manifestation of a delay in this stage.

Normally, ASTR and labyrinth reflexes are part of a single system. There is no point in separating them when we are talking about a normal function. And if a child shows an asymmetric cervico-tonic "reflex" - this means that this system fails (weakness of the labyrinth reflex, or weakness of regulatory mechanisms).

In very severe children, LM Zeldin sometimes observes a reaction that is opposite in terms of the construction of ASTR - in other words, the Asymmetric Labyrinth Tonic Reflex - ALTR.

It is also known that symptoms of anesthesia or damage to the posterior roots cervical regions C1-C3, disrupting proprioception of the neck, leads to nystagmus, ataxia and sensations of falling or tilting- which closely resembles the symptoms of a Wilson & Peterson labyrinthectomy

Cervical Vertigo

There is - a very controversial - diagnosis, "cervical vertigo" - cervical vertigo, controversial because it is a diagnosis of exclusion, and the list of exceptions there is long. Detailed good review in Russian can be found in the post laesus-de-liro, which provides a good definition of this condition - "a non-specific sensation of disorientation in space and balance, due to pathological afferent impulses from the neck."

In fact, this is a violation of the very interaction that is discussed in this article.

Links

• TDM Roberts: Biological Sciences: Reflex Balance 1973 I partially translate this work and analyze it in this article
• Lindsay, TDM Roberts & Rosenberg: Assymetric Tonic Labyrinth Reflexes and Their Interaction with Neck Reflexes in the Decerebrate Cat 1976
• Fredrickson, Schwarz & Kornhuber Convergence and Interaction of Vestibular and Deep Somatic Afferents Upon Neurons in the Vestibular Nuclei of the Cat 1966 are results identical and apparently independent experiments of Kornhuber's group. They also came to the conclusion that Magnus was wrong, but they also carried out additional destruction of the cerebellum, showing that this interaction does not depend on the cerebellum.
• Manzoni, Pompeiano, Andre: Neck Influences on the Spatial Properties of Vestibulospinal Reflexes in Decerebrate Cats: Role of the Cerebellar Anterior Vermis 1998 An article by masters of vestibulo- and cerebellar management that directly tests and builds on the results of TDM Roberts. It turned out that Roberts is right, but Kornhuber is not: the cerebellum is involved in the process.
• Luan, Gdowski et al: Convergence of Vestibular and Neck Proprioceptive Sensory Signals in the Cerebellar Interpositus 2013

Roberts apparatus for cats with rotation in three axes

Addition: Tonic lumbar reflexes

Forgotten works of the Japanese

精神神経学会雑誌 .

A rather detailed description can be found in Tokizane et al: Electromyographic studies on tonic neck, lumbar and labyrintine reflexes in normal persons written, thank God, in English.

In addition to a curious and rare description, the presence of a lumbar reflex raises the question of whether there is a similar coordinate transformation during movements relative to the waist. This is especially curious because (although the Japanese found a similarity here between humans and rabbits, but not between humans and dogs or cats), this transformation is much more important for bipedal people.

Personally, this seems to me somewhat controversial, but I can not find clear evidence. The Japanese article, it must be said, is rather flimsy in terms of technique: there are only four subjects, only one "deaf-mute" who is presented as a person with a bilateral loss of vestibular sense, but no data confirming this is given.

Basis for "hip strategy"?

Why is this reflex important? Movements in the lower back A-P direction, if we assume that they are perceived and interact with the vestibular flow in a similar way to ASTR, create an almost ideal substrate for constructing a hip strategy. See picture on the right.

The subtractive interaction of Tonic Lumbar Reflex and the Vestibular flow allows you to ignore the reafferentation from the execution of the strategy itself, compensate for head movements in antiphase to the center of mass, and receive a "clean" vestibular signal for maintaining the posture. This requires not a tonic vestibular flow, but a dynamic one, but the principle is close.

It is unfortunate that such experiments cannot be found.

Addendum 2: Proprioceptive return from limbs

Below I describe purely my speculation. Even the latest reviews. like The Vestibular System. A sixth sense. p. 220, describing numerous evidence of the reciprocal influence of somatosensory sensation on the vestibular nuclei, do not risk suggesting the function of this mechanism. For a description of the work on this return, see Somatosensory-Vestibular Integration.

However, if we assume that the function of integration of the vestibular and cervical reflexes described above is correct, and indeed helps to subtract neck movements from head movement, then it is quite obvious that the need for the same mechanism exists for locomotion.

Any locomotion leads to quite predictable, regular vibrations of the head. These oscillations can be called "locomotor inertial reafferentation". It would also be nice to be able to subtract this locomotor signal from head movement. This will allow the use of vestibular signals during locomotion. It is possible (especially hinted at by the difference between a decerebrated and a conscious cat) that such a mechanism is observed in the vestibular nuclei.

The second idea, which also has the right to life, is that the well-described effect of the absence of vestibular reflexes in muscles that do not play a postural role also logically requires somatosensory return to the vestibular nuclei (or such integration can be carried out in spinal networks).

Which of these is true is now decidedly impossible to say.

Function of the vestibular sensory system consists in providing the brain with information about the position of the head in space, about the action of gravity and the forces that cause linear or angular accelerations. This function is necessary to maintain balance, i.e., a stable position of the body in space, and for the spatial orientation of a person.

The vestibular system includes:

1) peripheral department, consisting of the vestibular apparatus located in the inner ear,

2) conducting paths,

3) the central section, represented by the vestibular nuclei of the medulla oblongata, the thalamus and the projection area of ​​the cortex in the postcentral gyrus.

Adequate stimuli of the vestibular system are gravity and forces that impart linear or angular acceleration to the body.. A specific feature of the vestibular system is that a significant part of the sensory information processed in it is used for automatic regulation of functions carried out without conscious control.

The vestibular system interacts at several levels of its hierarchical organization with the visual and somatosensory systems.; these three systems complement each other in providing a person with the information necessary for his spatial orientation.

In mammals, the inner ear includes:

The semicircular canals, which serve to receive angular acceleration,

Otolith organs for registration of linear acceleration,

The snail with the organ of Corti, which is the organ of frequency analysis of sound.

Three semicircular canals located in three mutually perpendicular planes: the horizontal channel in the horizontal plane, the anterior vertical channel - in the frontal plane and the rear vertical channel - in the sagittal plane. All three channels are connected in a cavity vestibule, from the Latin definition of which (vestibulum) comes the name of the vestibular apparatus. At the junction with the vestibule, the canals are dilated in the form of ampoules. They contain neuroepithelium, consisting of sensory cells, protruding inward in the form of a ridge or crista. Each crist is covered cupula, which is an amorphous jelly-like substance. It is pierced by hair-like processes of sensory cells.

Rice. Scheme of the structure of the organ of balance (scheme of the inner ear and cupula).

With angular accelerations, when the endolymph shifts due to inertia, the cupula also shifts, which leads to deformation of the hairs of secondary receptor cells immersed in it, followed by the appearance of a receptor potential in them.

There are also two extensions in the vestibule cavity: pouch (sacculus) and uterus (utriculus), representing otolith organs, used to measure linear accelerations. The receptor epithelium of the uterus and sac is located on small elevations - macula covered with an otolithic membrane that contains many small but heavy calcium carbonate crystals (otoliths or otokinia). The macula of the uterus is located in a horizontal plane (with the head in a vertical position), and the macula of the sac is oriented vertically. As a result, utricular receptors are sensitive to slight tilts of the head from its normal position and to linear accelerations that occur during movement in the horizontal plane. Saccular units, in contrast, are sensitive to dorsoventral acceleration, as occurs in jumps and falls.

The otolithic membrane is permeated with hairy processes (cilia) of sensory cells. Between the otoliths and the macula is a space filled with a jelly-like mass. Due to this, under the action of gravity or linear acceleration, the otolith slides over the macula and deforms the hairs of sensitive cells. The maximum displacement of the otolith along the macula is 0.1 mm for the sacculus and 0.005 mm for the utriculus.

Fig. Structure of the otolithic apparatus. 1 - otoliths; 2 - otolithic membrane; 3 - hairs of receptor cells; 4 - receptor cells; 5 - supporting cells; b - nerve fibers.

macula and cupula receptors presented hair cells, which are secondary mechanoreceptors and form synapses with peripheral endings vestibular ganglion neurons(primary sensory neurons). Each receptor has a bundle of 40-80 hairs - stereocilia, reaching a length of 50 microns, as well as one long hair located eccentrically with respect to the stereocilia - kinocilium. If the bundle of stereocilia tilts towards the kinocilium under the influence of a mechanical stimulus, the receptor depolarizes, and when the stereocilia deviate from the kinocilium, hyperpolarization of the receptor membrane occurs. As a result, when the bundle of stereocilia is bent in one direction, the hair cell is excited, and when the same bundle is bent in the opposite direction, it is inhibited, i.e., each hair cell has two functional poles. The direction of functional polarization changes from one cell to another, and the receptor epithelium as a whole contains a complete set of cells for registering stimuli acting in any possible direction.

Neurons of the vestibular ganglion, which form synapses based on receptors, have a spontaneous background activity, the nature of which changes under the influence of hair cell mediators, which can presumably be glutamate and/or GABA. The receptive fields of neurons of the vestibular ganglion include, on average, three hair cells of the ampullae of the semicircular canals or 4-6 receptors macula of uterus or sac.

Vestibular nerve fibers(processes of bipolar neurons) are sent to medulla. The impulses coming through these fibers activate neurons of the bulbar vestibular complex. From here, signals are sent to many parts of the central nervous system: spinal cord, cerebellum, oculomotor nuclei, cerebral cortex, reticular formation and ganglia of the autonomic nervous system.

Electrical phenomena in the vestibular system. Even at complete rest, spontaneous impulses are recorded in the vestibular nerve. The frequency of discharges in the nerve increases when the head is turned in one direction and is inhibited when it is turned in the other direction (detection of the direction of movement). Less commonly, the frequency of discharges increases or, conversely, is inhibited by any movement. In 2/3 of the fibers, an adaptation effect (a decrease in the frequency of discharges) is found during the ongoing action of angular acceleration. The neurons of the vestibular nuclei also have the ability to respond to changes in the position of the limbs, body turns, signals from internal organs, i.e., to synthesize information coming from different sources.

Complex reflexes associated with vestibular stimulation. The neurons of the vestibular nuclei provide control and management of various motor reactions. The most important of these reactions are the following: vestibulospinal, vestibulo-vegetative and vestibulo-oculomotor.

Vestibulospinal influences through the vestibulo-, reticulo- and rubrospinal tracts change the impulses of neurons at the segmental levels of the spinal cord. The vestibular nuclei are subcortical centers postural and statokinetic reflexes. With the help of them, a dynamic redistribution of skeletal muscle tone is carried out and the reflex reactions necessary to maintain balance are turned on. The cerebellum is responsible for the phasic nature of these reactions: after its removal, the vestibulospinal influences become predominantly tonic. During voluntary movements, vestibular influences on the spinal cord are weakened.

AT vestibulovegetative reactions are involved the cardiovascular system, digestive tract and others internal organs. With strong and prolonged loads on the vestibular apparatus, a pathological symptom complex occurs, called motion sickness, for example seasickness(kinetosis). It manifests itself as a change heart rate(increasing and then slowing down), narrowing and then vasodilatation, increased contractions of the stomach, dizziness, nausea and vomiting. An increased tendency to motion sickness can be reduced by special training (rotation, swing) and the use of a number of drugs.

Vestibulo-oculomotor reflexes(ocular nystagmus) consist in a slow movement of the eyes in the opposite direction to rotation, followed by a jump of the eyes back. The very occurrence and characteristics of rotational ocular nystagmus are important indicators of the state of the vestibular system; they are widely used in marine, aviation and space medicine, as well as in experiment and clinic.

Main afferent pathways and projections of vestibular signals. Conscious perception of changes in head position occurs as a result of sequential processing of information, first in the vestibular nuclei, then in the posterior ventral nuclei of the thalamus, which form a projection to the postcentral gyrus. Additional Information enters the projection cortex indirectly: from the vestibular nuclei to the cerebellum, and from it to the ventrolateral nuclei of the thalamus and the projection cortex. The primary projection area of ​​vestibular sensitivity is located in the postcentral gyrus, predominantly on the side of the body on which the vestibular apparatus is located.. Another projection, characterized by bilateral representation of vestibular sensitivity, is in the secondary motor cortex. Awareness of the spatial location and scheme of the body occurs with the participation of the posterior parietal regions of the cortex (fields 5 and 7), where the integration of the vestibular, visual and somatosensory sensitivity of a person takes place.

Functions of the vestibular system. The vestibular system helps the body to navigate in space during active and passive movement. During passive movement, the cortical sections of the system remember the direction of movement, turns and the distance traveled. It should be emphasized that under normal conditions, spatial orientation is provided by the joint activity of the visual and vestibular systems. Sensitivity of the vestibular system healthy person very high: the otolithic apparatus allows you to perceive the acceleration of rectilinear movement, equal to only 2 cm / s 2. The threshold for distinguishing the tilt of the head to the side is only about 1 °, and forward and backward - 1.5-2 °. Along with this, the receptor apparatus of the sac is highly sensitive to the action of vibration. The receptor system of the semicircular canals allows a person to notice accelerations of rotation of 2-3 degrees.