Vestibular Apparatus

vestibular apparatus [və′stib·yə·lər ′ap·ə‚rad·əs]

(anatomy)

The anatomical structures concerned with the vestibular portion of the eighth cranial nerve; includes the saccule, utricle, semicircular canals, vestibular nerve, and vestibular nuclei of the ear.

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Vestibular Apparatus 

the organ in vertebrate animals and man that perceives changes in position of the head or body in space and the direction of body movement; part of the inner ear.

The vestibular apparatus is the complex receptor of the vestibular analysor. The structural composition of the vestibular apparatus consists of a complex aggregate of ciliated cells of the inner ear, endolymph, calcareous formations enclosed within it called otoconia, and the gelatinous cupulae in the ampullae of the semicircular canals. The two types of signals that enter from the equilibrium receptors are static (associated with body position) and dynamic (associated with acceleration). Both types of signals arise when there is mechanical stimulation of the sensory hairs by movement of either the otoconia (or cupulae) or the endolymph. An otoconium usually has greater density than the surrounding endolymph and is supported by the sensory hairs. When there is a change of body position, the direction of the force exerted by the otoconium on the sensory hairs is changed. Experiments on fish have shown that the composite force directed parallel to the surface of the epithelium (the so-called shearing force) serves as the effective stimulating force acting on the sensory epithelium. This is probably the cause of the stimulation of the hair cells in other vertebrates as well. Acceleration of the movement of the whole body or of the head, acting in the plane of each canal, serves as the stimulating influence for the semicircular canals. Owing to the difference in the inertias of the endolymph and the cupula, a shift of the cupula occurs during acceleration and the friction resistance in the thin canals serves as a damper (suppressor) of the whole system. The oval sac (utricule) plays a leading role in the perception of body position and probably participates in the perception of turning. The round sac (saccule) complements the oval one and is apparently necessary for the perception of vibration.

The transmission of the excitation of the vestibular apparatus to the brain is accomplished by the vestibular branch of the auditory nerve. The centers of vestibular function are connected to the cerebellum, the nuclei of the oculomotor nerves, and the centers of the autonomic nervous system. The higher cortical centers of the vestibular apparatus are located in the temporal region of the large hemispheres of the brain. During stimulation of the vestibular receptors, a number of reflexes occur (changes in the tonus of the muscles of the neck, trunk, and extremities), which permit maintenance of equilibrium during changes of body position. These reflexes are accompanied by jerky movements of the eyes and autonomic reactions. In man, when there is intense stimulation of the vestibular apparatus, the motion-sickness syndrome develops (dizziness, disturbance of heart action and breathing rhythm, nausea, and vomiting), characteristic, for instance, of seasickness. With frequent repetition of intense vestibular stimulation, reaction to it is reduced. Vestibular conditioning, used in the physical training of sailors, aviators, and cosmonauts, is based on this fact.

Conditioning the vestibular apparatus includes movement that stimulates it (bending over, turns, jumps, exercises on the trampoline, crossbar, and so on) and also the repeated action on the body of oblique and rectilinear acceleration by means of revolving apparatus (centrifuges), swings, and others.

REFERENCE

Kisliakov, V. A., and I. V. Orlov. “Fiziologiia vestibuliarnoi sistemy (sovremennoe sostoianie problemy).” In the collection Voprosy fiziologii sensornykh sistem, [issue 1]. Moscow-Leningrad, 1966.

G. N. SIMKIN