Vestibular system:

The system that subserves the bodily functions of balance and equilibrium. It accomplishes this by assessing head and body movement and position in space, generating a neural code representing this information, and distributing this code to appropriate sites located throughout the central nervous system. Vestibular function is largely reflex and unconscious in nature.

The centrifugal flow of information begins at sensory hair cells located within the peripheral vestibular labyrinth. These hair cells synapse chemically with primary vestibular afferent nerve fibers, causing them to fire with a frequency code of action potentials that include the parameters of head motion and position. These vestibular afferents, in turn, enter the brain and terminate within the vestibular nuclei and cerebellum. Information carried by the firing patterns of these afferents is combined within these central structures with incoming sensory information from the visual, somatosensory, cognitive, and visceral systems to compute a central representation of head and body position in space. This representation is called the gravito-inortial vector and is an important quantity that the central nervous system employs to achieve balance and equilibrium. See also Brain; Nervous system (vertebrate); Postural equilibrium; Reflex.

The vestibular labyrinth is housed within the petrous portion of the temporal bone of the skull along with the cochlea, the organ of hearing (Fig. 1). The receptor element or primary motion sensor within the labyrinth is the hair cell (Fig. 2). Hair cells respond to bending of their apical sensory hairs by changing the electrical potential across their cell membranes. These changes are called receptor potentials, and the apical surface of the hair cell thus functions as a mechanical-to-electrical transducer. The frequency of the resulting action potentials in the VIIIth cranial (vestibulocochlear) nerve encodes the parameters of angular and linear motion. See also Biopotentials and ionic currents; Ear (vertebrate); Synaptic transmission.

The vestibular labyrinth is located within the inner ear. It communicates with the brain via the VIIIth nerve. Each of the three semicircular canals has an ampulla and a long and slender duct. The utricle primarily senses motion in an earth-parallel plane, while the saccule primarily senses motion and gravity in an earth-perpendicular plane.

Otolithic macula at rest. The arrows within the primary afferents or VIIIth nerve fibers indicate spontaneous activity in these fibers in the absence of motion of the otolithic mass relative to the hair cell stereocilia. (The otolithic membrane is not illustrated for clarity.)

Hair cells are the common sensory element in both the angular and linear labyrinthine sensors as well as within the cochlea. The particular frequency of energy that hair cells sense within these diverse end organs arises because of the accessory structures surrounding the hair cells. Thus, angular motion is sensed by the semicircular canals, linear motion by the otolith organs, and sound energy by the cochlea.

The primary afferents innervated by hair cells are the peripheral processes of bipolar neurons having cell bodies located in Scarpa's ganglion within the internal auditory meatus. The central processes of these cells contact neurons in the brainstem of the central nervous system. The vestibular nuclei complex is defined as the brainstem region where primary afferents from the labyrinth terminate. It is composed of four main nuclei: the superior, medial, lateral, and descending nuclei. The axonal projections of vestibular nuclear neurons travel to all parts of the neuraxis, including the brainstem, cerebellum, spinal cord, and cerebrum. See also Motor systems.

In all vertebrates, there is an efferent system that originates from cell bodies within the central nervous system and terminates upon labyrinthine hair cells and primary afferents. The efferent vestibular system is presently a subject of intense study but undoubtedly is in place to enhance vestibular function. It is interesting that evolution felt it necessary to modify incoming vestibular information before it could enter the central nervous system.