19.8:
The Vestibular System
The vestibular system detects the position and movement of the head through the labyrinth of the inner ear, which has three semicircular canals and two otolith organs.
The otolith organs contain vestibular hair cells that sit below the otolithic membrane, which is impregnated with otoconia, calcium carbonate crystals. When the head moves in certain ways, such as tilting forward this relatively heavy membrane shifts displacing the stereocilia on top of the hair cells generating a neural signal.
The semicircular canals also have vestibular hair cells, but their cilia are surrounded by a gelatinous cupula. Head movements such as rotations distort the cupula, moving the cilia.
For all vestibular hair cells, displacement towards the tallest cilium increases neurotransmitter release, while displacement in the opposite direction decreases it.
This information is then sent through the vestibular nerve to lower areas of the brain, such as the brain stem and cerebellum, so that gaze and balance can be rapidly adjusted to compensate for head movements. Some vestibular information travels further to the cerebral cortex, where it aids in the perception of body orientation.
19.8:
The Vestibular System
The vestibular system is a set of inner ear structures that provide a sense of balance and spatial orientation. This system is comprised of structures within the labyrinth of the inner ear, including the cochlea and two otolith organs—the utricle and saccule. The labyrinth also contains three semicircular canals—superior, posterior, and horizontal—that are oriented on different planes.
All of these structures contain vestibular hair cells—the sensory receptors of the vestibular system. In the otolith organs, the hair cells sit beneath a gelatinous layer called the otolithic membrane, which contains otoconia—calcium carbonate crystals—making it relatively heavy. When the head is tilted, the otolithic membrane shifts, bending the stereocilia on the hair cells.
In the semicircular canals, the cilia of the hair cells are contained within a gelatinous cupula, which is surrounded by endolymph fluid. When the head experiences movements, such as rotational acceleration and deceleration, the fluid moves, bending the cupula and the cilia within it.
Similar to the auditory hair cells, displacement towards the tallest cilium causes mechanically-gated ion channels to open, depolarizing the cell and increasing neurotransmitter release. Displacement towards the shortest cilium hyperpolarizes the cell and decreases neurotransmitter release compared to rest. In this way, head movements are transduced into neural signals.
Vestibular hair cells are oriented in different directions within the structures of the labyrinth—which are themselves oriented in different directions—allowing diverse types and directions of head movement to be detected. This information is sent from the labyrinth through the vestibular nerve to parts of the brain, such as the brainstem and cerebellum, as well as certain optic muscles. This enables rapid motor responses, such as the vestibulo-ocular reflex, which adjusts eye position to stabilize gaze while the head is moving.
In the brain, vestibular information from both ears is integrated with other types of sensory information—such as visual information—to aid in spatial orientation. Some vestibular information is sent through the thalamus to the cerebral cortex—aiding in the conscious perception of orientation in space.
Suggested Reading
Lewis, Richard F. “Advances in the Diagnosis and Treatment of Vestibular Disorders: Psychophysics and Prosthetics.” The Journal of Neuroscience 35, no. 13 (April 1, 2015): 5089–96. [Source]