AKA the vestibular system.
The vestibular system is made up of a peripheral portion and a central portion:
The vestibular system gives rise to a “sixth sense” that is critical both to automatic behaviors and to perception, with the consequence that balance, gaze stabilization during head movement, and sense of orientation in space are all adversely affected if the system is damaged.
The labyrinth is an elaborate set of interconnected chambers within the inner ear that are continuous with the cochlea. The labyrinth is similar to the cochlea in that they both use hair cells to transduce physical motion into neural impulse. However, in the labyrinth the hair cells respond to gravity from the translational and rotational movements of the head.
The labyrinth is buried deep in the temporal bone and consists of the two otolith organs (the utricle and saccule) and three semicircular canals (superior, posterior and horizontal)- together the organs and canals are referred to as the membranous labyrinth.
The membranous labyrinth is filled with a fluid called endolymph and surrounding this labyrinth, between it and the bony walls, there is another fluid called perilymph.
Special sensory cells, known as vestibular hair cells, are found inside the utricle and saccule, as well as inside three swollen areas called ampullae. The ampullae are located at the bases of the three semicircular canals, near the utricle.
Just like in the cochlea, tight junctions seal the top surfaces (apical surfaces) of the vestibular hair cells. These tight seals ensure that the hair-like structures (the hair bundles) are exposed only to the endolymph, while the bottom parts (the basal portions) of the hair cells are surrounded by perilymph. This separation is important for the proper functioning of the balance system.
The utricle and saccule are specialized primarily to respond to translational movements of the head and static head position relative to the gravitational axis (i.e., head tilts), whereas the semicircular canals are specialized for responding to rotations of the head.
As in the case of auditory hair cells, movement of the stereocilia toward the kinocilium in the vestibular end-organs opens mechanically gated transduction channels located at the tips of the stereocilia, depolarizing the hair cell and causing neurotransmitter release onto (and excitation of) the vestibular nerve fibers (figure A).