Zoology · Ch 10 — Neural Control and Coordination
Organ of Equilibrium
Organ of Equilibrium
Balance is a sense in its own right, part of what is called proprioception — the ability to sense the body's own position, movement and orientation. The dedicated organ for this is the vestibular system, sitting in the inner ear right next to the cochlea and built from a connected series of fluid-filled sacs and tubules. These structures are arranged as a membranous labyrinth (carrying endolymph) nested inside a bony labyrinth (carrying perilymph), and both fluids respond mechanically as the head changes position or accelerates.
Two membranous sacs closest to the cochlea, the utricle and saccule, each contain a patch of equilibrium receptor tissue called a macula, responsible for detecting the head's linear movement. Each macula is built of hair cells (acting as mechanoreceptors) embedded in a gelatinous otolithic membrane studded with tiny calcium-carbonate crystals called otoliths; the extra weight of this membrane increases inertia, so that when the head moves, the membrane lags slightly behind the hair cells beneath it, bending their stereocilia and generating a signal. …
What this figure shows. The bony labyrinth (containing perilymph) and, nested inside it, the membranous labyrinth (containing endolymph): the utricle and saccule sit in the central vestibule, three semicircular ducts (anterior, posterior and lateral) loop off it each with a swollen ampulla at one end, and the coiled cochlear duct extends from the vestibule, with the oval window and a secondary tympanic …
What this figure shows. A close-up of the macula of the utricle/saccule: a bed of supporting cells and hair cells, each hair cell bearing a bundle of stereocilia and one longer kinocilium projecting up into an overlying gelatinous otolithic membrane studded with small calcium-carbonate otoliths; vestibular nerve fibres are shown synapsing at the base of the hair cells to carry the signal gene …