otolitorgan

The otolith organ, also called the otolithic organ, refers to the sensory structures of the vestibular system in the inner ear that detect linear accelerations and head position relative to gravity. It comprises two membranous sacs, the utricle and the saccule. Each sac contains a macula, a sensory epithelium covered by an otolithic membrane that bears tiny calcium carbonate crystals called otoconia. The weight of the otoconia adds inertia to the membrane, so when the head moves, the membrane and otoconia lag behind the hair cells beneath, bending their stereocilia and kinocilium and altering neurotransmitter release to the vestibular nerve fibers.

The utricle and saccule have complementary orientations. The utricle’s maculae are oriented horizontally and detect horizontal linear accelerations (such as forward and backward movement) and head tilts in the horizontal plane, while the maculae of the saccule are oriented vertically and detect vertical accelerations (such as up and down) and tilting in the sagittal plane. This organization allows the system to sense head position and motion across multiple directions.

Functionally, signals from the otolith organs contribute to balance, gaze stabilization, and perception of verticality. Afferent information travels via the vestibular nerve to the brainstem vestibular nuclei and to the cerebellum, where it integrates with visual and proprioceptive inputs.

Clinical relevance includes disorders such as benign paroxysmal positional vertigo, which can arise when otoconia detach from the otolith membrane and migrate into semicircular canals, provoking vertigo with certain head positions. Tests like vestibular-evoked myogenic potentials assess otolith function.