The otolithic membrane is a fibrous structure located in the vestibular system of the inner ear. It plays a critical role in the brain's interpretation of equilibrium. The membrane serves to determine if the body or the head is tilted, in addition to the linear acceleration of the body. The linear acceleration could be in the horizontal direction as in a moving car or vertical acceleration such as that felt when an elevator moves up or down. Structure The otolithic membrane is part of the otolith organs in the vestibular system. The otolith organs include the utricle and the saccule. The otolith organs are beds of sensory cells in the inner ear, specifically small patches of hair cells. Overlying the hair cells and their hair bundles is a gelatinous layer and above that layer is the otolithic membrane. The utricle serves to measure horizontal accelerations and the saccule responds to vertical accelerations. The reason for this difference is the orientation of the macula in the two organs. The utricular macula lie horizontal in the utricle, while the saccular macula lies vertical in the saccule. Every hair cell in these sensory beds consist of 40-70 stereocilia and a kinocilium. The stereocilia and kinocilium are embedded in the otolithic membrane and are essential in the function of the otolith organs. The hair cells are deflected by structures called otoconia. Otoconia Otoconia are crystals of calcium carbonate and make the otolithic membrane heavier than the structures and fluids surrounding it. The otoconia are composite crystallites that overlie the macular sensory epithelium of the gravity receptors of most vertebrates and are required for optimal stimulus input of linear acceleration and gravity. Fishes often have a single large crystal called an otolith, but otoconia from higher vertebrates have numerous crystals, and each apparently single crystal in fact has multiple crystallites that are composed of organic and inorganic components. Ultra-high resolution transmission electron microscopy of rat otoconia shows that the crystallites are 50-100 nm in diameter, have round edges and are highly ordered into laminae. Biomineralization of otoliths and otoconia results mainly from the release of soluble calcium ions, which is in turn precipitated as calcium carbonate crystals. The mechanical coupling of the otoconia to the hair cell sensory stereocilia at the surface of the vestibular sensory epithelium is mediated by two layers of the extracellular matrix, each on with a specific role in the mechanical