An otolith, also known as an otoconium, is a tiny crystal of limestone found within a gelatinous layer that covers hair receptor cells in the utricle and saccule of the inner ear. The utricle and saccule are the areas within the ear which detect acceleration when one moves in a straight line, either horizontally or vertically. When acceleration occurs, each otolith is displaced and this movement is transferred to the hair cells below, triggering nerve impulses. The vestibular nerve carries the impulses to the brain where the information can be processed. Fish have much larger otoliths than humans.
The inner ear contains a series of spaces known as the bony labyrinth. These channels are full of fluid called perilymph, and within that, roughly following the shape of the bony labyrinth, is what is known as the membranous labyrinth. The membranous labyrinth forms an enclosed system of spaces, filled with a different fluid known as endolymph, and dilates to form two specialized areas, the utricle and the saccule. These are known as the otolithic organs. Other regions of the membranous labyrinth, the semicircular canals and the cochlea, do not contain otoliths.
Inside the utricle and saccule, a number of areas containing receptor cells are found. These cells, also called hair cells, are arranged so the minute hair-like processes projecting from their surfaces are embedded in the gelatinous material containing the otoliths. When the body accelerates, each otolith becomes displaced and the movement is transmitted through the gelatinous substance resulting in distortion of the hair cell processes. Hair cells are connected to nerve endings and the distortion triggers nerve impulses, which travel through branches of the vestibular nerve to reach the brain.
The utricle and saccule are responsible for detecting different types of linear acceleration, with the utricle mainly responsible for horizontal movement and the saccule, vertical. An individual otolith is made of calcium carbonate, or limestone, and protein. It is uncertain whether the body is able to replace an otolith if it becomes detached from its gelatinous support, but they are known to fall off. Otoliths also degenerate with age and may be damaged by certain drugs.
Fish have relatively large otoliths which they use to hear, balance and sense acceleration. As fish otoliths grow in layers similar to tree rings, scientists can study them to discover a fish's age and growth pattern. It is thought that rising carbon dioxide levels in the ocean may be leading to an increase in the size of otoliths in some fish.