Otolith

An otolith (Ancient Greek: ὠτο-, ōto- ear + λῐ́θος, líthos, a stone), also called otoconium, statolith, or statoconium, is a calcium carbonate structure in the saccule or utricle of the inner ear, specifically in the vestibular system of vertebrates. The saccule and utricle, in turn, together make the otolith organs. These organs are what allows an organism, including humans, to perceive linear acceleration, both horizontally and vertically (gravity). They have been identified in both extinct and extant vertebrates. Counting the annual growth rings on the otoliths is a common technique in estimating the age of fish.

Description Endolymphatic infillings such as otoliths are structures in the saccule and utricle of the inner ear, specifically in the vestibular labyrinth of all vertebrates (fish, amphibians, reptiles, mammals and birds). In vertebrates, the saccule and utricle together make the otolith organs. Both statoconia and otoliths are used as gravity, balance, movement, and directional indicators in all vertebrates and have a secondary function in sound detection in higher aquatic and terrestrial vertebrates. They are sensitive to gravity and linear acceleration. Because of their orientation in the head, the utricle is sensitive to a change in horizontal movement, and the saccule gives information about vertical acceleration (such as when in an elevator). Similar balance receptors called statocysts can be found in many invertebrate groups but are not contained in the structure of an inner ear. Mollusk statocysts are of a similar morphology to the displacement-sensitive organs of vertebrates; however, the function of the mollusk statocyst is restricted to gravity detection and possibly some detection of angular momentum. These are analogous structures, with similar form and function but not descended from a common structure. Statoconia (also called otoconia) are numerous grains, often spherical in shape, between 1 and 50 μm; collectively. Statoconia are also sometimes termed a statocyst. Otoliths (also called statoliths) are agglutinated crystals or crystals precipitated around a nucleus, with well defined morphology and together all may be termed endolymphatic infillings.

Mechanism The semicircular canals and sacs in all vertebrates are attached to endolymphatic ducts, which in some groups (such as sharks) end in small openings, called endolymphatic pores, on the dorsal surface of the head. Extrinsic grains may enter through these openings, typically less than a millimeter in diameter. The size of material that enters is limited to sand-sized particles and in the case of sharks is bound together with an endogenous organic matrix that the animal secretes. In mammals, otoliths are small particles, consisting of a combination of a gelatinous matrix and calcium carbonate in the viscous fluid of the saccule and utricle. The weight and inertia of these small particles causes them to stimulate hair cells when the head moves. The hair cells are made up of 40 to 70 stereocilia and one kinocilium, which is connected to an afferent nerve. Hair cells send signals down sensory nerve fibers which are interpreted by the brain as motion. In addition to sensing acceleration of the head, the otoliths can help to sense the orientation via gravity's effect on them. When the head is in a normal upright position, the otolith presses on the sensory hair cell receptors. This pushes the hair cell processes down and prevents them from moving side to side. However, when the head is tilted, the pull of gravity on otoliths shifts the hair cell processes to the side, distorting them and sending a message to the central nervous system that the head is tilted. There is evidence that the vestibular system of mammals has retained some of its ancestral acoustic sensitivity and that this sensitivity is mediated by the otolithic organs (most likely the sacculus, due to its anatomical location). In mice lacking the otoconia of the utricle and saccule, this retained acoustic sensitivity is lost. In humans vestibular evoked myogenic potentials occur in response to loud, low-frequency acoustic stimulation in patients with the sensorineural hearing loss. Vestibular sensitivity to ultrasonic sounds has also been hypothesized to be involved in the perception of speech presented at artificially high frequencies, above the range of the human cochlea (~18 kHz). In mice, sensation of acoustic information via the vestibular system has been demonstrated to have a behaviourally relevant effect; response to an elicited acoustic startle reflex is larger in the presence of loud, low frequency sounds that are below the threshold for the mouse cochlea (~4 Hz), raising the possibility that the acoustic sensitivity of the vestibular system may extend the hearing range of small mammals.

Comparative physiology In most vertebrates, otoliths are made of calcium carbonate, however in lampreys and hagfish, they are made of calcium phosphate. The polymorphs of calcium carbonate vary between different gnathostome groups. In Chondrichthyes it is of aragonite. In Chondrostei it is of vaterite (a rather unstable morph). In Holostei it is of a vaterite-aragonite mixture. In Teleostei, Coelacanthii, Dipnoi, and Amphibia, it is of aragonite. In most other tetrapods, it is of calcite, the most stable morph of calcium carbonate. In reptiles, it is of a mixture of calcite and aragonite.

Paleontology

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