| Kinocilium | |
|---|---|
 | |
| Details | |
| Identifiers | |
| Latin | kinocilium |
| TH | H1.00.01.1.01015 |
| FMA | 67323 |
| Anatomical terminology | |
Akinocilium is a special type ofcilium on the apex ofhair cells located in the sensoryepithelium of thevertebrateinner ear. Contrasting withstereocilia, which are numerous, there is only one kinocilium on each hair cell. The kinocilium can be identified by its apical position as well as its enlarged tip.[1]
Together with stereocilia, the kinocilium regulates depolarization andhyperpolarization of the hair cell, which is aneuron that can generateaction potentials. When the stereocilia and kinocilium move further apart, the cell hyperpolarizes. When they move closer together, the cell depolarizes and may fire an action potential.[1]
Kinocilia are found on the apical surface ofhair cells and are involved in both the morphogenesis of the hair bundle andmechanotransduction. Vibrations (either by movement or sound waves) cause displacement of the hair bundle, resulting in depolarization or hyperpolarization of the hair cell. The depolarization of the hair cells in both instances causessignal transduction vianeurotransmitter release.[1]
Each hair cell has a single,microtubular kinocilium. Beforemorphogenesis of the hair bundle, the kinocilium is found in the center of the apical surface of the hair cell surrounded by 20-300 microvilli. During hair bundle morphogenesis, the kinocilium moves to the cell periphery dictating hair bundle orientation. As the kinocilium does not move, microvilli surrounding it begin to elongate and form actinstereocilia. In many mammals, but not in humans,[1] the kinocilium will regress once the hair bundle has matured.[2]
The movement of the hair bundle, as a result ofendolymph[2] flow, will cause potassium channels on the stereocilia to open.[1] This is mostly due to the pulling force stereocilia exerts on its neighboring stereocilia via interconnecting links that hold stereocilia together (usually from tallest to shortest) and this leads to the depolarization of the hair cell. This pattern of depolarization should not be confused with the more common depolarization which involves the influx of Na+ into the cell while K+ channels stay closed. Endolymph composition resembles that of theintracellular fluid (more K+ and less Na+) more closely compared to its counterpart,perilymph which resembles theextracellular fluid (more Na+ and less K+ compared to intracellular matrix). This depolarization will open voltage gated calcium channels. The influx of calcium then triggers the cell to release vesicles containing excitatory neurotransmitters into a synapse. The post-synaptic neurite then sends an action potential to the Spiral Ganglia of Gard. Unlike the hair cells of thecrista ampullaris or the maculae of the saccule and utricle, hair cells of thecochlear duct do not possess kinocilia.[citation needed]
Kinocilia are present in the crista ampullaris of the semicircular ducts and the sensory maculae of theutricle andsaccule.[1] One kinocilium is the longest cilium located on the hair cell next to 40–70 stereocilia. During movement of the body, the hair cell is depolarized when the stereocilia move toward the kinocilium. The depolarization of the hair cell causes neurotransmitter to be released and an increase in firing frequency ofcranial nerve VIII. When the stereocilia tilt away from the kinocilium, the hair cell is hyperpolarized, decreasing the amount of neurotransmitter released, which decreases the firing frequency of cranial nerve VIII.[3]
The apical surface of a sensoryfish hair cell usually has numerousstereocilia and a single, much longer kinocilium.Deflection of the stereocilia toward or away from the kinocilium causes an increase or decrease in the firing rate of the sensoryneuron innervating thehair cell at its basal surface.[citation needed]
Hair cells in fish and some frogs are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion calledcupula. The hair cells therefore can not be seen and do not appear on the surface of skin of fish and frogs.[citation needed]
