is a special structure on the apex of hair cells
located in the sensory epithelium of the vertebrate inner ear
Anatomy in humans
In the human ear, each hair cell has a single non-motile cilium known as the kinocilium. Cochlear hair cells have kinocilia that degenerate following maturation of the stereociliary bundle. It is believed that the kinocilium is important in delineating the anatomical asymmetry of stereocilia or in making some mechanical connections with the hair cells in which they persist.
Anatomy in fish and frogs
The apical surface
of a sensory fish
hair cell usually has numerous stereocilia
and a single, much longer kinocilium
of the stereocilia toward or away from the kinocilium causes an increase or decrease in the firing rate of the sensory neuron
innervating the hair cell
at its basal surface.
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 called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin of fish and frogs.