Kinocilia

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The apical surface of a sensory fish hair cell usually has numerous stereocilia 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 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.

In the human ear, the kinocilium is a single, true cilium. Cochlear hair cells have kinocilia that degenerate during fetal development, indicating these processes play no essential role in transduction; instead kinocilia may be important in delineating the anatomical asymmetry of stereocilia or in making some mechanical connections with the hair cells in which they persist (Nolte, John. THE HUMAN BRAIN. 5th ed. Mosby. 2002: 340-41.)