1. any of the sensory receptors for hearing, located in the organ of Corti within the cochlea of the inner ear. They respond to vibrations of the basilar membrane via movement of the fine hairlike structures (stereocilia) that protrude from them. In humans, there are 12,000 outer hair cells and 3,500 inner hair cells. Almost all auditory nerve fibers communicate only with inner hair cells. The outer hair cells appear to be capable of movement and may provide amplification of the vibration that stimulates the inner hair cells.

2. any of the sensory receptors for balance, similar in structure to the cochlear hair cells. They are located in the inner ear within the ampullae of the semicircular canals (forming part of the crista) and within the saccule and utricle (forming part of the macula).