Figure V-2 illustrates schematically the major structural features of a hair cell. The cell is cylindrical or flask-shaped and is equipped at its apical end with a bundle of sensory hairs called stereocilia. Hair bundles occur in a variety of sizes and shapes and, for any given hair cell, may vary in number. The kinocilium is associated with an intracellular organelle, the basal body. This orientation of hair cells with respect to the kinocilium is referred to as morphological polarization.
Morphological polarization is related to the directional sensitivity of the hair cell.
In the receptor organ, hair cells rest on a basal lamina and are joined to one another by tight junctions. As epithelial cells, they lack axons and dendrites. Instead, hair cells make synapses onto afferent nerve fibers of the eighth cranial nerve and also receive efferent synaptic contacts from axons originating in the brainstem. Stereocilia consist of actin filaments within a tubular membrane. Because the filaments are crossbridged, each stereocilium behaves like a stiff rod which pivots at its base.