Six sensory regions of membranous labyrinth are composed of sensory hair cells and accessory supporting cells. These regions project from the wall of the membranous labyrinth into the endolymphatic space in each internal ear (see Fig. 25.7c):  Three cristae ampullaris (ampullary crests) are located in the membranous ampullae of the semicircular ducts. They are sensitive to angular acceleration of the head (i.e., turning the head).
 Two maculae, one in the utricle (macula of utricle) and the other in the saccule (macula of saccule), sense the position of the head and its linear movement.
 The spiral organ of Corti projects into the endolymph of the cochlear duct. It functions as the sound receptor.
FIGURE 25.8 • Diagrams of the human internal ear. a. This lateral view of the left bony labyrinth shows its divisions: the vestibule, cochlea, and three semicircular canals. The openings of the oval window and the round window can be observed.
b. Diagram of the membranous labyrinth of the internal ear lying within the bony labyrinth. The cochlear duct can be seen spiraling within the bony cochlea. The saccule and utricle are positioned within the vestibule, and the three semicircular ducts are lying within their respective canals. This view of the left membranous labyrinth allows the endolymphatic duct and sac to be observed. c. This view of the left membranous labyrinth shows the sensory regions of the internal ear for equilibrium and hearing. These regions are the macula of the saccule and macula of the utricle, the cristae ampullaris of the three semicircular ducts, and the spiral organ of Corti of the cochlear duct.
macula of saccule organ of Corti macula of utricle crista ampullaris of anterior semicircular duct crista ampullaris of lateral semicircular duct crista ampullaris of posterior semicircular duct
Hair cells are epithelial mechanoreceptors of the vestibular and cochlear labyrinth.
The hair cells of the vestibular and cochlear labyrinths function as mechanoelectric transducers; they convert mechanical energy into electrical energy that is then transmitted via the vestibulocochlear nerve to the brain. The hair cells derive their name from the organized bundle of rigid projections at their apical surface. This surface holds a hair bundle that is formed of rows of stereocilia called sensory hairs. The rows increase in height in one particular direction across the bundle (Fig. 25.9). In the vestibular system, each hair cell possesses a single true cilium called a kinocilium, which is located behind the row of longest stereocilia (Fig. 25.10). In the auditory system, the hair cells lose their cilium during development but retain the basal body. The position of the kinocilium (or basal body) behind the longest row of stereocilia defines the polarity of this asymmetric hair bundle. Therefore, movement of the stereocilia toward the kinocilium is perceived differently than movement in the opposite direction (see below).
Stereocilia of hair cells are rigid structures that contain mechanoelectric transducer channel proteins at their distal ends.