278 Introduction to Anatomy & Physiology Equilibrium People tend to take their equilibrium, or ability to balance, for granted. Only when they are dizzy or disoriented do they typically even appreciate balance. The ability to balance comes from specialized structures in the inner ear. Recall that the vestibule of the inner ear contains three semicircular canals. The hair cells in the semicircular canals are stimulated by movement of the endolymph in the canals, much like the hair cells in the organ of Corti are stimulated by motion of the endolymph to produce hearing. The hair cells in the semicircular canals, however, stimulate the vestibular nerve, which communicates with the cerebellum to provide information about the orientation and motion of the head. The semicircular canals play an important role in balance whether a person is stationary or moving. membrane of the oval window. This process of amplification and transmission sets in motion the fluids of the inner ear. Your ability to hear resides within the snail- shaped cochlea of the inner ear. The portion of the membranous labyrinth inside the cochlea is called the cochlear duct. Within the cochlear duct is the spiral organ of Corti, which contains hearing receptors called hair cells (Figure 7.14). The short, stiff hair cells are stimulated by high- pitched sounds the longer, more flexible hair cells are stimulated by low-pitched sounds. Motion of the endolymph in the cochlear duct causes the tectorial membrane to move. Movement of the tectorial membrane stimulates the hair cells, which in turn stimulate the cochlear branch of the vestibulocochlear nerve. The cochlear nerve then transmits electrical impulses to the auditory region in the temporal lobe of the brain. © Body Scientific International Figure 7.14 Anatomy of the cochlea. What role do different kinds of hair cells play in relaying sound messages to the brain? Cochlea Three-dimensional section of the cochlea Organ of Corti Cochlea Oval window Organ of Corti Cochlear nerve fibers Hair cells Tectorial membrane Perilymph Tectorial membrane Cochlear duct Wall of bony labyrinth (contains perilymph) Cochlear nerve Copyright Goodheart-Willcox Co., Inc.