As described on page 929, sound waves striking the tympanic membrane are translated into simple mechanical vibrations. The ossicles of the middle ear convey these vibrations to the cochlea.
In the internal ear, the vibrations of the ossicles are transformed into waves in the perilymph.
Movement of the stapes in the oval window of the vestibule sets up vibrations or traveling waves in the perilymph of the scala vestibuli. The vibrations are transmitted through
FIGURE 25.19 • Scanning electron micrograph of the spiral organ of Corti. This electron micrograph illustrates the configuration of stereocilia on the apical surfaces of the inner row and three outer rows of the cochlear sensory hair cells. 3,250.
the vestibular membrane to the scala media (cochlear duct), which contains endolymph, and are also propagated to the perilymph of the scala tympani. Pressure changes in this closed perilymphatic–endolymphatic system are reflected in movements of the membrane that covers the round window in the base of the cochlea.
As a result of sound vibrations entering the internal ear, a traveling wave is set up in the basilar membrane (Fig. 25.22). A sound of specified frequency causes displacement of a relatively long segment of the basilar membrane, but the region of maximal displacement is narrow. The point of maximal displacement of the basilar membrane is specified for a given frequency of sound and is the morphologic basis of frequency discrimination. High-frequency sounds cause maximal vibration of the basilar membrane near the base of the cochlea; low-frequency sounds cause maximal displacement nearer the apex. Amplitude discrimination (i.e., perception of sound intensity or loudness) depends on the degree of displacement of the basilar membrane at any given frequency range. Thus, coding acoustic information into nerve impulses depends on the vibratory pattern of the basilar membrane.
Movement of the stereocilia of the hair cells in the cochlea initiates neuronal transduction.
Hair cells are attached through the phalangeal cells to the basilar membrane, which vibrates during sound reception. The stereocilia of these hair cells are, in turn, attached to the tectorial membrane, which also vibrates. However, the tectorial membrane and the basilar membrane are hinged at different points. Thus, a shearing effect occurs between the basilar membrane (and the cells attached to it) and the tectorial membrane when sound vibrations impinge on the internal ear.
Because they are inserted into the tectorial membrane, the stereocilia of the hair cells are the only structures that connect the basilar membrane and its complex epithelial layer to the tectorial membrane. The shearing effect between the basilar membrane and the tectorial membrane deflects the stereocilia and thus the apical portion of the hair cells. This deflection activates MET channels located at the tips of stereocilia and generates action potentials that are conveyed to the brain via the cochlear nerve (cochlear division of the vestibulocochlear nerve, cranial nerve VIII).
Innervation of the Internal Ear
The vestibular nerve originates from the sensory receptors associated with the vestibular labyrinth.
The vestibulocochlear nerve (cranial nerve VIII) is a special sensory nerve and is composed of two divisions: A vestibular division called the vestibular nerve and a cochlear division called the cochlear nerve. The vestibular nerve is associated with equilibrium and carries impulses from the sensory receptors located within the vestibular labyrinth. The cochlear nerve is associated with hearing and conveys  impulses from the sensory receptors within the cochlear labyrinth (Fig. 25.23).