The cell bodies of the bipolar neurons of the vestibular nerve are located in the vestibular ganglion (of Scarpa) in the internal acoustic meatus. Dendritic processes of the vestibular ganglion cells originate in the cristae ampullaris of the three semicircular ducts, the macula of the utricle, and the macula of the saccule. They synapse at the base of the vestibular sensory hair cells, either as a chalice around a type I hair cell or as a bouton associated with a type II hair cell. The axons of the vestibular nerve originate from the vestibular ganglion, enter the brain stem and terminate in four vestibular nuclei. Some secondary neuronal fibers travel to the cerebellum and to the nuclei of cranial nerves III, IV, and VI, which innervate the muscles of the eye.
The cochlear nerve originates from the sensory receptors of the spiral organ of Corti.
Neurons of the cochlear nerve are also bipolar, and their cell bodies are located in the spiral ganglion within the modiolus. Dendritic processes of spiral ganglion cells exit the
FIGURE 25.20 • Electron micrograph of an inner and outer hair cell. a. Observe the rounded base and constricted neck of the inner hair cell. Nerve endings (NE) from afferent nerve fibers (AF ) to the inner hair cells are seen basally. IP, inner pillar cell; IPH, inner phalangeal cell. 6,300. b. Afferent (AF ) and efferent (EF ) nerve fiber endings on the base of an outer sensory hair cell are evident. Outer phalangeal cells (OPH) surround the outer hair cells basally. Their apical projections form the apical cuticular plate (ACP). Note that the lateral domains in the middle third of the outer hair cells are not surrounded by supporting cells. 6,300. (Reprinted with permission from Kimura RS. Sensory and accessory epithelia of the cochlea. In: Friedmann I, Ballantyne J, eds. Ultrastructural Atlas of the Inner Ear. London: Butterworth, 1984.)
FIGURE 25.21 • Structure of the outer phalangeal cell. a. This scanning electron micrograph illustrates the architecture of the outer phalangeal (Deiters’) cells. Each phalangeal cell cups the basal surface of an outer hair cell and extends its phalangeal process apically to form an apical cuticular plate that supports the outer sensory hair cells. 2,400. b. Schematic drawing showing the relationship of an outer phalangeal cell to an outer hair cell.
FIGURE 25.22 • Schematic diagram illustrating the dynamics of the three divisions of the ear. The cochlear duct is shown here as if straightened. Sound waves are collected and transmitted from the external ear to the middle ear, where they are converted into mechanical vibrations. The mechanical vibrations are then converted at the oval window into fluid vibrations within the internal ear. Fluid vibrations cause displacement of the basilar membrane (traveling wave) on which rest the auditory sensory hair cells. Such displacement leads to stimulation of the hair cells and a discharge of neural impulses from them. Note that high-frequency sounds cause vibrations of the narrow, thick portion of the basilar membrane at the base of the cochlea, whereas low-frequency sounds displace basilar membrane toward the apex of the cochlea near its helicotrema.