14 Hearing and Balance: The Eighth Cranial Nerve
The eighth nerve is the nerve of hearing and equilibrium. All of its receptive functions are accomplished by variations on a common theme; the different sensory information carried by different fibers in the nerve is simply the result of slight differences in the mechanical arrangement of receptors and accessory structures.
Auditory and Vestibular Receptor Cells Are Located in the Walls of the Membranous Labyrinth
Auditory and Vestibular Receptors Are Hair Cells
Hair cells, the characteristic receptor cells of the labyrinth, have a graduated array of specialized microvilli (stereocilia) and sometimes one true cilium (the kinocilium) on their apical surfaces. Each stereocilium is attached to its next tallest neighbor by a filamentous tip link protein, connected at one or both ends to a cation channel. The sensory hairs of the hair cells poke through the wall of the membranous labyrinth and are typically inserted into a mass of gelatinous material (Fig. 14-1). Movement of a hair bundle relative to the gelatinous material causes a depolarizing or a hyperpolarizing receptor potential, depending on the direction of deflection. Deflecting the hair bundle toward the tallest stereocilia stretches the tip links, opens the cation channels, and depolarizes the hair cell; deflecting in the opposite direction lets the tip links relax, and some channels that were open at rest close. This in turn causes an increase or a decrease in the release of an excitatory transmitter (probably glutamate), and a consequent increase or decrease in the firing rate of any eighth nerve fiber that innervates the hair cell (Fig. 14-2). The way in which the gelatinous material is arranged within the labyrinth plays a major role in determining the kind of mechanical stimulus to which a particular region of the labyrinth responds best.
The Cochlear Division of the Eighth Nerve Conveys Information about Sound
The Cochlea Is the Auditory Part of the Labyrinth
The cochlear duct is stretched as a partition across the cochlear part of the bony labyrinth. The partition is complete except for a small hole at the apex of the cochlea (the helicotrema) at which two otherwise separate perilymphatic spaces communicate with each other. Therefore when the stapes moves inward and outward, part of the resulting perilymph movement causes a traveling wave of deformation that moves along the cochlear duct. The deformation reaches a maximum amplitude at a site that depends on the frequency of the stapes vibration (Fig. 14-3); portions of the cochlear duct closer to the oval window are more sensitive to higher frequencies, and portions closer to the helicotrema are more sensitive to lower frequencies. This is, at least to a great extent, the result of gradual changes in the width and mechanical properties of the basilar membrane, which forms one wall of the cochlear duct. Cochlear hair cells are located in the organ of Corti (on the basilar membrane), with their sensory hairs embedded in the gelatinous tectorial membrane (Fig. 14-4). Deformation of the cochlear duct causes differential movement of the basilar and tectorial membranes and this deflects the sensory hairs, which in turn causes either a depolarizing or a hyperpolarizing receptor potential in the hair cells (depending on the direction of deflection).
Figure 14-3 Outer, middle, and inner ear, shown schematically as though the cochlea had been uncoiled. Vibrations transmitted through the tympanic membrane (TM), middle ear ossicles, and oval window (O) reach the perilymph of the inner ear. Very low frequencies and static pressure changes move perilymph through the helicotrema (H), but audible frequencies deform the cochlear duct. The dashed line indicates the plane of section in Fig. 14-4. R, Round window membrane.
Figure 14-4 A schematic cross section through one turn of the cochlea, showing the organ of Corti with its inner (i) and outer (o) hair cells. The tallest stereocilia of at least the outer hair cells are inserted in the gelatinous tectorial membrane (T). All three walls of the cochlear duct contain a diffusion barrier separating endolymph and perilymph. Reissner’s membrane does little more than this, but the stria vascularis is specialized as a secretory epithelium that produces endolymph. The perilymph of scala vestibuli (open to the vestibule) is continuous with that of scala tympani (ends at the round window, or secondary tympanic membrane) at the helicotrema. The dashed line indicates the plane of section in Fig. 14-3.
