Hearing and Balance : The Eighth Cranial Nerve

Keywords

endolymph, hair cells, vestibulocochlear nerve, ototoxicity, organ of Corti, acoustic reflex, conductive hearing loss, sensorineural hearing loss, vestibulo-ocular reflex, nystagmus, vertigo

Chapter Outline
Auditory and Vestibular Receptor Cells Are Located in the Walls of the Membranous Labyrinth, 86
- Endolymph Is Actively Secreted, Circulates Through the Membranous Labyrinth, and Is Reabsorbed, 86
- Auditory and Vestibular Receptors Are Hair Cells, 86
The Cochlear Division of the Eighth Nerve Conveys Information About Sound, 87
- The Outer and Middle Ears Convey Airborne Vibrations to the Fluid-Filled Inner Ear, 88
- The Cochlea Is the Auditory Part of the Labyrinth, 88
- Auditory Information Is Distributed Bilaterally in the CNS, 89
- Efferents Control the Sensitivity of the Cochlea, 89
- Conductive and Sensorineural Problems Can Affect Hearing, 90
The Vestibular Division of the Eighth Nerve Conveys Information About Linear and Angular Acceleration of the Head, 91
- Receptors in the Utricle and Saccule Detect Linear Acceleration and Position of the Head, 91
- Receptors in the Semicircular Ducts Detect Angular Acceleration of the Head, 91
- Vestibular Primary Afferents Project to the Vestibular Nuclei and the Cerebellum, 92
- The Vestibular Nuclei Project to the Spinal Cord, Cerebellum, and Nuclei of Cranial Nerves III, IV, and VI, 92
- Position Sense Is Mediated by the Vestibular, Proprioceptive, and Visual Systems Acting Together, 94

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

Key Concept

The membranous labyrinth is suspended within the bony labyrinth, a cavity in the temporal bone, that contains a special fluid termed endolymph.

Eighth nerve fibers innervate special receptors called hair cells , located in an elaborate end organ called the labyrinth . The labyrinth is two series of twisted tubes (hence the name labyrinth), one suspended inside the other. The outer tube, the bony labyrinth , is a continuous channel in the temporal bone. The bony cochlea is located anteriorly, the bony semicircular canals posteriorly, and the vestibule between the two. The inner tube (so to speak), the membranous labyrinth , is a second continuous tube suspended within the bony labyrinth; as explained a little later, the mechanical arrangement of the cochlear suspension is crucial to its function. The membranous labyrinth generally parallels portions of the bony labyrinth (i.e., there are cochlear and semicircular ducts ), except that the vestibule contains two parts of the membranous labyrinth—the utricle and the saccule .

Endolymph Is Actively Secreted, Circulates Through the Membranous Labyrinth, and Is Reabsorbed

The bony labyrinth is filled with perilymph , which is more or less equivalent to cerebrospinal fluid (CSF) and actually communicates with subarachnoid CSF. The membranous labyrinth, in contrast, is filled with endolymph , whose ionic composition more closely resembles that inside a cell (i.e., high [K ⁺], low [Na ⁺]). Endolymph is secreted by specialized cells in the walls of the membranous labyrinth, circulates through it, and is reabsorbed.

Auditory and Vestibular Receptors Are Hair Cells

Key Concepts

Hair cells have mechanosensitive transduction channels.
Subtle differences in the physical arrangements of hair cells determine the stimuli to which they are most sensitive.

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 and opens the cation channels, allowing potassium to flow in (remember endolymph is high in potassium), 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 auditory apparatus has three general areas—the outer , middle , and inner ears . The outer and middle ears (separated from each other by the tympanic membrane ) are air-filled cavities in or leading into the temporal bone; the inner ear is the fluid-filled labyrinth.

The Outer and Middle Ears Convey Airborne Vibrations to the Fluid-Filled Inner Ear

Sound vibrations are funneled through the outer ear and vibrate the tympanic membrane. This in turn vibrates the malleus , incus , and stapes (the middle ear ossicles ), and the stapes’ footplate vibrates the perilymph of the inner ear through the oval window . (Inward pushes and outward pulls of the stapes are accommodated by outward and inward bulges of the round window membrane .) This elaborate mechanism is necessary because sound does not cross an air-water interface very well, and there is essentially an air-water interface between the outside world and the perilymph of the cochlea. The slight mechanical advantage of the middle ear ossicles, together with the much larger area of the tympanic membrane relative to the oval window, results in a much greater force per unit area at the oval window than at the tympanic membrane.

The Cochlea Is the Auditory Part of the Labyrinth

Key Concept

Traveling waves in the basilar membrane stimulate hair cells in the organ of Corti, in locations that depend on sound frequency.

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 partition. The partition across the cochlear duct is triangular in shape with the inner part filled with endolymph, and it contains the components needed for hearing. 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 ), all within the endolymph partitioned region. 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).

Inner Hair Cells Are Sensory Cells; Outer Hair Cells Are Amplifiers.

There are two populations of cochlear hair cells all along the basilar membrane. Inner hair cells are closer to the center of the cochlea, less numerous, but heavily innervated by eighth nerve fibers; they are the principal source of the sound information conveyed by the eighth nerve. Outer hair cells are more numerous but sparsely innervated. The main job of the outer hair cells is not to transmit auditory information to eighth nerve fibers, but rather to lengthen and shorten very rapidly in response to the receptor potentials they produce when the basilar membrane vibrates. This movement in turn enhances the responses of nearby inner hair cells, making a major contribution to their sensitivity and frequency selectivity. In other words, the outer hair cells tend to modulate the activity received by the inner hair cells by modulating the movement of the basilar membrane.

The basilar membrane vibrations caused by outer hair cell movement are transmitted back along the middle ear ossicles and reach the tympanic membrane, turning it, in effect, into a tiny loudspeaker. The resulting otoacoustic emissions can be detected by a sensitive microphone in the ear canal, forming the basis of a clinical test of hair cell function.

Auditory Information Is Distributed Bilaterally in the CNS

We use our ears not only to identify sounds but also to localize them in space. This localization is achieved by comparing time and intensity differences between the sounds arriving at our two ears, with the comparison starting early in the CNS pathway. Cochlear primary afferent nerve fibers end ipsilaterally in the cochlear nuclei at the pontomedullary junction. The cochlear nuclei (i.e., second-order neurons for auditory) then project bilaterally in the brainstem, so that all levels rostral to the cochlear nuclei in the CNS of each ear are represented bilaterally ( Fig. 14.5 ). Therefore unilateral damage anywhere in the CNS past the cochlear nuclei does not cause deafness of only one ear. Rostral to the cochlear nuclei, the auditory pathway on each side is concerned not so much with one ear as with information from both ears relevant to the contralateral half of the auditory world, aiding in identifying and localizing sound.