The inner ear contains the neural structures for both the sense of balance (the vestibular system) and hearing. The auditory part of the inner ear consists of the cochlea, a tube that wraps around itself three or four times (depending on species) in a snail-like coil. The footplate of the stapes (the medial-most ossicular bone) pushes in on the oval widow at the base of the cochlea. The cochlear tube contains an inner tube, the cochlear partition, that is a sealed structure running the length of cochlea except at the top (apex), where there is an opening (the heliocotrema) between the cochlear partition and the end of the cochlea.
Figure 4 shows a schematic diagram of a cross section of the cochlear partition, the inner tube of the cochlea. Along the bottom membrane, the basilar membrane, lie the inner and outer hair cells, the neural elements for hearing, and their supporting structures. Figure 5 shows a view looking down on the top of the hair cells, revealing the hairs (stereocillia) from which the hair cells derive their name. In mammals there are three rows of outer hair cells and one row of inner hair cells. When the fluids of the inner ear are vibrated by
the ossicular chain, a wave motion occurs within the cochlea that causes a differential movement between the basilar membrane on which the hair cells sit and the tectorial membrane at the top of the hair cells. This differential movement causes the stereocillia to shear (a type of bending), and this shearing ignites a generator potential in the hair cell causing an action potential in the auditory nerve fiber attached to the hair cell at its bottom.
This cochlear wave motion is a crucial part of the biomechanical action of the inner ear. The wave is a traveling wave (Fig. 6), such that the region of maximum vibratory displacement along the cochlear partition from base to apex is frequency dependent. High frequencies produce a wave that travels only partway from the base to the apex, thus producing maximum displacement near the cochlear base. Low frequencies cause the wave to travel to the apex, producing maximum displacement at or near the apex. Thus, the area of maximum cochlear partition vibration is distributed along the cochlear partition according to the frequency content of the originating sound.
Since the hair cells are distributed from base to apex, those whose stereocillia will be maximally sheared depend on the sound's frequency content. Thus, hair cells are able to signal where, along the cochlear partition, the maximal displacement occurs and, hence, they code for the frequency content of sound. If the stereocilla of hair cells near the base are maximally sheared, the sound contains high frequencies, whereas maximal stereocillia shearing for hair cells near the apex code for low frequencies. Neural discharge rate is directly proportional to the amount of stereocillia shearing and, thus, to the displacement of the cochlear partition.
The frequency selectivity of this vibratory traveling wave is very sharp, the basilar membrane vibration is extremely sensitive, and the vibratory motion is a compressive nonlinear function of sound level. The exquisite frequency selectivity, high sensitivity, and important nonlinear function of the cochlear biome-chanical traveling wave are only possible because of the way in which the inner and outer hair cells function. The inner hair cells are the biological transducers that
provide the neural code transmitted to the central auditory system by the auditory nerve. Each auditory nerve fiber is connected to a few inner hair cells and 90% of the auditory nerve fibers innervate the inner hair cells. The outer hair cells change size (primarily length) as a result of the vibration of the cochlear partition and the consequential outer hair cell stereo-cillia shearing. Because the length of the outer hair cells changes, this presumably alters the connections between the basilar and tectorial membranes, which would affect the biomechanical vibration of the cochlea. If the outer hair cells are damaged, the frequency selectivity and the sensitivity of the biome-chanical vibration are compromised, suggesting the importance of outer hair cell motility in effective cochlear functioning. The fact that the outer hair cells are motile and this motility feeds back into the biomechanical action of the cochlea suggests that the outer hair cells provide an active mechanism that serves as a type of cochlear amplifier allowing the inner hair cells to provide a highly sensitive, very frequency selective, and compressively nonlinear code for the frequency, intensity, and timing of the acoustic input.
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