The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

The purpose of the present study was to describe the longitudinal and radial gradients of cochlear innervation in the cat. To this end, afferent and efferent terminals of both the inner (IHC) and outer hair cell (OHC) regions were reconstructed from serial ultrathin sections at six and eight cochlear locations, respectively, corresponding to roughly octave intervals of characteristic frequency (CF). Analysis of the afferent innervation of the IHCs showed 1) the number of radial fibers per IHC rises from 10 per IHC at the 0.25 kHz region to a maximum of 30 per IHC at the 10 kHz locus; 2) branching of radial fibers is essentially restricted to regions apical to the 1.0 kHz point; and 3) there are significant differences in synaptic-body morphology for synapses on different sides of the IHC, corresponding to known differences in afferent threshold and rate of spontaneous activity. With respect to efferent innervation in the IHC area, we found 1) that there were numerous vesicle-filled terminals contacting every IHC examined; however, those with obvious synaptic specialization were confined to the most apical regions; and 2) there were roughly the same numbers of efferent synapses per radial fiber at all cochlear locations; however, at each location, radial fibers contacting the modiolar side of the hair cell (corresponding to high-threshold afferents) showed significantly more efferent synapses than radial fibers contacting the pillar side. Analysis of the OHC afferent innervation showed 1) a clear rise in numbers of terminals per OHC from roughly 3 per cell in the base to 15 per cell in the apex, 2) no systematic differences in the numbers of terminals as a function of OHC row, and 3) that synaptic bodies at the OHC afferent synapse are common only apical to the 1.0 kHz locus. Counts of efferent terminals on OHCs revealed 1) maximal numbers (9 per OHC) between the 6 and 24 kHz regions and 2) striking decrease in terminal counts from first- to third-row OHCs. Ultrastructural data on efferent innervation were compared quantitatively with light-microscopic analysis of cochleas immunostained (with antibody to synaptophysin) to reveal all vesiculated terminals.

Frequency-threshold tuning curves were recorded in thousands of auditory-nerve fibers (ANFs) in chinchilla. Synthetic tuning curves with 21 characteristic frequencies (187 Hz to 19.04 kHz, spaced every 1/3 octave) were constructed by averaging individual tuning curves within 2/3-octave frequency bands. Tuning curves undergo a gradual transition in symmetry at characteristic frequencies (CFs) of 1 kHz and an abrupt change in shape at CFs of 3-4 kHz. For CFs [≤]3 kHz, the lower limbs of tuning curves have similar slopes, about -18 dB/octave, but the upper limbs have slopes that become increasingly steep with increasing frequency and CF. For CFs >4 kHz, tuning curves normalized to the CF are nearly identical and consist of three segments. A tip segment, within 30-40 dB of CF threshold, has lower- and upper-limb slopes of -60 and +120 dB/octave, respectively, and is flanked by a low-frequency ("tail") segment, with shallow slope, and a terminal high-frequency segment with very steep slope (several hundreds of dB/octave). The tuning curves of fibers innervating basal cochlear sites closely resemble basilar-membrane tuning curves computed with low isovelocity criteria. At the apex of the chinchilla cochlea, frequency tuning is substantially sharper for ANFs than for available recordings of organ of Corti vibrations. 10.1152/jn.90637.2008

Spontaneous activity and frequency threshold tuning curves were studied in thousands of auditory nerve fibers in chinchilla. The frequency distribution of spontaneous activity rates is strongly bimodal for auditory nerve fibers with characteristic frequency <3 kHz but only mildly bimodal for the entire sample. Spontaneous activity rates and thresholds at the characteristic frequency are inversely related. Auditory-nerve fibers with low spontaneous rate have tuning curves with lower tip-to-tail ratios and more sharply tuned tips than the tuning curves of fibers with high spontaneous rates. It is shown here that this dependence of tuning on spontaneous rates is consistent with a previously unnoticed nonmonotonic dependence on iso-velocity criterion of the frequency tuning of basilar membrane vibrations. 10.1152/jn.90639.2008

A phenomenological model was developed to describe responses of high-spontaneous-rate auditory-nerve (AN) fibers, including several nonlinear response properties. Level-dependent gain (compression), bandwidth, and phase properties were implemented with a control path that varied the gain and bandwidth of tuning in the signal-path filter. By making the bandwidth of the control path broad with respect to the signal path, the wide frequency range of two-tone suppression was included. By making the control-path filter level dependent and tuned to a frequency slightly higher than the signal-path filter, other properties of two-tone suppression were also included. These properties included the asymmetrical growth of suppression above and below the characteristic frequency and the frequency offset of the suppression tuning curve with respect to the excitatory tuning curve. The implementation of this model represents a relatively simple phenomenological description of a single mechanism that underlies several important nonlinear response properties of AN fibers. The model provides a tool for studying the roles of these nonlinearities in the encoding of simple and complex sounds in the responses of populations of AN fibers. �2001 Acoustical Society of America.