THE COCHLEAR AMPLIFIER: IS IT HAIR BUNDLE MOTION OF OUTER HAIR CELLS?

Medial olivocochlear (MOC) neurons are efferent neurons that project axons from the brain to the cochlea. Their action on outer hair cells reduces the gain of the “cochlear amplifier,” which shifts the dynamic range of hearing and reduces the effects of noise masking. The MOC effects in one ear can be elicited by sound in that ipsilateral ear or by sound in the contralateral ear. To study how MOC neurons project onto the cochlea to mediate these effects, single-unit labeling in guinea pigs was used to study the mapping of MOC neurons for neurons responsive to ipsilateral sound vs. those responsive to contralateral sound. MOC neurons were sharply tuned to sound frequency with a well-defined characteristic frequency (CF). However, their labeled termination spans in the organ of Corti ranged from narrow to broad, innervating between 14 and 69 outer hair cells per axon in a “patchy” pattern. For units responsive to ipsilateral sound, the midpoint of innervation was mapped according to CF in a relationship generally similar to, but with more variability than, that of auditory-nerve fibers. Thus, based on CF mappings, most of the MOC terminations miss outer hair cells involved in the cochlear amplifier for their CF, which are located more basally. Compared with ipsilaterally responsive neurons, contralaterally responsive neurons had an apical offset in termination and a larger span of innervation (an average of 10.41% cochlear distance), suggesting that when contralateral sound activates the MOC reflex, the actions are different than those for ipsilateral sound.

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Sensory Transduction and Adaptation in Inner and Outer Hair Cells of the Mouse Auditory System

Journal of Neurophysiology ◽

10.1152/jn.00914.2007 ◽

2007 ◽

Vol 98 (6) ◽

pp. 3360-3369 ◽

Cited By ~ 40

Author(s):

Eric A. Stauffer ◽

Jeffrey R. Holt

Keyword(s):

Hair Cells ◽

Slow Component ◽

Outer Hair Cells ◽

Fast Component ◽

Sensory Transduction ◽

Hair Bundle ◽

Sensory Cells ◽

Auditory Hair Cells ◽

Inner Hair Cells ◽

The Difference

Auditory function in the mammalian inner ear is optimized by collaboration of two classes of sensory cells known as inner and outer hair cells. Outer hair cells amplify and tune sound stimuli that are transduced and transmitted by inner hair cells. Although they subserve distinct functions, they share a number of common properties. Here we compare the properties of mechanotransduction and adaptation recorded from inner and outer hair cells of the postnatal mouse cochlea. Rapid outer hair bundle deflections of about 0.5 micron evoked average maximal transduction currents of about 325 pA, whereas inner hair bundle deflections of about 0.9 micron were required to evoke average maximal currents of about 310 pA. The similar amplitude was surprising given the difference in the number of stereocilia, 81 for outer hair cells and 48 for inner hair cells, but may be reconciled by the difference in single-channel conductance. Step deflections of inner and outer hair bundles evoked adaptation that had two components: a fast component that consisted of about 60% of the response occurred over the first few milliseconds and a slow component that consisted of about 40% of the response followed over the subsequent 20–50 ms. The rate of the slow component in both inner and outer hair cells was similar to the rate of slow adaptation in vestibular hair cells. The rate of the fast component was similar to that of auditory hair cells in other organisms and several properties were consistent with a model that proposes calcium-dependent release of tension allows transduction channel closure.

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