Medial-olivocochlear-efferent inhibition of the first peak of auditory-nerve responses: Evidence for a new motion within the cochlea

Abstract

Despite the insights obtained from click responses, the effects of medial-olivocochlear (MOC) efferents on click responses from single-auditory-nerve (AN) fibers have not been reported. We recorded responses of cat single AN fibers to randomized click level series with and without electrical stimulation of MOC efferents. MOC stimulation inhibited (1) the whole response at low sound levels, (2) the decaying part of the response at all sound levels, and (3) the first peak of the response at moderate to high sound levels. The first two effects were expected from previous reports using tones and are consistent with a MOC-induced reduction of cochlear amplification. The inhibition of the AN first peak, which was strongest in the apex and middle of the cochlea, was unexpected because the first peak of the classic basilar-membrane (BM) traveling wave receives little or no amplification. In the cochlear base, the click data were ambiguous, but tone data showed particularly short group delays in the tail-frequency region that is strongly inhibited by MOC efferents. Overall, the data support the hypothesis that there is a motion that bends inner-hair-cell stereocilia and can be inhibited by MOC efferents, a motion that is present through most, or all, of the cochlea and for which there is no counterpart in the classic BM traveling wave.

Auditory-Nerve Responses to Clicks at Low Levels, and the Initial Peak at High Levels, are Suppressed at Opposite Bias-Tone Phases

“…Similar to the ANIPr response, the ANSP response is also MOC inhibited, but more weakly [4]. A hypothesis that fits these data is that high-level clicks evoke a short-latency, quickly-decaying, oscillatory mechanical response that produces the ANIPr response by itself, and that the ANSP response is produced by a combination of this early mechanical response and the earliest part of the traveling wave (which at this high level receives little cochlear amplification).…”

“…Insight into the complex mechanical drives to inner-hair-cell (IHC) stereocilia can be obtained from auditory-nerve (AN) responses. In response to moderate-to-high-level rarefaction clicks (~75-100 dB pSPL), the AN initial peak (ANIPr) response is strongly inhibited by medial olivocochlear (MOC) efferents while much later peaks are not [4]. This indicates that the ANIPr response is due to active processes in outer hair cells (OHCs).…”

“…1B) which is the first response to condensation clicks in the 70-100 dB pSPL range. The ANSP response was looked at because it shows weak MOC inhibition [4]. Since MOC inhibition is not seen for peaks following ANSP, it has been suggested that ANIPr and ANSP could be one complete cycle of a decaying oscillatory motion generated by an active mechanism which is distinct from the active mechanism involved in traditional cochlear amplification [4].…”

“…Here we focus on the BT phase that produces the largest suppression. Recordings were made from >50 AN fibers in 8 anesthetized cats using methods described previously [4,5,7] and approved by our institutional animal review committee. To minimize ringing, clicks were produced by a reverse-driven condenser earphone.…”

How can the auditory efferent system protect our ears from noise-induced hearing loss? Let us count the ways

“…The underlying mechanism is not understood, but may be influenced by OHC effects that are not solely based on the classical traveling wave [23,24]. A similar effect is seen in human OAEs [11].…”