The location of the cochlear amplifier: spatial representation of a single tone on the guinea pig basilar membrane

Abstract

Acoustic stimulation vibrates the cochlear basilar membrane, initiating a wave of displacement that travels toward the apex and reaches a peak over a restricted region according to the stimulus frequency. In this characteristic frequency region, a tone at the characteristic frequency maximally excites the sensory hair cells of the organ of Corti, which transduce it into electrical signals to produce maximum activity in the auditory nerve. Saturating, nonlinear, feedback from the motile outer hair cells is thought to provide electromechanical amplification of the travelling wave. However, neither the location nor the extent of the source of amplification, in relation to the characteristic frequency, are known. We have used a laser-diode interferometer to measure in vivo the distribution along the basilar membrane of nonlinear, saturating vibrations to 15 kHz tones. We estimate that the site of amplification for the 15 kHz region is restricted to a 1.25 mm length of basilar membrane centered on the 15 kHz place.

Figure 1

(A) BM displacement as a function of intensity at the 13.25 kHz CF position at 3 frequencies around the CF; broken line, slope of 1. (B) Isoresponse frequency tuning curve at the 13.25 kHz position showing the intensity required to cause a 0.5 nm BM displacement [CF/bandwidth 10 dB from tip (Q_10dB) = 7.2]. (C) Locations of CF as a function of distance from the apex. ★, BM measurements; ⋄, IHC receptor potentials (15). Line represents the cochlear frequency-position function (14) (F = 0.35(10^0.1135d − 0.85). (D) BM displacement response to a 15 kHz tone measured at 15 positions along the BM from 12.5–27 kHz, over a range of intensities from 15–60 dB SPL in 5 dB SPL steps and from 60–100 dB SPL in 10 dB SPL steps. Functions at 5 dB SPL intervals have solid symbols. Broken line indicates the 15 kHz location. Frequency positions to the right of the dotted line (12.5–17 kHz) are from one preparation and those on the left (17–27 kHz) are each from separate preparations.

Figure 2

(A) BM displacement response as a function of intensity to a 15 kHz tone measured over a range of frequency positions about the 15 kHz point; broken line, slope of 1. (Lower) Slope of the initial region of the level function as a function of frequency position. (B) Phase as a function of position along the BM measured at several intensities (20–60 dB SPL). (C) Length along BM of the 15 kHz displacement envelope above the noise floor (0.5 nm) as a function of level. ⋄, total peak width; ○, length from the center of the peak to the apical end; •, length from the center to the basal end.

Figure 3

Each cylindrical OHC is supported at its base by a cup-like depression in a Deiters cell (D). Each Deiters cell extends a phalangeal process (PH), which extends at an angle and inserts into the reticular lamina one or two OHCs away toward the apex of the cochlea (indicated by arrow). (Scale bar = 10 μm.) (Scanning electron micrograph of the gerbil cochlea provided by A. Forge, Institute of Laryngology and Otology, University College of London)

Figure 4

BM displacement as a function of frequency recorded at four different levels from the 13.25 kHz point. The vertical dotted line indicates the CF (13.25 kHz).