Outer hair cells actively amplify and sharpen sound before inner hair cells fire

Evidence

The hair cells are of two types, inner and outer. The outer hair cells are active—that is, they can both respond to the shearing of the two membranes that they link and in turn affect the shearing by contracting, by a feedback mechanism. It is thought that this feedback plays two roles: (1) a kind of preamplifier for soft sounds, amplifying them for the inner hair cells, which create the signals sent to the brain, and (2) to sharpen frequency response, even when the sound is quite loud and needs no preamplification. Outer hair cells do not send neural signals to the brain. — Eric J. Heller, Why You Hear What You Hear- An…, 1960

At a given location, the available stimulus affects both the inner haircells and alsothrough the summation point and a phase shifter - the sensitive outer haircells. The latter operate as amplifiers with a saturating characteristic, approximated by the transfer function with idealized nearly symmetrical breakpoints, corresponding to a sound pressure level of about 30 dB. Although the process is modelled in electrical terms, the influence of the outer haircells on the inner haircells could be mechanical, electromechanical, electrical, biochemical, or all or any of the above in nature. A single inner haircell may be influenced by many outer haircells in its surroundings (an effect indicated by the resistors pointing in and out laterally). However, at high levels, the amplifiers (outer haircells) are saturated and the adequate stimulus operates almost exclusively on the inner haircells. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 241

Categorization

• DEFINE: Distinguishes the two types of hair cells and their division of labor — inner cells are the signal-senders, outer cells are the active mechanical processors.
• XREF: The cochlear amplifier concept connects to work on otoacoustic emissions and recent Nobel Prize in Physiology or Medicine 2024 research on mechanotransduction in the inner ear.
• corroborated: 4 independent passages support this claim — Eric J. Heller, Why You Hear What You Hear- An…, 1960; Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 241