Slow Electrically and Chemically Evoked Volume Changes in Guinea Pig Outer Hair Cells

Abstract

Our understanding of how the mammalian inner ear converts the mechanical vibrations of sound into neural energy has undergone a fundamental change during the past decade. Nearly ten years of experimental confirmation has convinced the hearing science community that the organ of Corti can produce sound as well as receive it. The most conspicuous evidence for this ability is that sound from the cochlea, or otoacoustic emissions, can be measured in the ear canal. The ear is thought to generate acoustic energy in order to better perform its role of spectrally analyzing the sound it receives (Patuzzi & Robertson, 1988). The inner ear can be considered to be an array of mechanical filters. Each element of this array vibrates at a magnitude determined by the amount of energy the ear receives in the frequency band to which that element is sensitive. A mechanical energy source in each of the elements results in each becoming an active, as opposed to a passive, mechanical filter. An active filter is both more sensitive and faster than a passive filter with the same bandwidth. The processing of auditory information requires speed and mammals appear to have evolved a mechanism that results in narrowly tuned active mechanical filtering. The inherent nonlinearity and instability of this feedback mechanism is thought to be the origin of spontaneous and evoked otoacoustic emissions. The cells responsible for generating the acoustic energy are the outer hair cells. There is one row of around 3000 inner and three rows of around 12000 outer hair cells in organ of Corti. The cylindrical outer hair cells possess a collection of flattened membranous sacs immediately below the cytoplasmic membrane of their lateral wall. These comprise an organelle called the laminated cisternal system that appears unique among eukaryotic cells and may be related to the mechanism that drives the cell’s distinctive electromotile response. Stereocilia-mediated mechanoelectrical modulation of intracochlear currents is thought to elicit electrically evoked outer hair cell length changes at acoustic frequencies and thereby provide for the active filtering that characterizes cochlear transduction.