3.20 - Cochlear Receptor Potentials

Abstract

Sensory processing in the cochlea is initiated when the hair cells respond to excitation by generating receptor potentials. Sensory transduction takes place in the organ of Corti, a strip of sensory epithelium consisting of a single row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs) sandwiched between two extracellular matrices, the basilar membrane (BM), and tectorial membrane (TM). Altogether, these structures comprise the cochlear partition. Hair cells are excited through sound-induced vibrations of the BM and the resultant reactions of their hair bundles with the overlying TM. IHCs share a division of labor with the OHCs; OHCs are sensory-effector cells that both detect and influence mechanical interactions between the TM and the reticular lamina (RL) of the organ of Corti. OHCs are most effective at exerting electromechanical feedback to the cochlear partition at frequencies close to the characteristic frequency of their location on the BM. Their role is to amplify responses to low-level sounds and compress responses to high-level sounds. They are therefore instrumental in determining the exquisite sensitivity and frequency tuning of the cochlea and define the enormous dynamic range of the auditory system. IHCs appear to be purely sensory, their hair bundles responding to the resultant interactions between the TM and RL, and their synapses transforming and transmitting the resulting receptor potentials to the afferent fibers, which conduct auditory responses in trains of nerve impulses to the hindbrain nuclei. OHCs also, presumably, transmit their responses to the type II afferent fibers, although what these responses look like and their purpose remain mysteries. Their roles in cochlear sensory processing being different from each other, IHCs and OHCs thus have different design briefs. IHCs are designed to detect and relay interaction between the major extracellular matrices of the cochlear partition. OHCs are designed to optimize this interaction and hence the sensitivity and frequency selectivity of the cochlea. This chapter is concerned with our understanding of the workings of the cochlea largely from the measurement of receptor potentials from individual hair cells in the cochlea.