Supporting sensory transduction: cochlear fluid homeostasis and the endocochlear potential

Abstract

The exquisite sensitivity of the cochlea, which mediates the transduction of sound waves into nerve impulses, depends on the endocochlear potential and requires a highly specialized environment that enables and sustains sensory function. Disturbance of cochlear homeostasis is the cause of many forms of hearing loss including the most frequently occurring syndromic and non-syndromic forms of hereditary hearing loss, Pendred syndrome and Cx26-related deafness. The occurrence of these and other monogenetic disorders illustrates that cochlear fluid homeostasis and the generation of the endocochlear potential are poorly secured by functional redundancy. This review summarizes the most prominent aspects of cochlear fluid homeostasis. It covers cochlear fluid composition, the generation of the endocochlear potential, K(+) secretion and cycling and its regulation, the role of gap junctions, mechanisms of acid-base homeostasis, and Ca(2+) transport.

Figure 1. Schematic diagrams illustrating the anatomy of the cochlea (centre), organ of Corti (left) and stria vascularis (right)

Figure 2. Schematic diagrams illustrating K⁺ secretion and the generation of the endocochlear potential in the cochlea

Left, cochlea cross-section. Right, stria vascularis. Strial marginal cells and basal cells of stria vascularis are each connected among themselves by tight junctions to form two barriers. The barrier between endolymph and intrastrial fluid consists of strial marginal cells and the barrier between intrastrial fluid and the extracellular fluid spaces in spiral ligament consists of basal cells. The endocochlear potential is generated across the basal cell barrier. Intermediate cells are connected by gap junctions to the inner membrane of basal cells. The outer membrane of basal cells is connected via gap junctions to fibrocytes of spiral ligament.

Figure 3. Schematic diagram illustrating the approximate location of expression of genes known or suspected to be associated with deafness

Data were collated from knowledge obtained in mice and humans.