The cochlea

Ears have evolved in parallel in vertebrates since a common divergence 300 million years ago. In birds, reptiles and amphibia, the hearing organ is strictly referred to as a papilla. The cochlea is the hearing organ of mammals and consists of a coiled cavity within the temporal bone on either side of the head. It contains the structures that separate the components of a complex sound by frequency and intensity. The cochlea acts as the sensory organ for the auditory system, signalling the information along the auditory nerve to the central nervous system. Hearing organs in different species differ in the range of sound frequencies to which they can respond. The sensory cell on which all designs rely is the hair cell: a polarized neuroepithelial cell with sensory transduction channels at the apical end and the primary sensory synapse at the basal end (Figure 1). The hair cell gets its name from the 100 or so actin-containing giant microvilli or ‘stereocilia’ that project from the apical membrane surface. Sounds entering the fluidfilled cochlea cause deflection of the hair cell stereocilla that lead to signals being sent to the central nervous system.