Development of tonotopy in the auditory periphery

Acoustic frequency analysis plays an essential role in sound perception, communication and behavior. The auditory systems of most vertebrates that perceive sounds in air are organized based on the separation of complex sounds into component frequencies. This process begins at the level of the auditory sensory epithelium where specific frequencies are distributed along the tonotopic axis of the mammalian cochlea or the avian/reptilian basilar papilla (BP). Mechanical and electrical mechanisms mediate this process, but the relative contribution of each mechanism differs between species. Developmentally, structural and physiological specializations related to the formation of a tonotopic axis form gradually over an extended period of time. While some aspects of tonotopy are evident at early stages of auditory development, mature frequency discrimination is typically not achieved until after the onset of hearing. Despite the importance of tonotopic organization, the factors that specify unique positional identities along the cochlea or basilar papilla are unknown. However, recent studies of developing systems, including the inner ear provide some clues regarding the signalling pathways that may be instructive for the formation of a tonotopic axis.