Auditory System

The auditory system can be divided into two large subsystems, peripheral and central. The peripheral auditory system converts the condensations and rarefactions that produce sound into neural codes that are interpreted by the central auditory system as specific sound tokens that may affect behavior. The peripheral auditory system is subdivided into the external ear, the middle ear, and the inner ear. Sound information, encoded as discharges in an array of thousands of auditory nerve fibers, is processed in nuclei that make up the central auditory system. Ambient sounds are collected by the pinna, visible portion of the external ear, and guided to the middle ear by the external auditory meatus, or ear canal. The auditory system includes both passive and active properties. The outer hair cells receive efferent output from the brain and actively modify the characteristics of the auditory system. The auditory nerve of the human contains about 30,000 fibers consisting of myelinated proximal processes of spiral ganglion cells.