The auditory system

Abstract

The ear or vestibulocochlear organ is composed of external, middle and inner parts. The external ear consists of the auricle and the external acoustic meatus with the outer layer of the tympanic membrane. The middle ear is formed by the tympanic cavity, the auditory ossicles and the inner layer of the tympanic membrane. The inner ear comprises the labyrinth, a series of fluid-filled spaces in the petrous part of the temporal bone. The auditory part of the inner ear consists of the cochlea with the organ of Corti, which contains hair cells as auditory receptors (Sect. 7.2). Receptors sensitive to high frequencies are located near the cochlear base and those sensitive to low frequencies near the apex of the cochlea. The hair cells are innervated by the peripheral processes of bipolar ganglion cells in the spiral ganglion. Their central processes form the cochlear division of the vestibulocochlear nerve and terminate in the cochlear nuclei (Sect. 7.3). The principal auditory pathway passes from the cochlea, via the cochlear nuclei, the inferior colliculus and the medial geniculate body (the MGB), to the contralateral auditory cortex on the dorsal surface of the superior temporal gyrus (Sect. 7.4). Each MGB is bilaterally innervated, so that each hemisphere receives cochlear input bilaterally. All of the components of the auditory pathway are tonotopically organized. Section 7.5 includes a brief discussion of the descending auditory system. The English terms of the Terminologia Neuroanatomica are used throughout. At birth, humans have about 20, 000 inner and outer hair cells in the organ of Corti which often do not last a lifetime as they do not regenerate when lost. By the age of 65-75 years, many individuals have a bilateral, high-frequency progressive hearing loss known as presbycusis associated with hair cell attrition. Hair cell loss is the most common cochlear defect causing hearing impairment in presbycusis and noise-induced hearing loss. Hearing disorders due to brain stem lesions are rare because of the bilateral projections of the central auditory pathways. Midline pontine tegmental lesions may result in impaired sound localization due to interruption of the input of the superior olivary complex. Disorders of auditory perception may follow strokes in the territory of the internal carotid arteries or of the vertebrobasilar system. The central disorders of auditory perception may result from lesions of either the right, the left or both cerebral hemispheres, usually involving parietotemporal cortical areas as illustrated in Clinical cases.