Central Auditory System, Anatomy

Abstract

Synonyms Acoustic nerve; Auditory brainstem response (ABR); Auditory nerve; Brainstem audio-evoked response; Cochlear nerve; Heschl's gyrus; Transverse temporal gyri Summary of the Central Auditory System The central auditory system begins as the cochlear nerve ascends from the cochlea to higher order auditory structures in the brainstem and functions to process information from the organ of Corti in the cochlea. Besides recognition of sounds, the central auditory system is responsible for localization of sound and discrimination of sound. The inner hair cells of the cochlea are innervated by the type I bipolar ganglion cells in the spiral ganglion, and the outer hair cells are innervated by the type II bipolar ganglion cells. The first point of information transfer in the central auditory system is to the cochlear nuclei. The cochlear nuclei are located at the pontomedullary junction, with the ventral cochlear nucleus extending laterally into the middle cerebellar peduncle and the dorsal cochlear nucleus lying lateral and dorsal to the inferior cerebellar peduncle. From the cochlear nuclei, auditory information is further processed into the superior olivary complex that sits in the trapezoid body where the ability to localize sounds based upon interaural differences is first delineated. Auditory fibers continue from the superior olivary complex and form a flattened fiber track ascending in the lateral lemniscus of the upper pons. Fibers from the lateral lemniscus converge on the mesencephalic inferior colliculus. The commissure of the inferior colliculus, located just dorsal to the aqueduct , crosses the midline and serves to carry information bidirectionally and contains ascending fibers of the crossed pathway from the colliculus. Fibers project from the inferior colliculus to the diencephalic medial geniculate body located in the dorsal thalamus. Finally, auditory nerve fibers converge on the auditory cortex located in the upper surface of the temporal lobe. Figure 1 illustrates the connections of the central auditory pathway between the auditory nerve and the geniculate body. Auditory Nerve The cochlear branch of the vestibulocochlear nerve leaves the cochleae and enters the brainstem at the pontomedullary junction where it terminates on the cochlear nuclei. More specifically, the auditory nerve enters the anteroventral cochlear nucleus on its ventro-medial surface. Axons from the basal end of the cochlea carry high-frequency sound information and project dorsally and medially, supplying the dorsal part of all cochlear nuclei divisions. Conversely, axons from the apical part of the cochlea carry low-frequency sound information and project ventrally ending in the venterolateral part of all cochlear nuclei divisions (Malmierca and Smith 2009). The auditory nerve fibers on entering the cochlear nucleus bifurcate into an ascending branch that supplies the anteroventral C 376 Cellular Immunity