Auditory pathway

First-order neurons: bipolar cells of the spiral ganglion in the modiolus; their central processes form the cochlear nerve, which enters the brainstem at the cerebellopontine angle (pontomedullary junction) with the vestibular nerve.

Second-order: all fibres synapse ipsilaterally in the dorsal and ventral cochlear nuclei on the inferior cerebellar peduncle. Each nucleus preserves tonotopy. Because every pathway above this level is bilateral, a unilateral lesion central to the cochlear nuclei does not cause unilateral deafness — unilateral hearing loss localises to the ear, cochlear nerve or cochlear nuclei.

Superior olivary complex (pons): receives input from both ears via the trapezoid body (the first site of binaural convergence). The medial superior olive computes interaural time differences (low frequencies) and the lateral superior olive interaural level differences (high frequencies) — the brainstem basis of horizontal sound localisation. It also gives rise to the olivocochlear efferents and the acoustic reflex. Fibres then ascend in the lateral lemniscus (with its nuclei) to the inferior colliculus (midbrain; obligatory relay and integration), then via the brachium of the inferior colliculus to the medial geniculate nucleus of the thalamus, and through the auditory radiation (sublentiform part of the internal capsule) to the primary auditory cortex — transverse temporal gyri of Heschl (Brodmann areas 41, 42) on the superior temporal plane, which is tonotopically organised (low frequencies anterolateral, high posteromedial). Surrounding belt/parabelt areas, including Wernicke's area (posterior superior temporal gyrus, area 22) in the dominant hemisphere, interpret speech.