Hearing, II: The retrocochlear auditory pathway

In contradistinction to the vestibular (Scarpa's) ganglion, which is located at the level of the internal auditory meatus, the cochlear (spiral) ganglion, the primary sensory ganglion for hearing, is located within the bony confines of the cochlea. Multiple cochlear fibers converge to form the cochlear nerve, which courses within the anterior-inferior quadrant of the internal auditory canal inferior to the facial nerve and immediately anterior to the inferior vestibular nerve. The cochlear nerve then traverses the cerebellopontine angle (usually descending slightly) to synapse in the dorsal (higher frequencies) and ventral (lower frequencies) nuclei in the upper medulla immediately superficial to the inferior cerebellar peduncle (restiform body). For imaging purposes, these nuclei have been referred to by Gebarski et al as the cochlear nuclear complex (CNC), which is tubular, 8 mm in length and 3 mm thick. These authors have exquisitely defined landmarks for identification of the CNC, which appears as a modest convexity along the posterolateral surface of the upper medulla bordered by the foramen of Luschka and its accompanying choroid plexus. The choroid plexus extends from the roof of the fourth ventricle to the cerebellopontine angle via the lateral recess. The root entry zone and cerebellar flocculus are also consistent landmarks at this level.