The central fibers from the cells of the spiral and vestibular ganglia travel in a common trunk, the eighth cranial nerve, which enters the cranial cavity through the internal auditory meatus (accompanied by the facial and intermediate nerves). They traverse the cerebellopontine angle and enter the lateral brainstem at the junction of the pons and medulla. Here the cochlear and vestibular fibers become separated. The cochlear fibers bifurcate and terminate almost at once in the dorsal and ventral cochlear nuclei. The fibers from each cochlear nucleus pursue separate crossing and ascending pathways; they pass to both inferior colliculi (mainly to the opposite side) via the lateral lemnisci. Secondary acoustic fibers project via the trapezoid body and lateral lemniscus to the medial geniculate bodies, a special component of the thalamic sensory system (Fig. 14-2). Some fibers terminate in the trapezoid body and superior olivary complex and sub-serve such reflex functions as auditory attention, sound localization, auditory startle, and oculopostural orientation to sound.
Both excitatory and inhibitory neurons are located at every level of these pathways. At all levels there are strong commissural connections through which auditory signals come to be represented bilaterally in the cerebrum. From the medial geniculate bodies, fibers project to the cortex via the auditory radiations—relatively compact bundles that course ventrolaterally through the posterior parts of the putamen before dispersing and ending in the transverse gyri of Heschl and other auditory cortical areas (Tanaka et al).
The auditory cortical field comprises the superior temporal gyrus and the upper bank of the sylvian fissure (Brodmann area 41; see Fig. 21-1), or primary auditory cortex, and the surrounding secondary and tertiary cortices in the adjacent temporal lobe. The latter are of particular importance in the interpretation of sound (Celesia) including spoken language. Bilateral temporal lobe lesions involving the geniculocortical fasciculi result in cortical deafness, although such lesions are rare. Unilateral cortical lesions do not affect hearing, but defects in function such as dichotic listening can be detected by specialized tests. At several levels of these ascending fiber systems, there is feedback to lower structures.
The vestibular fibers of the eighth nerve terminate in the four vestibular nuclei: superior (Bechterew), lateral (Deiters), medial (triangular, or Schwalbe), and inferior (spinal, or descending). In addition, some of the fibers from the semicircular ducts project directly to the cerebellum via the juxtarestiform body and terminate in the flocculonodular lobe and adjacent vermian cortex (consequently, these structures are called the “vestibulocerebellum,” as noted in Chap. 5). Efferent fibers from this portion of the cerebellar cortex, in turn, project ipsilaterally to the vestibular nuclei and to the fastigial nucleus; fibers from the fastigial nucleus project back to the contralateral vestibular nuclei, again via the juxtarestiform body. Thus each side of the cerebellum exerts an influence on the vestibular nuclei of both sides (Fig. 14-3; see also Chap. 4).