Anatomy of the auditory pathways, with emphasis on the brain stem.

The principal auditory leading to the cerebral cortex and therewith to conscious perception passes from the cochlea, via the cochlear nuclei, the inferior colliculus and the medial geniculate body to the contralateral auditory cortex in the temporal lobe. All components of this pathway are cochleotopically organized. The fibre stream leading from the cochlear nuclei to the contralateral inferior colliculus consists of both direct and multisynaptic components. The latter are interrupted one or more times in the superior olivary complex and/or the nuclei of the lateral lemniscus, cell assemblies that lie embedded into the main ascending fibre stream. The auditory cortex receives not only impulses from the contralateral cochlea, but also from the ipsilateral cochlea. The ipsilateral projection reaches the ipsilateral inferior colliculus after a synaptic interruption in the ipsilateral superior olivary complex. Fibres conveying impulses from the contralateral side back to the ipsilateral side are probably also involved. The auditory system of the brain stem is characterized by the presence of several well-developed commissures. Most of these contain, in addition to decussating components, true commissural fibres. The various auditory centres in the brain stem are not only way stations in the ascending auditory pathways, but also serve as relays in descending auditory projections. Most important among the latter is the cortico-cochlear projection which is synaptically interrupted in the inferior colliculus and the periolivary nuclei. The final link in this descending projection is formed by the bundle of Rasmussen, the fibres of which leave the central nervous system and innervate the inner and outer hair cells in the organ of Corti. The pathways for the auditory startle response, the auditory orientation reflex and the stapedius and tensor tympani reflexes are briefly described.