Anatomy of the auditory pathway

The human auditory system has inputs from both ears, which are connected by cross-over fibers from both ascending pathways. Both pathways from each ear have parallel organized processing but still with connection to the other site. Within both pathways distinct groups of neurons - nuclei - are embedded. These nuclei also have neuronal interconnections, allowing feed-back loops in the ascending as well as in the descending pathway.The uppermost fundamental property of the auditory nervous system is its tonotopic organization from the cochlea to the auditory cortex, which can continuously be recognized by anatomical findings and by physiological experiments.The auditory pathway includes 4 neurons from the hair cell to the auditory cortex. The spiral ganglion cell axons from the cochlea terminate in the cochlear nuclei, from which they project in one major pathway to the superior olive (SO) and in another pathway from the ventral cochlear nucleus (VCN) to the medial nucleus of the trapezoid body (TB) and the lateral superior olive. The trapezoid body forms the most important transverse auditory tract.From the superior olive fibers project via the lateral lemniscus to the nucleus of the lateral lemniscus and the inferior colliculus, where all of the brainstem auditory pathways converge. From here projections can be found to the thalamus (particularly the medial geniculate nucleus), from where they are relayed to the auditory cortex.Up to the auditory cortex the tonotopic (or cochleotopic) pattern can be followed in the cytoarchitecture. Nevertheless, the input from two ears and the cross-linked pathways allow the detection of differences in sound intensity and thus the localization of sound sources.