University of Wisconsin
The goal of the presentation is to provide an overview of the structure and function of the neuronal pathways in the mammalian brain stem. The auditory nerve conveys a tonotopic representation of sound to the brain. Information encoded in the timing of action potentials in a tonotopic array of auditory nerve fibers from each ear reaches the brain stem on the ipsilateral side in the cochlear nuclear complex. In innervating several distinct groups of neurons, each projecting to different nuclei (groups of cells), acoustic information is fed through at least six parallel, ascending pathways that converge upon the inferior colliculus. The integrative roles of these pathways are not completely understood. The evidence is compelling that the cross correlation of inputs from the two ears through spherical bushy cells and through the medial superior olivary nucleus (MSO) serve to measure interaural time and phase differences that are used to localize sound in the azimuth. It has been suggested that a pathway through the lateral superior olivary nucleus (LSO) detects interaural level differences through which animals can use head-shadowing to localize high frequencies in the azimuth. It is noteworthy, however, that hoofed mammals have large LSOs but are unable to localize high frequency sounds in the azimuth. The LSO detects decorrelation in sounds of the two ears. Localization in elevation is less well understood. In birds, a pathway through stellate cells has been implicated as carrying spectral information that allows the brain to make use of spectral distortions by the head and ears to localize sounds in elevation. In mammals, however, a pathway through fusiform and giant cells of the DCN has been shown to carry information that allows animals to orient to sounds in elevation and the role of stellate cells has been neither implicated or excluded from a role of sound localization in elevation. Little is known about what pathways extract meaning from patterns of sounds, including speech. Such function(s) are presumably largely monaural. It is tempting to speculate that this function is carried out at least in part in the monaural nuclei of the brainstem in which timing information converges. Octopus cells of the ventral cochlear nucleus detect coincidence in the firing of auditory nerve fibers and convey that coincidence with extraordinary precision to the superior paraolivary nucleus (SPN) and to the ventral nucleus of the lateral lemniscus (VNLL). From the inferior colliculus, acoustic information is carried to the medial genicular body (MGB) of the thalamus and thence to the auditory cortex.
In addition to the major ascending pathways through the brain stem there are at many stages descending pathways that form feedback loops. The descending pathways will not be considered in detail.