Ascending efferent projections of the superior olivary complex

The superior olivary complex conveys information about binaural time and in tensity to higher centers in the auditory pathway. This information is sent primarily to the subdivisions of the inferior colliculus and to the nuclei of the lateral lemniscus. Olivary projections are the predominant afferent s to the central nucleus of the inferior colliculus. Electron microscopic o bservations of axonal endings in the central nucleus suggest that the ipsil ateral medial superior olive and contralateral lateral superior olive make excitatory synapses. In contrast, the axons from the ipsilateral lateral su perior olive to the central nucleus contain glycine and have a morphology c onsistent with inhibitory synapses. Little is known about the transmitter t ypes used by olivary projections to the nuclei of the lateral lemniscus, bu t they are presumed to be similar to the collicular projections. Olivary as cending efferents are tonotopically organized and terminate in laminae in t he inferior colliculus. They combine with other laminar afferents and posts ynaptic neurons to create fibro-dendritic laminae in the colliculus. The ke y to the functional organization of the olivary efferents is the possible s egregation of excitatory olivary efferents from each other in "synaptic dom ains" located on the laminae. This segregation may be the major determinant of response properties in the colliculus. Olivary efferents may converge w ith other non-olivary afferents on the same postsynaptic neurons in the col liculus. Inhibitory efferents from the lateral superior olive are essential in shaping the response properties of neurons in the colliculus. Olivary e fferents to the nuclei of the lateral lemniscus are also key components of ascending pathways that inhibit neurons in the midbrain. Microsc. Res. Tech . 51:355-363, 2000. (C) 2000 Wiley-Liss, Inc.