Ascending efferent projections of the superior olivary complex

Abstract

The superior olivary complex conveys information about binaural time and intensity 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 afferents to the central nucleus of the inferior colliculus. Electron microscopic observations of axonal endings in the central nucleus suggest that the ipsilateral medial superior olive and contralateral lateral superior olive make excitatory synapses. In contrast, the axons from the ipsilateral lateral superior olive to the central nucleus contain glycine and have a morphology consistent with inhibitory synapses. Little is known about the transmitter types used by olivary projections to the nuclei of the lateral lemniscus, but they are presumed to be similar to the collicular projections. Olivary ascending efferents are tonotopically organized and terminate in laminae in the inferior colliculus. They combine with other laminar afferents and postsynaptic neurons to create fibro-dendritic laminae in the colliculus. The key to the functional organization of the olivary efferents is the possible segregation of excitatory olivary efferents from each other in "synaptic domains" located on the laminae. This segregation may be the major determinant of response properties in the colliculus. Olivary efferents may converge with other non-olivary afferents on the same postsynaptic neurons in the colliculus. Inhibitory efferents from the lateral superior olive are essential in shaping the response properties of neurons in the colliculus. Olivary efferents to the nuclei of the lateral lemniscus are also key components of ascending pathways that inhibit neurons in the midbrain.