Projections of the Inferior Colliculus in Insectivores and Primates; pp. 301–313

Abstract

The inferior colliculus is a nucleus of obligatory synapse in the ascending brain stem auditory pathways. Projections from the inferior colliculus in primates and insectivores are similar to that demonstrated in other mammals, with short direct projections to the external nucleus of the colliculus and adjacent regions of the central gray and superior colliculus ipsilaterally, and fibers crossing in the commissure of the colliculus to innervate the contralateral central and external nuclei. Long projection fibers enter the brachium of the inferior colliculus bilaterally, but the projection is markedly heavier on the side ipsilateral to the lesion. Brachial fibers supply terminals to two regions of the rostral midbrain tegmentum, the parabrachial area and the interstitial nucleus of the inferior brachium. On reaching the thalamus, brachial fibers bypass the caudal tip of the medial geniculate, but arborize to form a dense plexus of axons and terminals throughout the remainder of the principal nucleus of the geniculate and the entire internal division on the ipsilateral side. A lighter and more restricted projection to the opposite medial geniculate is seen in all primates, and particularly in the hedgehog. The small size of the contralateral tectothalamic pathway in the hedgehog is the only significant species difference noted in this study.