The efferent projections of the central nucleus and the pericentral nucleus of the inferior colliculus in the cat.

The efferent projections of the central nucleus of the inferior colliculus (ICC) and the pericentral nucleus of the inferior colliculus (ICP) were examined by placing restricted injections of anterograde tracers at electrophysiologically defined loci in the inferior colliculus (IC) of the cat. It was found that single loci in the ICC projected bilaterally onto the ventral division of the medial geniculate body (MGB) in the form of caudorostrally oriented sheets of terminals. The ICC loci also projected bilaterally onto the MGB in the form of caudorostrally oriented columns of terminals; these columns had their caudal aspects located in the medial and their rostral aspects located in the deep dorsal nucleus of the dorsal division. The caudal aspect of the sheets of terminals in the ventral division was folded and passed through both pars lateralis and pars ovoidea of the ventral division. Every component of the ICC-to-MGB projection was cochleotopically ordered. Periodic discontinuities of two types were noted in the projections of ICC onto pars lateralis (VI) of the ventral division. The one type of periodic discontinuity sometimes approximated bands oriented caudorostrally in the caudal aspect of VI. The second type of discontinuity was of very thin parallel columns of more intense labeling oriented rough dorsoventrally and oblique or normal to the first type of discontinuity. Injections in the ICP produced autoradiographic labeling in the caudal dorsal nucleus (Dc) of the dorsal division of the MGB. Thus the ICC-to-MGB and ICP-to-MGB projections are segregated. The efferent connections of the IC with other brainstem auditory structures were noted.