Topographical organization of the inferior collicular projection and other connections of the ventral nucleus of the lateral lemniscus in the cat.

Abstract

The topographic distribution of projections from the ventral nucleus of the lateral lemniscus (VNLL) in the cat was investigated with the autoradiographic tracing method. The origin of minor projections was verified by retrograde tracing methods. Small injections of tritiated leucine were placed in restricted zones of VNLL. A major afferent fiber system to the inferior colliculus was labeled in all cases. From the injection site labeled fibers coursed through and around the nuclei of the lateral lemniscus to enter the ipsilateral inferior colliculus. Regardless of the position or small size of the injection, labeled fibers distributed widely in the inferior colliculus. Fibers ended in the central nucleus and deeper layers of the dorsal cortex in most cases. There was also labeling in the ventrolateral nucleus, but very few fibers ended as lateral as the lateral nucleus. A small number of labeled fibers passed from the inferior colliculus into the nucleus of the brachium of the inferior colliculus and adjacent tegmental areas. Some labeled fibers entered the commissure of the inferior colliculus where they were traced into the dorsal cortex and rostral pole of the inferior colliculus on the side contralateral to the injection site. Though the projections labeled in individual cases were similar in their divergent pattern within the central nucleus of the inferior colliculus, specific variations in the pattern were found. The dorsal zone of VNLL projected more heavily to the deeper layers of the dorsal cortex and an adjacent field in the central nucleus than the other zones. Dorsal injections in the middle zone of VNLL, on the other hand, labeled the medial part of the central nucleus more heavily, whereas ventral injections in the middle zone resulted in heavier lateral labeling. The ventral zone of VNLL projected heavily to a central field in the central nucleus. In addition to this major afferent system of VNLL to the inferior colliculus, a smaller descending projection was found. The descending projection ended mainly in the dorsomedial periolivary region and ventral nucleus of the trapezoid body. However, in some cases a few fibers were traced to the cochlear nuclei. Finally, we observed projections to the medial geniculate body from the dorsal and ventral zones of VNLL that ended diffusely in the medial division of the medial geniculate body. Possibly some fibers from the dorsal zone contribute to a broader projection of the lateral tegmentum to the dorsal division of the medial geniculate body.