Nucleus sagulum: projections of a lateral tegmental area to the inferior colliculus in the cat

The nucleus sagulum, an area of the midbrain tegmentum, has been considered a component of a lateral tegmental system within the ascending auditory pathway to the thalamus. In this study, connections of the nucleus sagulum within the midbrain were investigated in adult cats. Tracing methods using anterograde and retrograde axonal transport of markers were employed. The nucleus sagulum was identified as a region of principally small neurons (261 +/- 79 micron2) at the margin of the midbrain and neighboring the nuclei of the lateral lemniscus. Injections of tritiated leucine in the nucleus sagulum labeled axons that ended in dense patches within the superficial layers of the caudal portion of the dorsal cortex of the inferior colliculus on the ipsilateral side. Retrograde experiments confirmed this connection. Other axonal projections labeled in the anterograde studies included fibers ending in the dorsomedial nucleus, the superficial layers of the dorsal cortex, and the rostral nucleus of the inferior colliculus with some bilateral distribution. Outside of the inferior colliculus, sagulum injections labeled other axons ending in the ventral intercollicular tegmentum on both sides and in a dorsal and rostral region of the contralateral nucleus sagulum that appeared contiguous with the dorsal nucleus of the lateral lemniscus. The latter region included a population of larger neurons (340-540 micron2) and had different connections with the inferior colliculus. The distribution of axonal labeling after injections in the nucleus sagulum was contrasted with the distribution of projections from several neighboring areas of the lateral tegmentum, including the dorsal nucleus of the lateral lemniscus. None of these areas exhibited connections with the superficial layers of the caudal cortex of the inferior colliculus, which was the major target in the inferior colliculus of the nucleus sagulum. Thus, the results indicated that the nucleus sagulum is distinguished from adjacent regions of the lateral tegmentum by its connectivity. Its association with midbrain auditory pathways is supported by these connections as well as ascending ones to the auditory thalamus.