The distribution and origin of the ipsilateral descending limb of the brachium conjunctivum. An autoradiographic and horseradish peroxidase study in the rat.

The distribution, organization and origin of the ipsilateral descending limb of the Brachium Conjunctivum (B.C.), have been studied in the rat by using anterograde and retrograde tracing techniques. After injections of tritiated leucine/proline into the lateral cerebellar nucleus, covering both its medial part, corresponding to the dorsolateral hump (DLH) of Goodman et al. (1963) and its lateral part, (designated here as the lateral dentate, LD), and the neighboring interposed nucleus (NI), emerging fibres are numerous and leave laterally from the B.C. On the contrary, injections restricted to LD reveal very few such fibers. Within the lateral parvocellular reticular formation (LPRF) terminal labelling is heavy, and moderate to sparse within the adjacent trigeminal complex. Rostro-caudally, silver grain accumulation within the LPRF extends from the level of the motor trigeminal nucleus (VM) to the pyramidal decussation, exhibiting a cephalocaudal decrease of grain density. Within the trigeminal complex, labelling occurs in the caudal VM, the dorsal portion of the principal sensory nucleus, and within and around the trigeminal spinalis oralis. In addition, the area surrounding the VM (in part corresponding to the supratrigeminal region of Lorente de Nó 1922, 1933) is moderately labelled. After injections of HRP into various levels of the ipsilateral descending B.C.'s projection field, retrogradely labelled cells are numerous within the DLH. A slightly lesser amount of labelled cells are found in the lateral half of the NI, primarily concerning the nucleus interpositus posterior. Within the LD, only a few labelled cells are observed: these are mainly restricted to the dorsal portion at rostral levels of the nucleus. The results obtained by both the anterograde and retrograde studies suggest an absence of a topographic organization within this descending B.C. component. The possible functional meaning of these results is discussed.