Pallidal projections to the mesencephalic locomotor region (MLR) in the cat.

The entopeduncular nucleus (EN) in the cat, homologue of the primate internal globus pallidus and main output of the basal ganglia, is known to project to the mesencephalic tegmentum. We have been able to elicit antidromic responses in single EN neurons from a site in the posterior mesencephalon, then transect the brainstem (precollicular-postmamillary) and elicit locomotion and rhythmic movements of the limbs by stimulation of the same site in the same animal. These studies demonstrate the existence of a direct projection from the EN to the mesencephalic locomotor region (MLR). However, this is not a particularly large pathway since fewer than 5% of the EN cells appear to project to the MLR. In a parallel series of anatomical experiments, injections of fluorescent dyes into the area of the MLR induced retrograde labeling of cell bodies in the EN and motor cortex. Injections of tritiated amino acids into the motor cortex resulted in labeling in the area anterior to the MLR. We assume that these connections may be involved, in part, in the sequencing and ordering of series of voluntary movements in which locomotion is involved.