Pallidal inputs to thalamocortical neurons projecting to the supplementary motor area: an anterograde and retrograde double labeling study in the macaque monkey.

The relationship between thalamocortical neurons projecting to the supplementary motor area (SMA) and pallidothalamic projection fibers was examined with an anterograde and retrograde double labeling technique in macaque monkeys (Macaca fuscata). In each monkey, Fast Blue (FB) was injected into the hand-arm area of the SMA after mapping the somatotopy using intracortical microstimulation, and horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) was injected into the ipsilateral internal segment of the globus pallidus (GPi). As a result, numerous projection neurons labeled with FB were distributed in pallidal terminal areas labeled with WGA-HRP in the ventral nuclear group of the thalamus. The present findings indicate that the SMA receives strong indirect projections from the GPi via the thalamus.