Spatial distribution of thalamic projections to the supplementary motor area and the primary motor cortex: A retrograde multiple labeling study in the macaque monkey

The exact knowledge on spatial organization of information sources from the thalamus to the supplementary motor area (SMA) and to the primary motor cortex (MI) has not been established. We investigated the distribution of thalamocortical neurons projecting to forelimb representations of the SMA and the MI using a multiple retrograde labeling technique in the monkey. The forelimb area of the SMA, and the distal and proximal forelimb areas of the MI were identified by electrophysiological techniques of intracortical microstimulation and single neuron recording. Injections were made into these three representations with three different dyes in the same animal (horseradish peroxidase conjugated to wheat germ agglutinin, diamidino yellow, and fast blue), and the thalamic neurons were retrogradely labeled. Injections into the SMA densely labeled thalamic neurons in nuclei ventralis lateralis pars oralis (VLo), ventralis lateralis pars medialis (VLm) and ventralis lateralis pars caudalis (VLc), but not in nucleus ventralis posterior lateralis pars oralis (VPLo). Injections into the MI labeled thalamic neurons primarily in VLo, VLc, and VPLo. We found that the distribution of projection neurons to the three areas was largely separate in the thalamus. However, in the middle part of VLo, and in a limited portion of VLc, thalamic neurons projecting to the SMA partially overlapped with those to the distal forelimb area of the MI. They overlapped little with those to the proximal forelimb area of the MI. We noted no overlap between the distributions of thalamic projection neurons to the distal and proximal forelimb areas of the MI. These findings suggest that the SMA and MI receive separate information from the thalamus, while sharing minor sources of common inputs.