An autoradiographic study of cortical projections from motor thalamic nuclei in the macaque monkey.

The special areal and laminar distributions of cortical afferent connections from various thalamic nuclei in the monkey (Macaca fuscata) were studied by using the anterograde axonal transport technique of autoradiography. The following findings were obtained. The superficial thalamocortical (T-C) projections, terminating in the (superficial half of) cortical layer I, arise mainly from the nucleus ventralis anterior, pars principalis (VApc) and nucleus ventralis lateralis, pars oralis (VLo), and possibly from the nucleus ventralis lateralis, pars medialis (VLm) and nucleus ventralis anterior, pars magnocellularis (VAmc). The VApc gives rise to the superficial T-C and deep T-C projections onto the postarcuate premotor area around the arcuate genu and spur, and onto the dorsomedial part of the caudal premotor area as well as the supplementary motor area (SMA). The VApc also gives rise to only deep T-C projections onto the remaining premotor area and onto the rostral bank of the arcuate sulcus as well as the ventral bank of the cingulate sulcus at the level of the premotor area. The VLo gives rise to the superficial T-C projections onto the ventrolateral part of the motor area (mainly to the forelimb motor area) and onto the dorsomedial part to the mesial cortex at the rostral level of the motor area. The VAmc gives rise to the superficial T-C projections onto the banks of the arcuate genu and adjacent region of area 8. Area X, the nucleus ventralis posterolateralis, pars oralis (VPLo), nucleus ventralis posterolateralis, pars caudalis (VPLc), nucleus ventralis posteromedialis (VPM) and possibly the nucleus ventralis lateralis, pars caudalis (VLc) send only deep T-C projections. The dorsal and medial parts of the VLc project onto the premotor area, the rostral part of the motor area and the SMA, and also the ventral bank of the cingulate sulcus. Area X projects onto the premotor area, the SMA, and the caudal part of area 8. The thalamic relay nuclei projecting onto the frontal association cortex were found to be the VAmc, medial VLc and area X.