The red nucleus of the monkey. Topographic localization of somatosensory input and motor output.

The topographic organization of somatosensory input to the primate red nucleus was investigated by studying receptive fields of rubral neurons, and that of the motor output by delivering trains of microstimulating pulses to evoke movements. A receptive field was identified in 191 of 208 rubral neurons. Most neurons (172) responded to passive movement of one or two joints including digits but some (26) had a cutaneous input. Neurons in both the parvocellular (RNpc) and magnocellular (RNmc) divisions of the nucleus had receptive fields. Neurons which responded to stimulation of the forelimb were located in the dorsomedial part of the nucleus. Those responsive to stimulation of the hindlimb were in the ventrolateral part. Thin regions on the dorsal and ventrolateral borders of the nuclei, respectively, contained neurons responsive to face and tail stimulation. Within the regions representing each limb, neurons receiving an input from the extremity (hand or foot) formed a core surrounded by neurons with an input from more proximal segments. This core extended uninterrupted throughout the RNpc and RNmc. Movements of individual limb segments including digits were readily evoked by microstimulating in the RNmc with thresholds as low as 3 microA. In most cases, movements were evoked in the direction opposite to the passive movement which drove the neurons at the stimulating site, although fibers of passage limited the analysis of the sensory input-motor output organization with stimulation. We conclude that there is topographic localization of somatosensory input and motor output in the macaque red nucleus. Furthermore, the red nucleus of monkeys contributes to the control of independent movements of limb segments including digits, although the number of axons it sends to the spinal cord is less than 1% of the number of corticospinal axons.