CONTRASTING LOCATIONS OF PALLIDAL-RECEIVING NEURONS AND MICROEXCITABLE ZONES IN PRIMATE THALAMUS

1. In two awake, juvenile male Macaca fascicularis monkeys, microstimu lation was applied in ventralis anterior (VA), ventralis lateralis (VL ), or ventralis posterior lateralis (VPL) of the thalamus. Thalamic re cording was used to identify the region that contained pallidal-receiv ing (PR) thalamic neurons, cells that responded orthodromically to sti mulation in the internal pallidal segment (GPi). Thalamic stimulation was used to elicit motor responses. Penetrations in the thalamus and t he pallidum focused on areas with activity related to contralateral ar m movement. Fifty one PR cells were identified electrophysiologically in VL oralis (VLo), VL caudalis (VLc) and VA pars parvocellularis (VAp c). 2. With a subject at rest, trains of stimuli were applied through the thalamic microelectrode. Palpable or visible muscle twitches or jo int movements were evoked by short trains (12 pulses) of stimuli appli ed in VLc and VPL oralis (VPLo); thresholds there ranged from 5 to 75 mu A. In VA and VLo, areas where PR neurons were located, even longer trains (24 pulses) of stimuli with currents up to 200 mu A usually fai led to evoke movement. In the caudal portions of VLo, near VPLo, there were some microexcitable sites found near PR cells where stimuli at s imilar to 50 mu A elicited movement. 3. From microstimulation studies combined with histological reconstruction, Ashe and co-workers hypothe sized that microexcitable zones were cerebellar receiving areas (CR) a nd nonexcitable zones were PR areas. Our data support theirs and add e lectrophysiological identification of PR areas. Further, we injected w heat germ agglutinin-horseradish peroxidase (WGA-HRP) into the thalamu s at one of the more rostral microexcitable sites, just caudal and lat eral to identified PR cells. The tetra-methyl benzidine-reacted HRP la bel was found in the contralateral deep cerebellar nuclei (DCN) but no t in ipsilateral GPi, showing that even this rostral microexcitable zo ne was a CR area. 4. Together with evidence from the literature, the d ata are consistent with the hypothesis that PR cells have relatively w eak access to spinal-destined motor outputs, whereas thalamocortical n eurons from VPLo and VLc have more secure access. In addition to chara cteristics of cell discharge and responses to somatosensory stimulatio n, microexcitability may be a further aid in tentative electrophysiolo gical identification of PR versus CR areas of the motor thalamus witho ut necessarily recording thalamic neuronal responses to stimulation in GPi or the DCN.