MICROSTIMULATION OF PRIMATE MOTOR THALAMUS: SOMATOTOPIC ORGANIZATION AND DIFFERENTIAL DISTRIBUTION OF EVOKED MOTOR RESPONSES AMONG SUBNUCLEI. VITEK, JERROLD L., JAMES ASHE, MAHLON R. DELONG, AND YOSHIKI KANEOKE. Department of Neurology, Emory University School of Medicine, Atlanta, Georgia 30322, Department of Neurology, University of Minnesota and Brain Sciences Center, VAMC, Minneapolis, Minnesota 55417.
APStracts 2:0372N, 1995.
SUMMARY AND CONCLUSIONS
1. The functional organization of motor responses to microstimulation throughout the primate "motor" thalamus including nucleus ventralis lateralis, pars oralis (VLo), nucleus ventralis posterior lateralis, pars oralis (VPLo), ventralis lateralis, pars caudalis (VLc), and portions of ventralis anterior (VA) and Area X, was systematically studied in awake monkeys. A total of 2021 sites were examined for their response to microstimulation. Of these, 1123 were histologically verified as to their location within the motor thalamus. At or near each site, isolated neurons were examined for their responses to somatosensory examination and active movement (n = 1272). This study was carried out as part of a larger study examining the responses of neurons in the motor thalamus to somatosensory examination, torque-induced limb perturbations, and active movement in a visuomotor step-tracking task. 2. Microstimulation at 40 [mu]A or less evoked movements in the contralateral limbs, trunk, or face. Evoked movements of the limb were generally maximal about a single joint. 3. There was a differential response to microstimulation between subnuclei of the motor thalamus. In order of decreasing frequency, the percentages of sites within each subnucleus from which movements were evoked were as follows: VPLo; 93% (449/483), VLo; 21% (57/272), VLc; 11% (15/140), VA; 1% (1/85), Rt 0% (0/65). In VPLc, 44 % (34/78) of sites examined were microexcitable. However, these were almost all within 500 [mu]m of the border of VPLo, suggesting they may have occurred as a result of current spread to adjacent VPLo. Although area X was not sampled in its entirety, it did not appear to be microexcitable. 4. Microexcitable responses had a somatotopic organization, similar to that reported for neuronal responses to sensorimotor examination (Vitek et al.1994), with leg responses found most lateral and arm and face found progressively more medially. 5. Zones in VPLo generally ranging from 500 - 1500 [mu]m were found in which microstimulation resulted in the same motor response. These microexcitable zones resemble those described for the striatum and were termed thalamic microexcitable zones (TMZ). TMZs also resemble cortical efferent zones in that both are somatotopically organized, may affect a single muscle or group of muscles, have low thresholds for microstimulation with sharp boundaries that lie adjacent to other microexcitable zones with the opposite effects, and are of approximately the same dimension. 6. This study suggests that a fundamental unit of motor organization, i.e., single muscle or joint, is preserved at the thalamic level in the form of TMZ's, and that these fundamental units of organization may contribute to the modular organization of the cortex. 

Received 21 December 1995; accepted in final form 11 December 1995.
APS Manuscript Number J797-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95