Late muscular responses to arm perturbations persist during supraspinal dysfunctions in monkeys

This study was conducted to determine which supraspinal structures, if any, are essential for generation of late (M2 and M3) EMG responses to perturbations. EMGs were recorded during active arm movements made by Cebus monkeys trained to resist perturbations applied to a handle and/or during passive movements imposed on sedated or anesthetized animals. EMGs were tested during active movements after ablation of the arm areas of the sensory and caudal motor cortices, during cooling part of the sensorimotor cortex along the central sulcus, and during cooling the arm area in the ventral lateral (VL) thalamus. EMG responses to passive movements were tested after decerebration and decerebellation, after lesions of the motor and/or sensory cortices, during surface cooling the same, and during cooling the VL arm area and the cerebellar dentate nucleus. M1, M2, and M3 were not abolished in any monkey. Decreases in the magnitudes of these responses occurred in some animals, but no reproducible changes in the sizes of M2 and M3 (in comparison to M1) were observed following any procedure. The effects of cortical lesions on M1, M2, and M3 responses to passive movements were found to depend on the level of arousal of the animal. These results demonstrate that M1, M2, and M3 can be produced in the primate spinal cord and brainstem and that the sensorimotor cortex can facilitate these responses.