Pyramidal and non-pyramidal motor cortical effects on distal forelimb muscles of monkeys

Summary Monkeys with chronic unilateral lesions of the pyramidal tract were investigated with respect to motor cortical effects on distal forelimb muscles. The forelimb area of the pre-central gyrus was stimulated with long single pulses or with repetitive pulses at 50/sec on both sides, one cortex having an intact pyramidal projection (control) and the other cortex having a lesion in its pyramidal projection varying from 30–100%. Repetitive stimulation at 50/sec of the intact cortex elicited, after a summation period of variable length, synchronized discharges following each stimulus pulse. Long single pulses (3 msec, 1/sec) were likewise effective to activate the distal forelimb muscles. Stimulation of the motor cortex with a lesion in its pyramidal projection was no longer effective to elicit discharges in the distal muscles when single pulses of up to 6 mA were used, even in the animal with a 30% lesion. With repetitive stimuli (1 msec, 50/sec) some activity was evoked in all animals including the monkey with a 100% lesion. The pattern of activation tended to be diffuse, i.e. not time-locked to the individual stimulus pulses. In the animals with smaller lesions the mean latency of discharges following the repetitive stimuli was increased as compared with the control side. The motor cortical effects in a 10 month old monkey were the same as in the adult monkey with intact pyramidal projection. The results of motor cortex stimulation in a 7 week old monkey were, however, similar to those obtained in adult monkeys with pyramidal lesions.