Responses of monkey precentral neurones to passive movements and phasic muscle stretch: relevance to man.

Single cell recordings were made from movement-related neurones from the precentral cortex of two monkeys, trained to perform a simple lever-pulling task. They were also trained to remain relaxed while the arm was explored with passive movements at different joints, cutaneous stimuli and during the application of two types of phasic muscle stretch: percutaneous vibration and percussion of muscle tendons. Recordings were made of the responses of cortical neurones both to the 'natural' stimuli and to vibration of specific muscle tendons or percussion of the triceps tendon. Both tendon percussion and vibration excited neurones within area 4 with an average latency for tendon percussion of 21.0 msec. There was a high degree of consistency in the effects on single neurones of tendon percussion and vibration at the same site. Although long-term facilitation was not seen. vibration-induced discharge in the motor cortex should be considered as a potential mechanism of its effects in intact man. In contrast to the similarity of the effect of the two forms of phasic stretch, the relationship between a single neurone's response to either tendon percussion or vibration and to passive movement was complex. The dissociation seen between the effects of phasic muscle stretch and that of passive movement may underlie the failure, in man, to find uniformly increased long-latency stretch reflexes in clinical states of extrapyramidal rigidity.