Long-latency component of the stretch reflex in human muscle is not mediated by intramuscular stretch receptors

Reflex responses to mechanical stimulation of muscle (brief imposed movemen t) were investigated. Reflexes were elicited in the forefinger, recording f rom the first dorsal interosseous (FDI), and in the foot, recording from so leus. These responses typically consisted of a short-latency component (M1) and a long-latency component (M2) at 33 ms and 53 ms, respectively, after the stimulus in the case of FDI, and 37 ms and 68 ms, respectively, in sole us upon stimulation of the sole of the foot. Normally, when a muscle is str etched by a mechanical stimulus (either naturally or by an experimentally i mposed movement), both skin receptors and muscle stretch receptors are acti vated. It is possible, however, to devise stimulation parameters where this is not the case. Fixating the finger with plasticine enables the effects o f skin stimulation to be studied without stretching the FDI muscle. On the other hand, tapping a long tendon allows muscle stretch receptors to be act ivated without involving skin or subcutaneous structures. Component M1 was always abolished by finger fixation in 40 trials on 10 subjects, with M2 be ing essentially unchanged in latency, duration, or amplitude. Reflex respon ses were obtained in soleus muscle in nine experiments by prodding the sole of the foot (thereby stimulating both skin and muscle stretch receptors). Alternatively, the tendo achilles was prodded (which solely activates stret ch receptors in the muscle). In the former, M1 and M2 were generated. In th e latter, only M1 was produced. It is concluded that the long-latency compo nent of the stretch reflex, M2, originates in skin and/or subcutaneous nerv e terminals and that no part of M2 originates in muscle stretch receptors.