EVIDENCE THAT LOW-THRESHOLD MUSCLE AFFERENTS EVOKE LONG-LATENCY STRETCH REFLEXES IN HUMAN HAND MUSCLES

1. The aim of the present study was to identify the type of spinal afferents involved in the generation of the long-latency response in intrinsic human hand muscles. Position-controlled extensions were imposed on the index finger or on the wrist of healthy subjects who were exerting a steady voluntary flexion force at the relevant joint. Averaged surface electromyographic (EMG) responses of the first dorsal interosseus muscle (FDI) or of the wrist flexors were evaluated with respect to latency and size. 2. Small transient angular displacements of the index finger (1-degrees, as measured at the metacarpophalangeal joint), which are supposed to excite primary rather than secondary afferents, evoked two clearly discernible EMG responses with mean latencies of 32.3 ms (M1 response) and 54.7 ms (M2 response), respectively. The size of the M2 response exceeded the size of the M1 response by 60%. In the wrist flexors, transient stretch (1-degrees) gave rise to a large M1 response (latency 22.8 ms) and a small, inconstent M2 response. 3. Small-amplitude vibration of the index finger elicited EMG responses in the FDI that were qualitatively and quantitatively similar to those seen in response to small transient stretches of the index finger. This was also true for fast ramp-and-hold stretches (stretch velocity 400-degrees/s, amplitude 5-degrees), whereas slow ramp-and-hold stretches (125-degrees/s, 5-degrees) elicited predominantly M2 responses. 4. In the FDI, the mechanical threshold of the M1 and M2 response to the transient angular displacement was approximately 0.15-degrees, with a tendency for the M2 response to appear at a lower threshold. 5. The latency difference, M2 - M1, was similar in the distal FDI and the more proximal wrist flexors, arguing against slowly conducting afferents as a possible source of the M2 response. A similar result was obtained by investigating the EMG response of the FDI of 77 adults with body heights ranging from 1.42 to 2.06 m. The latency of both M1 and M2 was positively correlated with body height, but the latency difference M2 - M1 was not. This result indicates that the excess latency of M2 over M1 is due to a central rather than a peripheral delay. 6. Complete anesthesia of the radial aspect of the hand, sparing only the ulnar nerve supply to the FDI, diminished the M2 response of the FDI to small transient stretches only slightly. This argues against a major contribution of fast-conducting skin afferents to the M2 response. 7. We conclude that in intrinsic hand muscles primary muscle spindle afferents play a major role in the generation of long-latency responses to muscle stretches, being transmitted with a long central delay.