Gating of segmental and transcortical reflexes to human hand muscles depends on the mode of innervation

We investigated the influence of the innervation pattern on the transmissio n of reflexes to a hand muscle. Reflexes were elicited by repetitive median nerve stimulation and recorded from the abductor pollicis brevis muscle. B oth H and long loop reflexes were exclusively attenuated by irregular isoto nic thumb abduction. In 10 of 15 experiments the long loop reflex was block ed completely. Inhibition intercalated between the H and long loop reflexes also decreased considerably. The observed attenuation of both spinal and s upraspinal reflexes is suspected to take place mainly at the spinal level. Among the possible mechanisms, reciprocal inhibition is the most likely. (C ) 2000 Elsevier Science Ireland Ltd. All rights reserved.