Long-latency responses in human thenar muscles mediated by fast conducting muscle and cutaneous afferents

Electrical stimulation of median nerve at the wrist near motor threshold evokes two distinct reflexes in voluntarily innervated thenar muscles. The first is the Hoffmann reflex (HR) and the second is the long-latency reflex (LLR). Stimulation of the motor branch of the median nerve at the thenar sub-threshold for motor fibers evokes the same pattern suggesting that group IA muscle afferents are mediating the LLR. Stimulation of pure cutaneous afferents evokes a LLR of similar latency without a preceding HR. Statistical analyses of the reflex latencies of HR and LLR are in favor of a fast conduction velocity for the cutaneous afferents involved. Hence for muscular and cutaneous LLR a long-lasting central conduction time has to be assumed.