Long-latency stretch reflexes of two intrinsic muscles of the human hand analysed by cooling the arm.

1. The arm was cooled to examine the effect of slowing nervous conduction on the latency of various reflex responses of the abductor digiti minimi (ADM) and the first dorsal interosseus (FDI) muscles. A reflex dependent upon slow afferents acting with a short central delay should be slowed by more than one of comparable initial latency, but dependent upon fast afferents acting with a long central delay. The F wave seen in the EMG on stimulating the ulnar nerve was used to assess the effect of cooling in slowing fast fibres. 2. Cooling showed that the prominent long-latency excitation evoked on stretching one or other muscle by displacing the appropriate finger is medicated by fast afferent fibres. 3. Both fast cutaneous and group I muscles afferents were found to be capable of contributing to the long-latency responses. The muscle afferent contribution was consistently dominant for slow 'stretches', while a cutaneous afferent contribution was sometimes recognizable for finger taps. 4. The distinction proved possible because the cutaneous afferents from the relevant finger run in the median nerve for FDI and the ulnar nerve for ADM. The cooling produced much greater slowing of conduction for the ulnar nerve. 5. The prior suggestion that spindle group II afferents are responsible for the major part of the long-latency component of the human stretch reflex thus fails to stand; however, some contribution has not been finally excluded.