Analysis of human long-latency reflexes by cooling the peripheral conduction pathway; which afferents are involved?

This chapter first outlines current views on the afferents of origin of human "long-latency stretch reflexes", and especially whether they are fast or slow. Attention is concentrated on methodology; other approaches can be found elsewhere with appropriate bibliography (Marsden et al., 1983; Matthews, 1985, 1986a; Wiesendanger, 1986). Recent experiments involving cooling of the human arm are then described. They were performed on the abductor digiti minimi and first dorsal interosseus muscles. The arm was cooled from wrist to axilla by circulating cold water through a tube wrapped round it. Cooling slows conduction by a constant proportion, hence the conduction delay introduced by cooling a segment of nerve is greater for small slow axons than for large fast ones. On this basis it was concluded that group I muscle afferents (presumably Ia) can elicit a long-latency reflex with a long central delay, as also can fast cutaneous afferents. No evidence was found for a long-latency reflex with the delay introduced peripherally by conduction along slow axons (i.e. spindle group II afferents). However, the co-existence of such a mechanism has yet to be excluded.