Group III muscle afferents evoke reflex depressor responses to repetitive muscle contractions in rabbits

Repetitive twitch contraction of the hindlimb muscles in anesthetized rabbi ts consistently evokes a reflex depressor response, whereas this type of co ntraction in anesthetized cats evokes a reflex presser response in about on e-half of the preparations tested. Rapidly conducting group III fibers appe ar to comprise the afferent arm of the reflex are, evoking the depressor re sponse to twitch contraction in rabbits because electrical stimulation of t heir axons reflexly decreases arterial pressure. In contrast, electrical st imulation of the axons of slowly conducting group III and group TV afferent s reflexly increases arterial pressure in rabbits. In the present study, we examined the discharge properties of group III and IV muscle afferents and found that the former (i.e., 13 of 20), but not the latter (i.e., 0 of 10) , were stimulated by 5 min of repetitive twitch contraction (1 Hz) of the r abbit triceps surae muscles. Moreover, most of the group III afferents resp onding to contraction appeared to be mechanically sensitive, discharging in synchrony with the muscle twitch. On average, rapidly conducting group III afferents responded for the 5-min duration of l-Hz repetitive-twitch contr action, whereas slowly conducting group III efferents responded only for th e first 2 min of contraction. We conclude that rapidly conducting group III afferents, which are mechanically sensitive, are primarily responsible for evoking the reflex depressor response to repetitive-twitch contractions in anesthetized rabbits.