The effects of antidromic discharges on orthodromic firing of primary afferents in the cat

This study investigated the effects of antidromically conducted nerve impul ses on the transmission of orthodromic volleys in primary afferents of the hindlimb in decerebrated paralyzed cats. Two protocols were used: (A) Singl e skin and muscle afferents (N = 20) isolated from the distal part of cut d orsal rootlets (L7-S1) were recorded while stimulation was applied more cau dally. The results showed that during the trains of three to 20 stimuli, th e orthodromic firing frequency decreased or ceased, depending on the freque ncy of stimulation. Remarkably, subsequent to these trains, the occurrence of orthodromic spikes could be delayed for hundreds of ms (15/20 afferents) and sometimes stopped for several seconds (10/20 afferents). Longer stimul ation trains, simulating antidromic bursts reported during locomotion, caus ed a progressive decrease, and a slow recovery of, orthodromic firing frequ ency (7/20 afferents), indicating a cumulative long-lasting depressing effe ct from successive bursts. (B) Identified stretch-sensitive muscle afferent s were recorded intra-axonally and antidromic spikes were evoked by the inj ection of square pulses of current through the micropipette. In this case, one to three antidromic spikes were sufficient to delay the occurrence of t he next orthodromic spike by more than one control inter-spike interval. If the control inter-spike interval was decreased by stretching the muscle, t he delay evoked by antidromic spikes decreased proportionally. Overall, the se findings suggest that antidromic activity could alter the mechanisms und erlying spike generation in peripheral sensory receptors and modify the ort hodromic discharges of afferents during locomotion. (C) 1999 Elsevier Scien ce B.V. All rights reserved.