CENTRAL DELAY OF THE LASER-ACTIVATED RAT TAIL-FLICK REFLEX

The latency of the heat-activated rat tail-flick (TF) reflex is depend ent upon 4 variables, none of which has previously been determined: ac tivation of cutaneous nociceptors (T-N); afferent conduction to the do rsal horn (T-A); conduction within the central nervous system (CNS) (c entral delay); and conduction from the ventral horn (VH) to, and activ ation of, tail muscles (T-E). Using a CO2 infrared laser (10 W, 45 mse c) to produce synchronous activation of tail-skin nociceptors, TF late ncy (EMG response) was measured in 10 awake rats. Based on shifts in r esponse latency from points of stimulation near the tip and base of th e tail, conduction velocity in the afferent limb of the reflex was est imated to be 0.76 +/- 0.11 m/sec. This indicates that the response is mediated by C fibers. The rats were then anesthetized with pentobarbit al and multiple-unit activity and evoked potentials (EPs) were recorde d from the superficial dorsal horn at spinal segments S-3-Co-1 during laser or high-intensity electrical(10 mA, 1 msec) stimulation of the t ail. Unit activity and EPs elicited by both stimuli consisted of two d istinct components, corresponding to activation of A and C fibers. The difference in latency between laser and electrical evoked activity in dicated that 60.00 +/- 7.33 msec was required for activation of nocice ptors by the laser. Electrical stimulation of the VH at S-3-Co-1 in 3 rats produced a TF (EMG) response in 4 msec, Central delay, calculated as total TF time minus (T-N + T-A + T-E), was 82.3 +/- 13.08 msec. Th is represents the time frame during which modulation of the reflex by an intrinsic, pain-activated, supraspinal system could occur.