Central delay of the laser-activated rat tail-flick reflex

Abstract

The latency of the heat-activated rat tail-flick (TF) reflex is dependent upon 4 variables, none of which has previously been determined: activation of cutaneous nociceptors (TN); afferent conduction to the dorsal horn (TA); conduction within the central nervous system (CNS) (central delay); and conduction from the ventral horn (VH) to, and activation of, tail muscles (TE). Using a CO2 infrared laser (10 W, 45 msec) to produce synchronous activation of tail-skin nociceptors, TF latency (EMG response) was measured in 10 awake rats. Based on shifts in response latency from points of stimulation near the tip and base of the tail, conduction velocity in the afferent limb of the reflex was estimated to be 0.76 +/- 0.11 m/sec. This indicates that the response is mediated by C fibers. The rats were then anesthetized with pentobarbital and multiple-unit activity and evoked potentials (EPs) were recorded from the superficial dorsal horn at spinal segments S3-CO1 during laser or high-intensity electrical (10 mA, 1 msec) stimulation of the tail. Unit activity and EPs elicited by both stimuli consisted of two distinct components, corresponding to activation of A and C fibers. The difference in latency between laser and electrical evoked activity indicated that 60.00 +/- 7.33 msec was required for activation of nociceptors by the laser. Electrical stimulation of the VH at S3-CO1 in 3 rats produced a TF (EMG) response in 4 msec. Central delay, calculated as total TF time minus (TN+TA+TE), was 82.3 +/- 13.08 msec. This represents the time frame during which modulation of the reflex by an intrinsic, pain-activated, supraspinal system could occur.

Discharge of Raphe Magnus on and offCells Is Predictive of the Motor Facilitation Evoked by Repeated Laser Stimulation

“…The motor withdrawal evoked by laser heat is analogous to that elicited by slower forms of heat stimulation (Willer et al, 1979;Schouenborg et al, 1992;Bragard et al, 1996;Plaghki et al, 1998) but has the advantage that more is known about its underlying circuitry (Danneman et al, 1994). Initiation of the laser Figure 3.…”

“…These two perceptual components are likely caused by the activation of A␦ and C fiber nociceptors, respectively, by the laser stimulus Treede, 1983, 1984;. In rats as in humans, laser heat stimulation activates nociceptors, evokes a motor withdrawal, and elicits a response in primary somatosensory cortex (Devor et al, 1982;Kalliomaki et al, 1993;Danneman et al, 1994).…”

The ITFR, impulsive tail flick reflex by short duration nociceptive stimuli

“…In the case of thermal stimulation with CO 2 laser pulses, we calculated that the afferent fibres responsible for the occurrence of the late and ultralate components had a mean conduction velocity in the range of C fibres (0.95 ± 0.49 m/s), which also happens to be very similar to the value found in a previous paper (Danneman et al, 1994) describing direct extracellular recordings from the dorsal horns of the spinal cord (0.76 ± 0.11 m/s). In our experiments at low intensity (7.5 W) only the latter components were elicited.…”

“…Nevertheless, these two types of stimulation have seldom been used for algometry in animal models (Le Bars et al, 2001) and have only been employed for research on the mechanisms underlying the response. Because they allow the assessment of the epochs of time needed for: (i) receptor activation, (ii) C or A␦ fibre afferent conduction, (iii) conduction within the spinal segment (central delay), (iv) efferent motor conduction (Danneman et al, 1994), they can be exploited to investigate the mechanisms of action by neuropathies and analgesic drugs. Furthermore, the possibility of monitoring the reflex response by electromyogram (EMG) or strain gauge permits not only its evaluation within the time domain, but also amplitude and energy domains can be taken into consideration for a more detailed analysis of the phenomenon.…”

CO₂ Laser Pulse-Evoked Nocifensive Behavior Mediated by C-Fibers

“…These findings suggest that LEPs may be used as objective correlates of pain perception. Laser-evoked cortical responses can be classified into two groups [Danneman et al, 1994, Isseroff et al, 1982, Kalliomaki et al, 1993. The first group, mediated by Aδ-fibers, is more sensitive to pentobarbital-induced anesthesia [Shaw et al, 2001] and laser pulse energy level [Kalliomaki et al, 1993].…”