Modulation of heat evoked nociceptive withdrawal reflexes by painful intramuscular conditioning stimulation

Convergence between cutaneous heat nociceptors and muscles afferents was investigated by applying a phasic, conditioning electrical stimulus to the tibialis anterior muscle (a train of five 1 ms pulses over 21 ms) at varying time intervals relative to a thermal test stimulus used for evoking the withdrawal reflex in humans. The 200 ms thermal stimulus was applied on the dorsum of the foot at an intensity of two times the pain threshold. The conditioning electrical stimulus was applied at an intensity of two times the pain threshold via a set of intramuscular needle electrodes. The conditioning-test interval was varied between –400 ms and 8,000 ms at 17 different intervals. The mean reflex onset latency of reflexes evoked by thermal stimuli alone was 354 ± 9 ms. A facilitation of the reflex was seen when the conditioning stimulus was applied 275 ms (174 ± 30% compared to control) and 300 ms (162 ± 32% compared to control) after the test stimulus onset indicating sensory convergence between muscle afferents (group I–III) and cutaneous Aδ heat nociceptors arriving simultaneously at the spinal cord.