Peripheral suppression of first pain and central summation of second pain evoked by noxious heat pulses

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Abstract
Psychophysical experiments were carried out on 6 human subjects to determine how 1st and 2nd pain were influenced by peripheral receptor mechanisms and by CNS inhibitory and facilitatory mechanisms. The brief, natural, painful stimuli delivered to the hand were a train of 4-8 constant waveform heat pulses generated by a contact thermode (peak temperature = 51.5.degree. C). The magnitude of 1st and 2nd pain sensations was estimated using cross-modality matching procedures and reaction times were determined. The latter confirmed the relationship between 1st and 2nd pain and impulse conduction in A.delta. and C noxious heat afferents, respectively. The intensity of 1st pain decreased with each successive heat pulse when the interpulse interval was 80 s or less. This decrease was most likely the result of heat induced suppression of A.delta. heat nociceptors since it did not occur if the probe location changed between successive heat pulses. In contrast, 2nd pain increased in intensity with each successive heat pulse if the interval was 3 s or less. This summation was most likely due to CNS summation mechanisms since it also occurred after blockage of 1st pain by ulnar nerve compression and when the location of the thermode changed between heat pulses. These observations and their interpretations were supported by recording of responses of single A.delta. heat nociceptive afferents, C polymodal nociceptive afferents and warm afferents of rhesus monkeys to similar trains of noxious heat pulses. Their responses to these heat pulses showed a progressive suppression. Wide dynamic range dorsal horn neurons showed summated responses to repeated volleys in C fibers (> 1/3 s). These spinal cord summation mechanisms could account for the summation of 2nd pain.