MECHANISMS OF FIRST AND SECOND PAIN IN THE PERIPHERAL AND CENTRAL NERVOUS SYSTEMS

Brief (< 2.0 sec) noxious heat pulses (peak temp = 51.5°C) programmed by a computer and generated by a contact thermode produced first and second pain in human subjects. Measurements of reaction time confirmed that these first and second pains were related to the conduction of impulses in Aδ heat nociceptive and C polymodal nociceptive afferents respectively. Estimates of psychophysical magnitude showed that when four identical heat pulses were applied to the same spot on the hand (interstimulus interval ≤ 80 sec), the first pain progressively decreased in perceived intensity whereas the second pain increased with interstimulus intervals of 3sec or less. The first pain did not decrease if the location of the thermode was changed between each stimulus whereas the summation of the second pain increased under these conditions. Identical trains of noxious heat pulses partially suppressed the responses of Aδ and of C nociceptive afferents. These psychophysical observations and physiologic records indicate that the temporal suppression of heat-induced first pain is related to the suppression of Aδ heat nociceptors and that prolonged temporal summation of second pain is related to summation within the central nervous system. Aδ heat nociceptive afferents and C polymodal nociceptive afferents converge on two types of spinothalamic tract neurons: (1) wide dynamic-range neurons that receive input from non-nociceptive and nociceptive afferents. and (2) nociceptive-specific neurons. Since both types show summation of responses to repeated C fiber stimulation, they can account for summation of second pain