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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2641-2648
Copyright ©1999 by the American Physiological Society
Department of Oral and Craniofacial Biological Sciences, University of Maryland Dental School, Baltimore, Maryland 21201
Andrew, David and
Joel D. Greenspan.
Peripheral Coding of Tonic Mechanical Cutaneous Pain:
Comparison of Nociceptor Activity in Rat and Human
Psychophysics. J. Neurophysiol. 82: 2641-2648, 1999. These
experiments investigated temporal summation mechanisms of tonic
cutaneous mechanical pain. Human volunteers provided psychophysical
estimates of pain intensity, which were compared with discharge
patterns of rat cutaneous nociceptors tested with identical stimulus
protocols. Human subjects made either intermittent or continuous
ratings of pain intensity during stimulation of the skin between the
thumb and first finger. Stimulus intensities of 25, 50, and 100 g
were applied with a probe of contact area of 0.1 mm2 for 2 min. Pain perception significantly increased during stimulation (temporal summation) for the 50- and 100-g stimulus intensities. Sequential conduction block of the myelinated fibers supplying the
stimulated skin was used to investigate the role of A-fiber mechanoreceptors and nociceptors in this temporal summation. Conduction block of the A
fibers resulted in an increase in mechanically evoked
pain estimates and an increase in temporal summation, consistent with
loss of A
-mediated inhibition. When only conduction in the unmyelinated fibers remained, pain estimates were reduced to the preblock levels, but temporal summation was still present.
Electrophysiological recordings were made from filaments of the sciatic
nerve supplying receptors in the plantar skin of
barbiturate-anesthetized rats. Forty units fulfilled the identification
criteria for nociceptors: 20 A-fiber and 20 C-fiber nociceptors. Each
unit was characterized by recording its responses to graded mechanical
and heat stimuli. Nociceptors were also tested with stimuli identical
to those applied to the human subjects. The responses of all units to
sustained mechanical stimuli were adaptive
that is, they exhibited a
gradual decline in response with time. However, the time course of
adaptation varied among units. All the C-fiber nociceptors and one-half
of the A-fiber nociceptors had rapidly adapting responses. The
remainder of the A-fibers displayed slowly adapting responses.
One-third of all units also showed short-duration increases in firing
rate during stimulation. The latency after stimulus onset of this rate acceleration was inversely related to stimulus intensity. Despite the
apparent disparity between perceptual temporal summation and nociceptor
adaptation, central and peripheral mechanisms are proposed that can
reconcile the relationship between nociceptor activity and pain perception.
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