Human intramuscular and cutaneous pain: psychophysical comparisons.

We used psychophysical methods to compare the central processing of nociceptive inputs from skin and muscle in ten normal humans. Both intramuscular electrical and infrared CO2 laser cutaneous stimulation showed increasing but decelerating (downward concave) stimulus-response curves and similar temporal summation characteristics. Intramuscular stimulation was rated significantly more unpleasant than cutaneous stimulation. The results are consistent with a common mode of central nociceptive processing for skin and muscle pain intensity but suggest a relatively larger activation of affective mechanisms by muscle afferents.