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Evidence is presented that the signal that damage has occurred to an animal begins with the activation of receptors which respond specifically to noxious stimuli. In fact, different types of nociceptors are found which respond selectively to different types of damage. The activity of nociceptive sensory fibers influences neurons in the spinal cord which are not activated by other types of somatic stimuli and are thus specific. At higher levels of the nervous system less is known about the physiology of pain and such fundamental questions as the degree to which the cerebral cortex is involved in pain perception have not been answered. It is not known to what extent the mechanisms at higher levels are specific and the significance of convergent systems in which an individual neuron can be excited by a number of different stimuli, both noxious and innocuous, has not been resolved. However, it is argued that the evidence at present most strongly supports the concept that the neural system involved in pain is specific; the activity of neurons in this system either causes pain, or if the level of activity is insufficient, no sensation. Ways in which the activity of this specific system may be modulated are discussed in the context of counterirritation and acupuncture analgesia.

HIGH-THRESHOLD MECHANORECEPTORS and their centrally projecting myelinated fibers make up a functionally distinct group of cutaneous sensory units that have been suggested as part of the afferent apparatus for pain resulting from mechanical damage to the skin (3, 17). The argument for their relation to pain was based on two points: 1) the ability of such afferent elements, unique among those with medullated fibers, to provide signals differentiating noxious from innocuous mechanical events affecting the skin; and 2) the well-established correlation between pain and activity in thin myelinated afferent fibers

The present study was concerned with the masking and pain-attenuating effect of vibration at different levels of intensity of noxious stimulation. Forty Ss were given noxious stimulation in the form of increasingly painful electric shocks in trials where such shocks were presented with and without concurrent vibratory stimulation. The masking or pain-attenuating effect of the vibration was greatest at the lowest level of noxious stimulus intensity and decreased as the noxious stimulation intensity increased. At the highest level of noxious stimulation the effect of vibration was not significant although there was a tendency for Ss with higher pain tolerance to show summation. The possible relevance of the intensity of the vibratory stimulus to this pattern of results was discussed.

Cutaneous nerve activity was recorded in response to noxious stimuli. An analog cross-correlator was used to quantify the activity occurring in the A, alpha-beta; A, delta-gamma; and C fiber groups. By comparing various noxious stimuli with non-noxious stimuli it was shown that the amount of small myelinated fiber activity responding to noxious stimuli can be accounted for by the mechanical deformation of the skin. It was clearly demonstrated that C fiber activity is greatly increased by noxious stimuli with burning, which produced the most tissue damage, giving the largest amount of activity. The technique can not distinguish between the presence of specific C fiber receptors for pain or an increased firing of mechanoreceptors innervated by C fibers. However, it can be stated definitely that in the cat, noxious stimuli are associated with an increase in C fiber activity with no detectable change in delta fiber activity.