Physiological pain may be defined as a specific sensorial sensation induced in normal tissues when external stimuli are applied with sufficient intensity to endanger tissue integrity. Because pain may be induced by a wide variety of stimuli, it has not always been accepted as constituting a sensorial sensation. Considerable evidence exists, however, to indicate that physiological pain is a specific sensation, similar to the other sensorial sensations. One indication that pain is a proper sensorial sensation lies in the fact that its has its special nerve system.

Blix (5) and Goldscheider (6) found that certain areas of the skin were sensitive to painful stimuli from a pinprick while others were not. Strughold (7) demonstrated that, in various areas of skin, pain points were concentrated in varying degree. Microscopic study of areas of skin showing high aggregations of spots of specific forms of sensibility has indicated that special sensory nerve and organ structures are apparency associated with different types of sensation. Thus, Krause's corpuscles are considered as receptors for cold; Ruffini's endings and Golgi Mazzoni corpuscles for heat; and Meissner's corpuscles, Merkel's discs and the basket endings around hair roots for touch. (8) Woolard (9) described unmyelinated, finely beaded, branched free endings as the specific nerve end organs believed to be responsible for the reception of pain impulses. Certain areas such as the cornea and the mucous membrane of the nose, which are generally considered sensitive to pain alone, have been shown to have these free endings as the characteristic nerve endings at these sites. Weddell has found only this type of end structure in areas of skin sensitive solely to pain during nerve regeneration.

That pain constitutes a specific form of sensation is further indicated by the evidence that its impulses are carried along definite nerve pathways to special centers in the thalamus. By temporary asphyxia, by cocainization, or by cooling, differential interference with conduction of the special sensations of pain, touch, heat and cold along a nerve can be produced. The existence of individuals without the sensation of pain, but with sensations of touch, cold and heat, has confirmed this view of pain as a proper sensorial sensation.

It is characteristic of the sensation of pain that it may be elicited by a wide variety of stimuli. Below the pain threshold, the incitation induces specific sensorial sensations according to the stimulus used. Above this threshold, the sensation felt is pain. When different noxious stimuli produce pain, the subject cannot distinguish the nature of the incitation. In effect.

when it is below the threshold, the incitation informs about the nature of the stimulus; above the threshold, the individual is conscious of another fact: that the stimulus is of such intensity as to endanger the integrity of the tissues. Pain thus appears to be the sensorial sensation of a specific character of stimuli—sufficient intensity to represent a danger for the tissues, and it is this which differentiates pain from the other sensorial sensations, and puts it into a special category. Pain is independent of the nature of the stimuli. By constituting a warning to the body, that its tissues are in jeopardy, physiological pain induces a general response involving brisk, rapid movements, a rise in pulse rate, and a sense of invigoration. (10)

The fact that sensorial pain results from the intensity of the external incitations has prompted investigators to study this kind of pain largely in terms of the threshold of incitation. It must be emphasized that for each stimulus there exist two thresholds, one for intensity values required to produce specific sensations, and the second for intensity needed to produce pain. There is a considerable difference, for example, between the heat intensity necessary to produce a sensation of warmth and the amount that will produce a sensation of pain.