There are currently two opposing schools of thought concerning pain:
1. The specificity theory cf von Frey which asserts that pain is a modality of somesthesis subserved by specific peripheral receptors and particular fibers.
2. The pattern theory of W´eddel and Sinclair which denies specific receptors and fibers and maintains that any noxious stimuli cause impulses characterized by spatially and temporally dispersed pattern. The former better explains the capacity of the brain to localize cutaneous stimuli and the latter has advantages in relating the quality of the pain sensation to the characteristics of the pattern which might be detected by the sensory cortex. Although the two theories appear to be mutually ,exclusive both contain valuable concepts that supplement one another.
In 1965, Melzack and Wall proposed a gate control theory on pain mechanism. According to this theory, inputs through large or small fibers cause two opposing effects on the substantia gelatinosa activity. Subs tantia gelatinosa cells act as a "gate control system" upon the afferent inputs by modulating the membrane potential of afferent terminal arborizations. Thus balance between large and small fiber activity determines the central transmission process of afferent signals.
The gate control theory was proposed with following experimental evidences:
1. Recent neuroanatomic studies on the dorsal root entry zone revealed that both small and large fibers terminate in the substantia gelatinosa.
2. The dorsolateral tract of Lissauer was shown to be the propriospinal tract of the substantia gelatinosa, refuting the conventional description given by Ranson(1931).
3. The hitherto unknown ´origin of the dorsal root potential is known to be substantia gelatinosa.
4. Large, and small fiber inputs were selectively con-trolled by means of anodal block technique and the two opposing effects of large and small fiber inputs on dorsal root potential were observed.
5. Selective stimulation of either large and small afferent fibers and the record of single unit firing in the dorsolateral column suggested two opposing effecsts upon substantia gelatinosa activity.
The theory has a number of factors which aid the explanation of various pain problems and its therapeutic implications are an encouraging aspect. However, be-cause of certain controversies regarding portions of the evidences the theory requires extended study and confirmation.
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