ack and Wall produced evidence which showed that dull, slow, diffuse, burning pain is carried from the periphery by small unmyelinated fibers, while the sensation of light touch is carried by large myelinated fibers (4:5). They also postulated that the sensory signals carried along these two sets of fibers were modulated in the substantia gelatinosa of the dorsal horns within the spinal cord (4:5). This modulation consisted of the different outputs acting upon each other in a negative feedback mechanism (4:5).
Despite these hypotheses, however, the actual mechanisms by which pain relief is obtained from cutaneously applied electrical stimulation are, for the most part, unknown. Research on TENS is often based on case reports or poorly designed studies (22:28). Evaluating it can be confusing (23:3). Factors which contribute to the complexity surrounding TENS research include a large amount of variability in the terminology, the methods of application, the characteristics of the electrical current, and the different types and placements of the electrodes (23:3). In additional , the innumerable etiologies of pain, the chronicity of different types of pain, and the different evaluation measures may also add to the confusion (23:3). All of these variables act to interfere with any attempt to make a clear comparison between the different TENS studies (23:3). Regardless though, a number of tenable theories on possible TENS mechanisms do currently exist (18:10).
One is that the pain relief is merely a placebo response (1:10). Contradicting this, however, is the fact that in both long and short-term studies success rates with TENS have been significantly greater than those measured using placebos (18:10 11).
There are basically three possible effects that TENS can have on the peripheral nervous system (23:4). The first of these involves presynaptic inhibition at the spinal cord (23:4). Campbell and Taub have suggested bran...