In 1965, Ronald Melzack from McGill University in Montreal Canada and Patrick Wall from University College London UK, published their seminal paper which
proposed a gating mechanism in the central nervous system to regulate the flow of nerve signals from peripheral nerves en-route to the brain.
According to this Gate-Control Theory of Pain, activity in large diameter low threshold mechanoreceptive (touch-related) nerve fibers could inhibit the transmission of action potentials from small diameter higher threshold nociceptive (pain-related) fibers through pre and post synaptic inhibition in the dorsal horn of spinal cord. Humans utilise this mechanism whenever they rub their skin to relieve pain. Because nociceptive fibers (A-delta and C-fibers) have a higher threshold of activation than mechanoreceptive fibers (A-beta fibers) Melzack and Wall proposed that it would be possible to selectively stimulate mechanoreceptive fibers by titrating the amplitude of electrical currents delivered across the surface of the skin (ie TENS). This would prevent signals from nociceptive fibers from reaching higher centres of the brain, thus reducing pain (Figure 2). In essence, TENS electrically rubs pain away.
O Tashani, O and Johnson, MI. Transcutaneous Electrical Nerve Stimulation (TENS) A Possible Aid for Pain Relief in Developing Countries? Libyan J Med. 2009; 4(2): 62–65.
Melzack, R. and P. Wall, Pain mechanisms: A new theory. Science, 1965. 150: p. 971-79.