The neurophysiological basis of the analgesic action of TENS remains under investigation, yet there are two popular theories to explain how TENS successfully decreases or eliminates pain.

• The Gate Control Theory is the most commonly advanced explanation, suggesting that by electrically stimulating sensory nerve receptors, a gate mechanism is closed in a segment of the spinal cord, preventing pain-carrying messages from reaching the brain and blocking the perception of pain.

• The Endorphin Release Theory suggests that electrical impulses stimulate the production of endorphins and enkaphalins in the body. These natural, morphine-like substances block pain messages from reaching the brain, in a similar fashion to conventional drug therapy, but without the danger of dependence or other side effects.