What are the two primary mechanisms by which conventional TENS achieves analgesia?

• A. Gate control via large-diameter afferent fibers and activation of endogenous opioid systems at higher intensities or longer durations.
• B. Local tissue heating and increased blood flow.
• C. Mechanical disruption of nerve fibers.
• D. Inhibition of alpha motor neurons to reduce muscle spasm.

Answer: (C)

Conventional TENS reduces pain mainly by modulating how pain signals are processed in the nervous system through two complementary mechanisms. First, stimulating large-diameter A-beta fibers activates inhibitory circuits in the dorsal horn, which dampen transmission from smaller nociceptive fibers—a fast, gate-like effect that lowers pain perception as long as the unit is on. Second, when stimulation is at higher intensities or applied for longer durations, TENS can engage the endogenous opioid system, producing pain relief via supraspinal pathways and spinal mu- receptor–mediated inhibition that can last beyond the stimulation period. This combination explains why TENS can feel instantly helpful through gating and sometimes have longer-lasting effects when higher intensity or longer sessions are used. Mechanical disruption of nerve fibers is not how TENS achieves analgesia, and heating or muscle relaxation effects are separate modalities or secondary phenomena rather than the primary analgesic mechanisms of conventional TENS.