Acute Effects of Transcutaneous Electrical Nerve Stimulation at Acupoints on Nociceptive Transmission: A Mechanism Study Using Pain-Related Evoked Potentials

The goal of this clinical trial is to learn if transcutaneous electrical nerve stimulation (TENS) applied to acupoints can modulate pain transmission and cortical processing in healthy adult volunteers. The main questions it aims to answer are: * Does TENS at Zhongzhu (SJ3) and Hegu (LI4) change pain-related evoked potentials (PREPs)? * Does acupoint electrical stimulation alter nociceptive pathways reflected by PREP waveform modulation? Researchers will compare PREPs recorded before and after TENS at the two acupoints to see if TENS produces measurable effects on cortical nociceptive responses. Participants will receive low-frequency TENS applied to Zhongzhu (SJ3) and Hegu (LI4), and undergo PREP assessments before and after the intervention to measure cortical responses to standardized noxious stimuli.