The Pain-Relieving Effects Induced by Electroacupuncture with Different Intensities at Homotopic and Heterotopic Acupoints in Humans

The purpose of the present study was to investigate the relationship between the distribution of the analgesic area when different intensities of stimulation were applied to homotopic and heterotopic acupoints. The experiments were performed on volunteers; the left sural nerve was stimulated by a volley of eight rectangular pulses delivered at a frequency of 400 Hz. Electromyographic reflex responses (EMGs) were recorded from the ipsi-lateral biceps femoris muscle. Electroacupuncture stimuli were given on the skin overlying the Zusanli point (St 36), and the strengths chosen were times of the threshold eliciting a nociceptive reflex (T RIII ) response (0.6, 0.8, 1.0, T RIII and T supra-RIII , respectively). The effects of homotopic and heterotopic stimuli applied to St 36 on the pain sensation and the R III reflex elicited by stimulation of the left sural nerve were observed to explore the pain-relief and R III reflex-inhibition produced by stimulation of the St 36 point with different intensities. Both the nociceptive reflex and painful sensation evoked by stimulating the sural nerve were similarly inhibited by electroacupuncture at less than T RIII applied to the ipsi-lateral acupoint. In other words, acupuncture with an innocuous intensity can produce homotopic pain-alleviating effects and reflex suppression. With contra-lateral electroacupuncture at the St 36 acupoint, innocuous intensities cannot produce heterotopic pain-relieving effects; these inhibitions were only observed at electroacupuncture intensities similar to the T RIII threshold. These results suggest that local acupuncture-induced analgesia is effective with activation of large afferent fibers, whereas heterotopic acupuncture-induced analgesia is only effective with intensities strong enough to excite small afferent fibers. Local analgesic effects of acupoint stimulation involve segmental inhibition, whereas systemic analgesic effects of acupoint stimulation are involved in contra-lateral effects. The latter may recruit the diffuse noxious inhibitory controls (DNIC) system. The specific function of an acupoint is determined by the anatomical relationship between the disease focus and the segmental location of the acupoint.