Effect of Acupuncture Stimulation on the Long Latency Reflex

Abstract

[Objectives] In this study, we examined the effects of acupuncture stimulation on short latency reflexes (SLR) and long latency reflexes (LLR) to determine the site of acupuncture stimulation in modulating motor reflexes. Further, we investigated the relationship between changes in LLR and changes in the N20 somatosensory evoked potential (SEP) component induced by acupuncture stimulation and examined changes in central motor conduction time (CMCT). [Subjects and Methods] Sixteen healthy and right-handed adults (11 males and 5 females; 28.9 ± 6.6 years old; upper limb length 54.9 ± 3.2 cm) participated in this study. The experiments were performed under three conditions: (1) control (no acupuncture stimulation), (2) acupuncture stimulation of right-sided Hegu (LI4), and (3) acupuncture stimulation of left-sided LI4. An acupuncture needle (0.18 mm in diameter) was inserted up to a depth of 10 mm at the right- or left-sided LI4. Electrical stimulation was delivered to the median nerve in the right hand joint at a 120% intensity compared with the threshold to produce an M-wave. SLR and LLR were recorded from the opponens pollicis muscle of the right hand. The frequency and amplitude ratio of SLR (latency, approximately 20-30 ms) and LLR (latency, approximately 40-70 ms) were analyzed. SEP was produced by electrical stimulation delivered to the median nerve. The amplitude from baseline and mean latency of N20 waves were measured. F-wave in the evoked electromyography was evoked by electrical stimulation of the median nerve of the right hand at supramaximal intensity to elicit an M-wave and recorded from the opponens pollicis muscle of the same hand. We analyzed the mean latency and calculated the CMCT using the mean latencies of LLR, N20, F-wave, and M-wave. [Results] The frequency and amplitude ratio of SLR were reduced by acupuncture stimulation of left- and right-sided LI4, respectively. LLR frequency and amplitude ratio were reduced by acupuncture stimulations on either side. A correlation was observed between changes in the LLR amplitude ratio and changes in the N20 SEP amplitude ratio induced by acupuncture stimulation. No effect of acupuncture stimulation was observed on CMCT. [Discussion and Conclusion] SLR is the reflex potential of the spinal cord, and LLR is the motor reflex of the central nervous system via supraspinal pathways. These findings suggest that acupuncture stimulation inhibits motor nerve reflexes via both spinal and supraspinal modulation systems.