In order to be widely accepted by both patients and physicians, the beneficial effects of acupuncture should be proven by our Western medicine terminology and methodologies. Acupuncture treatment involves the insertion of thin needles into the skin and underlying muscle layer. In a traditional acupuncture procedure, the inserted needles are twisted right and left at 0.5-1 second intervals. Most recently, acupuncture needles are stimulated by electricity under various frequencies of 1-100 Hz (electro-acupuncture; EA). Thus, the procedure may stimulate the somatic afferent nerves of the skin and muscles (Takahashi 2011).
The spinal–supraspinal pathways responsible for somatosensory stimulation mainly comprise the posterior spinal cord column pathway and the spinothalamic pathway. These impulses are further relayed to the thalamus, and ultimately sent to the primary somatosensory cortex. In addition, these impulses are also relayed to other brain areas, including the brain stem, periaqueductal gray (PAG) and hypothalamus, via collateral connections.
The nucleus tractus solitarius (NTS) is the primary brain stem relay for visceral information from cardiovascular, respiratory and GI systems. In addition, recent studies indicate that the NTS also receives somatic afferent inputs. Neurons of the NTS and the spinal cord were labeled by injection of neuroanatomical tracers to the acupoints of hind limb (Stomach-36; ST-36) in rats.
Somatic stimulation induced by acupuncture is conveyed to the NTS. The NTS is adjacent to the dorsal motor nucleus of vagi (DMV) and both compose the dorsal vagal complex (DVC). The DVC integrates vago-vagal reflex which plays a major role in regulation of GI motor function. The NTS neurons also project to the rostral ventrolateral medulla (RVLM) of the brain stem. The RVLM neurons provide drive to the sympathetic preganglionic neurons in the intermediolateral nucleus of the spinal cord (Tada 2003).
When NTS neurons are stimulated by acupuncture, autonomic nerve function is altered via the activation of the DMV and/or RVLM.
In addition to the NTS, DMV and RVLM, acupuncture may activate some other nuclei in the brain, such as PAG and hypothalamus (Iwa 2007). Once the PAG is stimulated, opioid is released. Once the hypothalamus is stimulated, oxytocin (OXT) is released. Opioid and OXT mediate analgesic effect and anti-stress effect.
It has been well demonstrated that analgesic effect of acupuncture is mediated via an endogenous opioid (Stux et al., 1995; Diehl, 1999) and that anti-stress effect of acupuncture is mediated via an endogenous OXT (Yoshimoto 2012).