Nociception
Three categories of acupuncture mechanisms have been described; peripheral, spinal, and supraspinal (Lundeberg 1998). Firstly, on needling, nociceptive afferents are stimulated and release vasodilatory neuropeptides into the muscle and skin they innervate, forming the basis of the local or peripheral effects of acupuncture (Sato et al 2000). This phenomenon, an axon reflex, releases neuropeptides into human skin such as calcitonin gene-related peptide (CGRP) and substance P (Weidner et al 2000). Sensory neuropeptides modulate immune responses and hence will assist in tissue healing (Brain 1997). Secondly, acupuncture will act within the spinal cord, known as spinal effects or segmental effects. To initiate spinal effects, the sensory stimulus must be applied to tissues that share an innervation with the appropriate spinal cord level (Fig. 1.1). Dorsal horn neurons activated by painful inputs may be inhibited by acupuncture via a gate control mechanism, producing a spinally mediated analgesic response. Neurons of the ANS efferent fibres can be influenced and both sympathetic and parasympathetic activity may be affected, depending on the position of the needles.
ā¢ High-intensity (HI) needling may immediately increase sympathetic outflow to tissues supplied by the segment, which is then followed by a decrease in outflow.
ā¢ Low-intensity (LI) or non-painful input could reduce sympathetic outflow in the segment (Sato et al 1997).
Lastly, acupuncture may influence alpha-motoneurons housed in the ventral horn of the spinal cord to alter reflex activity in muscles supplied by the segment (Fig. 1.1). At present the effect on motoneurons is still unclear: an immediate change in excitability has not been demonstrated in contrast to clinical observations (Chan et al 2004).
Supraspinal effects
Acupuncture can influence neuronal structures within the brain (Stener-Victorin et al 2002) and these are known as supraspinal effects. Analgesic pathways such as diffuse noxious inhibitory controls (DNIC) and beta-endorphin mediated descending pain inhibitory pathways from the hypothalamus will be activated with appropriate needling (Stener-Victorin et al 2002). Autonomic outflow is also under central control via the medullary vasomotor centre and can be influenced by the acupuncture stimulus.
Neurohormonal responses
Responses affecting the immune, endocrine, and reproductive systems of the body can be affected by acupuncture (Carlsson 2002, Stener-Victorin et al 2002; White 1999). Recent advances in brain imaging technologies such as functional magnetic imaging (fMRI) and positron emission tomography (PET) have allowed investigations of the brain and have elucidated the effect of acupuncture on the CNS. Several analgesic points in the extremities will stimulate blood flow to cortical and subcortical brain regions (Lundeberg 1998). Activation is relatively non-specific and closely related to areas activated by painful stimuli, through what is known as the pain matrix (Lewith et al 2005). Studies show an increase in blood flow in the hypothalamus (Table 1.1) and a decrease in the limbic system (Table 1.2), a brain region where affective and emotional responses to pain are integrated with sensory experience. However, most of the brain regions activated by acupuncture are closely related to those areas mediating placebo analgesia and expectation (Lewith et al 2005), and it is unclear how much of the change is due to the acupuncture stimulus and how much is due to non-specific effects. Recently studies using transcranial magnetic stimulation (TMS) have shown that acupuncture modulates motor cortical excitability and that the effect (excitation or inhibition) is specific to the investigated muscle and the site of needle placement (Lo et al 2005; Maioli et al 2006). Maioli et al (2006) showed that changes lasted for fifteen minutes following the removal of the needle stimulus, suggesting longer term plastic changes in motor cortical excitability.
Table 1.1 Suggested points to stimulate blood flow to hypothalamus
Meridian | Points |
Large intestine | LI4 |
Lung | LU5 |
Gall bladder | GB34, GB40 |
Spleen | SP6 |
Stomach | ST36 |
Liver | LIV3 |
Biella et al (2001); Fang Kong et al (2004); Hsieh et al (2001); Hui et al (2000); Wu et al (1999, 2002); Yan et al 2005; Zhang et al (2003)
Table 1.2 Suggested points for deactivation of limbic system
Meridian | Points |
Large intestine | LI4 |
Gall bladder | GB34 |
Spleen | SP6 |
Stomach | ST36 |
Liver | LIV3 |
Hsieh et al (2001); Hui et al (2000, 2005); Kong et al (2002); Wu et al (1999,2002); Zhang et al 2003