Local (nerve) effects of acupuncture
Acupuncture stimulation involves stimulation of myelinated Aβ (mechanoreceptor) and Aδ (nociceptor: sharp focussed pain) nerve fibres. These nerves suppress thin, non-myelinated C fibres (nociceptor responsible for dull, burning, aching pain). This mechanism is called the Gate Control Theory. (Melzack and Wall 1965) TENS (Transcutaneous Electrical Nerve Stimulation) and massage are thought to have this mechanism as their main mode of action and therefore have relatively short lasting effects. The Gate Control Mechanism probably plays some role in acupuncture but it has many longer lasting modes of actions as well. (covered later)
Acupuncture mostly targets the facia, connective tissue and nerves. After insertion, needles require activation by winding them, which is thought to wind collagen fibres around the needle and this can lead to a short pulling sensation. This technique is only advised on horses and dogs, not in cats as they do not tolerate it. In humans this activation is often felt like a dull aching, or a feeling of pressure, or tingling, and it is thought to increase efficacy of treatment. (Langevin et al. 2002)
In a study in mice, it has been found that stimulation near an inflamed ankle, reduces neuropathic pain and inflammation. It was hypothesised that locally acting substances, such as adenosine released as a result of needle stimulation, are the main cause.(Goldman et al, 2010)
Another study found acupuncture reduced allodynia and nociceptive behaviour. They also found that gene expression was changed in 236 genes around the acupuncture site. This change in gene expression leads to changes inside cells as well as in the extracellular environment. This in turn is thought to lead to local analgesic effects as well as conduction of signals through ascending nerves. (Park et al. 2014)
Spinal cord effects:
When pain signals reach the spinal cord they are magnified by Wide Dynamic Range Interneurons, causing an increased discharge of dorsal horn cells. This causes a reduced threshold, amplification of pain signals, as well as continuous activity, independent of stimulation. This process is called central wind up or central sensitisation. In this case the peripheral neurons behave normally but the spinal neurons are hypersensitive.
Electro acupuncture causes endogenic opioids such as enkephalins, endorphins and dynorphin to be released in the spinal cord, reducing central sensitisation. (Kim et al, 2009). It also counteracts hyperalgesia by preventing the activation of the N-methyl-D-aspartate (NMDA) and nitric oxide (NO) in the spinal cord. (Garrido- Suarez et al 2009)
Effects on the Autonomic Nervous System:
The parasympathetic nervous system is involved in conservation and restoration of energy and nutrients and reduces the blood pressure and heart rate. It facilitates absorption of nutrients and the excretion of waste products. The sympathetic nervous system on the other hand induces the fear fight flight mechanism and increases heart rate, blood pressure and cardiac output.
The autonomic nervous system will often not function optimally during stress. During stressful situations the sympathetic tone can increase by more than 300%. This excessive sympathetic tone can lead to T-cell proliferation, natural killer cytotoxic response and interleukin2 and interferon γ production. (D’Arbe et al 2002) There is evidence that acupuncture can regulate the activities of certain areas in the brain that have an autonomic nervous system imbalance. This is probably the reason why acupuncture in humans has shown to effectively treat cardiovascular disease, epilepsy, anxiety, cardiac disorders, nausea, etc. by correcting autonomic dysfunction. (Li et al, 2013)
The mechanisms by which acupuncture targets the autonomic nervous system, influencing various organs is not completely understood. Most likely these occur through somatovisceral reflexes. Somatovisceral reflexes occur when (acupuncture) stimulation of a certain area cause nerve activity of a sympathetic efferent nerve and thereby change organ function. (Sato A, 1995) It is by this same mechanism that we can feel referred pain from a particular organ, in particular areas in the skin. This mechanism can be utilised by stimulation of the following nerves:
- The spinal neuro-vascular bundle emerging between the longissimus and iliocostalis muscles on the back
- The distal spinal nerve branch on the ventral midline or the costochondral junction.
The brain: The Limbic system and the Default Mode Network (DMN)
The limbic system deals mainly with emotions, memory and arousal. It is located above the brainstem and connects it to the cerebrum. It contains the thalamus (relaying info from senses), the hypothalamus (regulating autonomic nervous system) and the amygdala (involved in memory and fear). The Default Mode Network is involved in stress, mental illness and chronic pain. Recent studies have found that acupuncture is associated with deactivation of the limbic system (Hui et al, 2010) This is one of the most important mechanisms that explains why acupuncture can have a positive effect on chronic pain, fear and anxiety and the autonomic nervous system. Animals have a relatively large limbic system. This is probably why they often respond so strongly to acupuncture by literally falling asleep during treatment, and often show a good response to acupuncture stimulation
Electro-acupuncture for nerve injury
Peripheral nerve injuries can result in (partial) loss of sensory, motor and autonomic function due to interruption of axons and the degeneration/death of distal nerve fibres/neurons. (Allodi et al. 2012) Compensation takes place due to regeneration of injured axons or collateral branching of undamaged axons. Plasticity of central connections may compensate for lack of targeted re-innervation but has little effect on sensory localisation or fine motor control, and may result in neuropathic pain or hyperreflexia (Navarro, 2009)
One of the main limiting factors in nerve regeneration is lack of specificity of nerve regrowth through path-finding and target re-innervation. In experiments with rodents, nerve-crush injury was studied and it took about 3 weeks before regeneration started. Allodynia set in at the same time and took a long time to resolve (Vogelaar et al. 2004)
Electro-acupuncture has in various studies shown to improve regaining of neurological function as well as preventing allodynia. (Ho et al. 2013) Having said this, not all electrical stimulation seem to be beneficial. Transcutanious electrical nerve stimulation (TENS) actually seems to delay regeneration (Baptista et al. 2008)
Acupuncture has local, efferent nerve, spinal cord and brain effects, and has shown to alter gene expression involved in the development of chronic pain. The effects of treatment often build up and the treatment interval can often be reduced to once monthly or less. This is contrary to manual manipulation or TENS which only work via the gate control mechanism and usually need continual weekly treatments. The different levels on which acupuncture can affect the nervous system also explains the wide range of conditions that can be treated. Lastly, electro-acupuncture has shown to have a positive effect on regeneration of functional nerve cells as well as the prevention of allodynia and neuropathic pain.
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- Langevin HM, Yandow JA (2002) Relationship of Acupuncture Points and Meridians to Connective Tissue Planes. The Anatomical Record (NEW ANAT.) 269:257–265.
- Goldman N, Chen M, Fujita T, et al (2010) Adenosine A1 receptors mediate local anti- nociceptive effects of acupuncture. Nat Neurosci. 13:883–8
- Park J-Y, Park JJ, Jeon S, et al (2014) From Peripheral to Central: The Role of ERK Signaling Pathway in Acupuncture Analgesia. J Pain. 15(5): 535–549. doi:10.1016/j.jpain.2014.01.498.
- Garrido-Suárez BB1, Garrido G, Márquez L et al (2009) Pre-emptive anti-hyperalgesic effect of electroacupuncture in carrageenan-induced inflammation: role of nitric oxide. Brain Res Bull. 79(6):339-44. doi: 10.1016/j.brainresbull.2009.04.014. Epub 2009 May 3.
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- Li Q, Shi GX, Xu Q, et al (2013) Acupuncture Effect and Central Autonomic Regulation. Evid- Based Complement and Altern Med, Volume 2013, Article ID 267959, 1-6. http://dx.doi.org/10.1155/2013/267959
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- Hui K, Marina O, Liu J, et al (2010) Acupuncture, the Limbic System, and the Anticorrelated Networks of the Brain. Auton Neurosci; 157(0): 81-90.
- Allodi I, Udina E. and Navarro X. (2012) Specificity of peripheral nerve regeneration: interactions at the axon level. Prog. Neurobiol. 98, 16-37.
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- Vogelaar CF, Vrinten DH, Hoekman M.F, et al (2004) Sciatic nerve regeneration in mice and rats: recovery of sensory innervation is followed by a slowly retreating neuropathic pain-like syndrome. Brain Res. 1027, 67-72.
- Baptista AF, Gomes JRS, Oliveira JT, et al (2008) High- and low-frequency transcutaneous electrical nerve stimulation delay sciatic nerve regeneration after crush lesion in the mouse. J. Peripher. Nerv. Syst. 13, 71-80.