Acupuncture: The Scientific Perspective

Acupuncture: The Scientific Perspective
Western Veterinary Conference 2006
Narda G. Robinson, DO, DVM, DABMA, FAAMA
Colorado State University College of Veterinary Medicine and Biomedical Sciences
Fort Collins, CO, USA

Objectives

  • Describe how acupuncture treats pain, reviewing pain modulation pathways and the effects of acupuncture on the peripheral, central, and autonomic nervous pathways.
  • Describe how acupuncture treats systemic dysfunction (organ, immune, and circulatory disorders) by extending the neurophysiologic discussion to these areas.

Key Points

  • Acupuncture treats pain via activation of a series of neuromodulatory effects engendered by mechanical stimulation following introduction of an acupuncture needle into precise locations on the body that demonstrate specific neurovascular characteristics.
  • Pain relief occurs through segmental and non-segmental effects which dampen pain transmission and the conscious recognition of pain.
  • Acupuncture stimulates physiologic restoration mainly by its effects on the autonomic nervous system and reduction of sympathetic tone.

Overview

Acupuncture affects the nervous system in many complex ways. Not all of the neurophysiologic effects of acupuncture are understood. However, describing the local and systemic effects that are understood, using present-day scientific terminology rather than the metaphoric and archaic language from ancient China, improves communication and understanding of the mechanisms involved. Contrary to popular belief, there is no evidence that acupuncture works by moving visible energy through invisible lines on the body called meridians. Unfortunately, most practicing acupuncturists in the West mistakenly believe that acupuncture affects this energy, which has created an unfounded mystique and unfair dismissal of this ancient medical practice. The idea that energy travels through invisible lines near the body surface arose as a result of a mistranslation of the Chinese word “qi” into “energy”, rather than “vital air”, and of “jingluo” into “meridians” rather than a system of vascular channels. It was George Soulié de Morant, a French bank clerk working in China, who provided some of these earliest translations of Chinese acupuncture texts in the 1930’s, though his errors in word selection still haunt us today. Because Soulié de Morant lacked a medical background but instead held a personal belief that acupuncture worked through pathways other than standard physiologic means, he created a metaphysical foundation for acupuncture that not only failed to exist previously, but one that would misinform future acupuncturists for generations.1 2 What follows is a physiologic description of the effects of acupuncture, starting with the insertion of an acupuncture needle, following the various routes and reflexes that produce changes in local and systemic function.

The Acupuncture Point and Its Morphological Relationship to Nerves

Acupuncture points occur at over 360 sites found on the human body. Until only recently, however, there were no veterinary atlases with lines and dots depicting acupuncture channels and points. The ancient acupuncture atlases of large animals only described locations of certain isolated points over vessels that were used for bloodletting. Acupuncture points have a higher density of neural and fine vascular components than do non-acupuncture points; ninety-five percent of all points have a nerve trunk or large nerve branches within 1.0 cm of the point.3 Acupuncture points also occur near blood vessels. Thus, needle stimulation of acupuncture points often excites both somatic afferents and autonomic (predominantly sympathetic) nervous system fibers contained within the walls of blood vessels.

Tissue Responses to Needling

The introduction of a thin, sterile, metal acupuncture needle into body tissue provokes inflammatory, immunomodulatory, and circulatory changes at the site. In humans, a visible reaction often appears on the skin surrounding the shaft, characterized as a small flare response, occurring shortly after insertion. This tissue response involves nociceptors, sympathetic afferents, and an interaction between the blood coagulation system and the immune complement system. Cutaneous immunity displays functional integration between mast cells, bradykinin, somatic afferents, and sympathetic fibers associated with blood vessels. The neural, vascular, and immunologic elements associated with the skin provide the anatomic and physiologic substrates through which acupuncture needling stimulates ascending (dromic) neurologic events in the central nervous system (CNS), and antidromic changes in the form of axon reflexes. Eventually, needling affects the hypothalamic-pituitary-adrenal axis.

Acupuncture needles have a fine, tapered tip, in contrast to the abrupt beveled edge of hypodermic needles. Nevertheless, inserting acupuncture needles does cause a small amount of tissue damage that activates neurologic responses and healing activity. Cell products produced by the trauma of needling consist of collagen fibers, elastic fibers, fibroblasts, adipocytes, and mast cells.1 Collagen, microfibrils, and pieces of basement membrane activate Hageman factor (Factor XII of the blood coagulation pathway). Activated Hageman factor initiates the blood coagulation pathway, and results in the production of plasmin, which triggers the alternative pathways of the immune complement system. The complement pathway causes vasodilation at the needling site, helping clear the products of needle-damage. Activation of the complement pathway by plasmin results in the production of more C3, and C5. C3 and C5 are potent vasodilators that cause mast cells (in tissue) and basophils (in plasma) to degranulate, releasing histamine, heparin, and cytokines that attract neutrophils and eosinophils to the needling site.

The phases of needling reaction can be separated into six discrete, time-dependent intervals:1

  1. Capillary dilation at the needling site, initiation of a nonspecific immune defense response.
  2. Nociceptor stimulation and sensory afferent neural stimulation.
  3. Immune cell attraction to the needling site resulting from cytokine release.
  4. Local release of blood solubility products which improve blood flow.
  5. Tissue repair stimulation to heal the microtrauma caused by needling.
  6. Control phase comprised of both local tissue processes and supraspinal control, which inhibits inflammation, normalizes blood flow, and inhibits sensory afferent fibers.

Neural Receptors

Somatosensory receptors associated with the mechanical introduction of an acupuncture needle include mechanoreceptors and nociceptors. Sensory receptors transduce the physical stimulus of needling into an electrical signal (“receptor potential”) that encodes the physical stimulation and local tissue damage into neural information. This neural signal can then be further propagated throughout the nervous system as an action potential, or remain a receptor potential that modulates action potential frequency.

The most prominent subjective responses to needling are local soreness, distention, numbness, pinprick pain, and sometimes a tingling sensation that seems to travel in a longitudinal direction. Nociceptors convey information related to the pinprick sensation (via lightly myelinated skin A-? and tissue Group III fibers) and the slower, duller pain following release of inflammatory mediators (communicated via unmyelinated skin C and tissue Group IV fibers). Muscle spindle afferents participate in the reflex known as “propagated sensation” (described below).

Spinal Cord

Nociceptors deliver information from sensory afferent neurons to second-order neurons in the dorsal horn of the spinal cord. These second-order neurons may communicate with neurons at other levels of the spinal cord via the propriospinal tract. Axons from neurons in the propriospinal tract interconnect at several levels of the spinal cord. These axons may traverse the entire length of the spinal cord. For example, the rat spinal cord contains propriospinal neurons that project from the C1-C2 spinal segments down to the lumbosacral cord segments.4 Interconnections from afferents of different sources and segments produce the phenomenon called “propagated sensation” (PS). PS is a propagated muscle reflex mediated by the propriospinal system, and gives the sensation of a tingling sensation that is “traveling” along the body surface. Practitioners treating human patients may seek to elicit this sensation, as they believe it enhances the clinical effectiveness of needling therapy.1

Brain

Signals ascend from the spinal cord to the pons, medulla, and midbrain, thalamus, and hypothalamus, via various spinal pathways. The spinothalamic tract carries information related to the conscious recognition and localization of painful stimuli. Fibers in the spinoreticular tract are important for changes in attention level in response to pain. Presumably, it would be this pathway that is most closely involved in positive effects seen in patients regaining consciousness after coma, and perhaps in animals successfully resuscitated with strong stimulation of GV-26, in the cartilage underlying the nasal philtrum. Spinomesencephalic fibers synapse in centers in the brainstem that activate endogenous pain control pathways that participate in pain modulation by releasing neurotransmitters (endogenous opioids) within the dorsal horn of the spinal cord. This helps to suppress the transmission of pain information by spinothalamic neurons. Finally, there are nociceptive projections to the hypothalamus via the spinohypothalamic tract. The hypothalamus is involved in systemic autonomic nervous functioning, emotional responses to pain, drive-related behaviors, pituitary function, and homeostatic function (feeding and drinking behavior, temperature regulation, gut motility, sexual activity, etc.).

Technological advances now allow us to visualize brain function in response to stimuli, via functional brain imaging techniques such as functional magnetic resonance imaging (fMRI). A developing database connecting acupuncture points with their cortical activation sites offers an intriguing visual display of the ways in which acupuncture needling influences brain function. Because fMRI images reflect minute changes in site-specific cerebral blood oxygenation, acupuncture must therefore be capable of influencing vascular supply to neural centers based on a complex assortment of somatic and autonomic reflexes, beginning at the periphery and arriving at the cerebral cortex. Determining the commonalities in arterial supply of the main acupuncture channels and the brain may shed some insight into the mechanisms underlying these relationships.1

Autonomic Nervous System

Somatovisceral reflex responses have been measured in several internal organs and major vessels, resulting from cutaneous, muscular, and articular sensory afferents.1 Efferent effects occur in both the sympathetic and parasympathetic nervous system arenas. The specific type of somatovisceral reflex that occurs depends upon which organ is stimulated, along with which acupuncture points and method of stimulation the practitioner selects. The different factors involved may relate to the presence of functionally distinct reflex pathways unique to each organ and its vascular supply.

Investigations into the effects of acupuncture point stimulation on certain organs, have demonstrated spinal cord convergent input for the heart, stomach, gallbladder, bladder, and uterus1. Acupuncture can also have general effects on overall sympathetic and parasympathetic function, which can then alter autonomic tone throughout the body and, consequently, internal organ function.

Additional Detail

Research at Colorado State University is currently underway to ascertain the precise, neuroanatomically based acupuncture point locations in non-humans, in order to develop accurate transpositional veterinary acupuncture databases and atlases.

Summary

Acupuncture offers a unique and effective means of stimulating endogenous pain control mechanisms and restoring homeostasis. In order to best understand and improve the clinical effectiveness of acupuncture treatments, a working knowledge of the local and systemic neurologic ramifications of acupuncture needling is vital. Informed consent involves effectively communicating to clients about the ways in which a treatment works, along with its safety and benefits profiles. Basing discussions about acupuncture on science, rather than metaphysics, facilitates communication and deepens understanding of this ancient medical intervention.

References

1. Kendall DE. Dao of Chinese Medicine. Hong Kong: Oxford University Press, 2002.

2. Unschuld PU. Chinese Medicine. Brookline, MA: Paradigm Publications, 1998.

3. Shaozong C. Modern acupuncture theory and its clinical application. International Journal of Clinical Acupuncture. 2001;12:149-157.

4. Miller KD, Douglas VD, Richards AB, Chandler MJ, and Foreman RD. Propriospinal neurons in the C1-C2 spinal segments project to the L5-S1 segments of the rat spinal cord. Brain Research Bulletin. 1998;47:43-47.

5. Windhorst U and Kokkoroyiannis T. Interaction of recurrent inhibitory and muscle spindle afferent feedback during muscle fatigue. Neuroscience. 1991;43:249-259.

6. Bergenheim M, Johansson H, and Pederson J. The role of the gamma-system for improving information transmission in populations of Ia afferents. Neuroscience Research. 1995;23:207-215.

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