Introduction

With the multi-faceted regulatory issues surrounding opioids (opioid epidemic, mercurial availability issues and increased regulation incentivizing abusers to pursue veterinary sources), as well as the ugly underbelly of opioid agonists being increasingly revealed (opioid induced hyperalgesia, very poor absorption and utility of oral opioids in dogs, opioid-induced immune suppression and the neuro-inflammatory effects of opioids, to name a few), non-opioid options for treating pain become increasingly desirable. This opens the door to physical medicine techniques: defined as techniques that utilize and modify the endogenous chemistry of an organism to treat pain. At the forefront of these modalities, especially where pain is concerned, is acupuncture—a modality with deep roots and deep potential but plagued with equally deep misconceptions and consequent controversy.

Acupuncture describes the stimulation of distinct anatomic points, often with fine needles. Other methods of stimulation of these points (such as injections, laser, deep pressure massage, trigger point needling) exist, and are encompassed in the overall field of acupuncture-related modalities. The key differentiation between acupuncture with needles, and other treatments, is the source of force applied to the region of the acupuncture point—mechanical deformation (needle), thermal, electrical, chemical, photons, etc.

Acupuncture is Applied Anatomy

Acupuncture points have distinct anatomic underpinnings that can be understood based upon the integral homeostasis of living organisms. These points were found by trial and error as well as being in the vicinity of neurovascular structures (bleeding from these areas was important traditionally). They have since been linked into “lines” or “meridians” that partially relate to anatomic similarities, such as major nerve pathways, dermatomes, sclerotomes, or sensations related to axon anatomy, but partially are not explained by these. As an over-arching heading for the effects of acupuncture, it is most accurate to describe acupuncture as a tool that can help modify the endogenous homeostasis of the various body systems of an organism. The major homeostatic systems modified by acupuncture include nerves: sensory, motor and autonomic; neurotransmitter biochemicals; blood vessels; fascial sheets and lymphatic beds. It is a combination of these systems that provide organism homeostasis, and a combination of these effects, that arise from an acupuncture treatment.

Cutaneous Structures

Microscopic analysis repeatedly demonstrates that acupuncture points contain somatic anatomic structures such as: free nerve endings, encapsulated cutaneous receptors, and musculo-tendinous sensory receptors (muscle spindles and Golgi tendon organs). It has long been recognized that acupuncture points are found in cutaneous regions that show high levels of diverse innervation (Zhao 2008). This occurs where groups of nerves emerge through bone, muscle and fascia from their origins, where they branch or join, and where they attenuate distally. These nerve fibers are made up of Aβ, Aδ (myelinated) and C (non-myelinated) fibers, as well as autonomic fibers, and acupoints generally have greater neural density compared to non-acupoint regions (Zhang 2012).

These peripheral components help to explain why neuromodulation is the primary pathway for acupuncture mechanisms, and these components occur in the skin as well as along peripheral nerve bundles. Acupuncture stimulates afferent nerve fibers. This has been shown with both manual acupuncture as well as electroacupuncture. Both manual stimulation and electrical stimulation have been found to stimulate all four types of nerves: Aα Aβ, Aδ and C-fibers, although these occur with unique discharge frequencies.

With insertion and rotation of an acupuncture needle, collagen fibrils pull on the associated fibroblast, causing remodeling of the cellular structure within 10 minutes, as well as release of purines from both the fibroblasts and the mast cells that are caught up in this wave of mechanical stretching. This is followed within 90 minutes by increased mechano-transduction and up-regulation of genes related to muscle and sensory homeostasis. Unlike other forms of collagen deformation, such as massage and stretching, the acupuncture needle creates a microscopic “whorl” of collagen within the connective tissue, which may result in a biochemical modification and changes in connective-tissue tension regulation that can last for hours to days, as compared to other forms of stretch (Gray 2014).

Corridor Structures

Fascia

Changes in the “loose” connective tissue of the body have a dramatic influence on fluid movement through the tissues, as the appendicular connective tissue is the home of the lymphatic system. The lymphatic system is the home of immune homeostasis, as is demonstrated by the coalescing of lymphocytes and other immune cells into the lymph nodes along these lymphatic chains. Thus, the fibroblasts, and by extension acupuncture, are integral to immune homeostasis as well as neurological processing homeostasis (Yin 2018).

When an acupuncture needle is placed, a dynamic and self-sustaining change occurs in the fascia, and this can be identified microscopically but also by measuring chemical mediators and transcription. This change likely ripples along the fascial network of an organism, creating far-reaching change. Of note, this impact also self-sustains, as fibroblasts will pull on their neighbors, changing the community neurochemistry and creating a new army of activated fibroblasts to subsequently pull on their neighbors, spreading the signal over both space and time (Gray 2014). Thus, the fascia provides a critical component of the efficacy of acupuncture treatment. In the periphery this is primarily tied into the microcirculation and microscopic nerve and vasodilatory influences. Along the body this is tied more into a macroscopic structure, tying together nerve and motor communication and body awareness, and providing for the delivery and removal of life-sustaining fluids to the extra-vascular body compartments.

Trigger Points

Even for acute pain conditions, trigger points in muscles are a common and important component of treatment. In addition to named points that are treated proximate to the pain, there are points proximal and distal to the pain along the nervous system, and points that are primarily homeostatic, as well as paired points on the opposite limbs. Needling these regions will disinhibit muscles that are restricted by poor bloodflow, algogenic substances, and reduced function. This is accomplished through inserting needles into the damaged, contracted region to bring in nutritive blood-flow, reduce pain, and restore function and collaboration with neighboring muscle groupings (Tang 2018).

Reflex Loops

In addition to the release of trigger points from muscle groups, acupuncture can modify the sensory and reflex-loop portion of the motor unit. The sensory portion of muscles are the muscle spindles, and these are located diffusely throughout motor units. These sensory structures are more abundant in muscles that have a larger proprioceptive role. Acupuncture over muscles can interact with the muscle spindle units, both modifying the sensations experienced by the muscles, and also interacting with the reflex loops that increase or decrease muscle contraction in the presence of stretch or fibrosis.

Golgi tendon organs are deep structures, and like the muscle spindle points, they have use in managing musculo-skeletal conditions where pain, damage or musculotendinous shortening has taken place. Needle placement in points with strong tendon input will feed into the reflex relaxation of the associated myotendinous structures and produce an immediate reduction in local pain.

Any one acupuncture point may generate input from both cutaneous sources and deeper sources, such as trigger points, muscle spindles and Golgi tendon organs. In general, data exist to indicate that deeper needling generates a greater biological response, so a summation of input from a combination of both cutaneous and deeper structures is likely ideal. At the same time, any needled point carries the cutaneous sensory and fascial input.

This input is part of why acupuncture performs poorly in placebo-controlled studies, as some biological response will be seen whether a true acupuncture point or any other point on the body is used. The responses to sham versus verum acupuncture have been shown to differ in magnitude, as acupuncture points are universally located in regions with potent neurochemical underpinnings, but differences in magnitude are difficult to detect in clinical studies.

Spinal Structures

The dorsal horn of the spinal cord is the receiving zone for afferent impulses ascending from the periphery. Significant diversity of receptors, pain fibers and neurochemical compounds contributed to the magnitude and type of signal being seen at the dorsal horn. At the level of the dorsal horn, this signal can be transduced to nerve tracts that ascend to the central nervous system, but there is vast opportunity to modify this signal here as well.

At the dorsal horn, a variety of biochemicals can modify the likelihood that the signal will cross the synapse and create an action potential in the second order nerve. Moment-to-moment modulation occurs through changes in GABA, serotonin, norepinephrine, CGRP, SP, endorphins, and cannabinoids. Acupuncture effects emanating from the periphery have been shown to exert at least some modification on each of these compounds in various laboratory studies (Zhao 2008). Acupuncture has been shown to influence the activity of glia, potentially reversing some of the negative effects of glial stimulators (like the opioids) and decreasing the long-term central neuro-inflammation resulting from pain and opioid treatments alike.

In addition to the modifications possible at the first synapse, the topography of the spinal cord provides for interneurons that can inhibit or amplify the incoming afferent signal. Concepts such as diffuse noxious inhibitory control (DNIC) are being utilized to assess amplified pain states as well as to detect therapies, such as acupuncture, that work to decrease central pain amplification. These can be modified by the local neurochemical milieu but also by descending input from the midbrain. Acupuncture input has also been shown to aid in the modification of descending inhibitory mechanisms, and this method of testing the nervous system may significantly improve the clinical data needed to demonstrate acupuncture efficacy. The somatotopic layout of the spinal cord is integral to the function of acupuncture to modify deep tissues and organs. Deeper tissues can be modified by interacting with somatic sites that share the same spinal innervation network. Examples of this used by modern medicine include “sea bands” for nausea (over PC-6 acupuncture point) and electrical stimulation over the tibial nerve (KI-3 acupuncture point) to aid in urinary retention.

Modification of autonomic outflow is most likely near accessible portions of the sympathetic chain and parasympathetic ganglia. Examples of sympathetic proximity include the cervico-thoracic junction (start of the sympathetic chain) and the lumbosacral junction (sacral sympathetic outflow). Examples of parasympathetic ganglia are in the thoraco-lumbar region (stellate) and at the thoracic inlet (cranial cervical ganglion). In addition to modification of autonomic function through the vagal nerve, acupuncture can influence sympathetic outflow at certain points and parasympathetic outflow at others. Spinal reflex loops, in addition to the modification of sympathetic/parasympathetic balance more globally, have been shown to contribute to some of the organ effects seen with acupuncture, such as regulation of cardiac activity.

Brain Structures

Studying acupuncture’s central effects has traditionally been limited to laboratory animals, due to the invasiveness of such studies. Decades of data show influences on neurotransmitters (as we previously discussed regarding the spinal cord), especially endogenous opioids. This hasn’t answered the lingering questions about why acupuncture appears so effective for mood and behavior. However, neuroimaging studies have come of age and have provided vast amounts of data, although there remains controversy as to how to value and interpret these data. In general, imaging studies have shown complex activation and deactivation of many areas of the brain. In general, verum acupuncture needles have shown a larger effect than sham, and having a deep needling treatment with adequate tissue grab (deqi) appears important.

The periaqueductal grey and ventrolateral medulla show consistent responses to acupuncture for pain. This is one of the important effects when using acupuncture in acute and peri-operative pain.

References

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