The classical theories of acupuncture are thousands of years old; dating back to the primitive society--prior to the 22nd century BCE (1). For the next 17 centuries different theories were developed as humans tried to understand the world in which they lived. Theories such as naturalism, which provides the root of Yin and Yang for all Chinese medical practices, Daoism, which tried to understand and explain how nature works, and Confucianism which strived to define man's role in society and commitment to future generations, were all prevalent during these centuries. When one looks at current Chinese medical philosophy the influence of all three of these philosophies can be seen. As we move into the 5th century BCE Chinese scholars recorded their observations and ideas about the inner working of the physical body. The vast majority of the literature is regarding humans but it is understood that veterinary medicine applied virtually the same principles to animal species. During the Shang Dynasty (16th-11th century BCE) animals were an important part of daily life, especially horses and oxen. During the Western Zhou Dynasty to the Spring and Autumn period (11th century-477 BCE) veterinary knowledge was developed further and veterinarians were full-time employees of government.(1) During the Han Dynasty much of the classical literature was written for both human and veterinary Chinese medicine. The classic work, the Huang Di Nei Jing (The Yellow Emperor's Classic of Internal Medicine), was written and this provides the basis for not only human acupuncture and Chinese medicine but also veterinary.(1)
As we explore the new modern understandings of acupuncture we must remind ourselves and respect the evolution of this medical system rooted in Chinese philosophy; a philosophy born through centuries of objective observation of nature, humans and animals; studying cause and effect, response to certain stimuli, such as environmental changes and internal alchemy. As one studies Chinese medicine often one discovers overlap between ancient Chinese medical ideas and current western physiology. An example, one Chinese medical theory states that the Kidneys are the mother of the bone and are important in the formation of blood. Before 1980 any western trained physician would have viewed this statement with partial skepticism. The kidney's role with bone and calcium and phosphorus regulation could be understood, but how was the kidney relevant to the production of blood. With the discovery of erythropoietin we now have a direct link between the kidney, the bone marrow and the production of blood.
Acupuncture points (AP) have been most consistently described as areas of lower skin electrical resistance (LSR).(2,4) The normal electrical resistance of the skin is 300-2000 kOhms; the measurement of acupoints is between 7-70 kOhms. Compared to the surrounding skin the acupoint is also characterized by an increased sensitivity to pressure and higher temperature.(2) There is no one unique characteristic of all AP. In general they are closely associated with cutaneous nerves; they are supplied by a high number of free nerve endings, nerve bundles, plexi; contain mast cells, lymphatics, capillaries and venules.(4) The two most common characteristics are an association with a nerve or a nerve apparatus--bundle, Golgi tendon, motor point(4, Egerbacher 3), etc. Many have a similar connective tissue structure; a thin walled vascular structure organized in closed loops surrounded by a network of unmyelinated cholinergic nerves (autonomic nervous system).(4) Electron microscopy has also shown a high concentration of micro vesicles and perineural cells situated at a contact zone between sympathetic nerve terminals and large blood vessels.(4) Hemoacupuncture was also a prominent part of classical use of acupuncture and many points do reside over or on blood vessels. (One study showed that 71% AP had significant vessels beneath them.)
Another type of important point that was first described by Travell and Renzler was the Trigger Point. In classical terminology many of these points could also be termed Ah shi points. Ah shi points are non-specific acupoints that reflect an abnormality in some part of the body. Trigger points are also non-specific points that consist of chronically contracted muscle spindle fibers, they can be highly sensitive to pressure and pressure results in pain in adjacent or remote (referred) areas. Needling the Ah shi point or the Trigger point will result in the relief of pain.
Although acupuncture had been used for thousands of years the anatomic structure that is the meridian has not been definitively found. There are many proposals of what the classical Chinese scholars were referring to but there is no one study that can define a single anatomic entity. Numerous studies have used the concept of LSR to trace the meridians.(2,5,6) Additional studies have measured the propagated sensation along channels.(7) The measured speed of the propagated sensation is 1-10 cm/sec; this is slower than the slowest C fibers; 0.5-2m/ sec.(4) A recent study in sheep measured the acoustic emission signals (AES) propagated along 14 meridians and found similar readings; 6.67 cm/sec.(8) This study also tried to determine the correlation of the AES and specific tissues. Three different acupoints were selected and the AES at the most distal point was measured as the most proximal point was stimulated. Different tissue layers were transected at the center acupoint and the AES recordings were determined after each transection. The AES was adversely affected at each tissue layer transection but was most affected when the muscle/nerve layer was transected. The Huang Di Nei Jing states that the meridians are located among muscles; this study would confirm this ancient text.
Another recent study has found a structure that resides within lymphatic vessels that could be an anatomic representation of the meridians.(8) In the 1960's a Korean researcher, Bong Han Kim, discovered a special anatomic structure at AP and also an anatomic structure within the lymphatic vessels that followed meridian trajectories. He reported that the ducts floated in the lymphatic and blood vessels and on the surface of internal organs.(9) Unfortunately, their were no photographs and for years the study was not reproducible. In recent years two additional studies were finally able to show the existence of these structures with the use of special stains.(10,11) This most recent study also confirms the presence of a structure within lymphatic vessels using fluorescent magnetic nanoparticles. Due to the nature of the nanoparticles it is postulated that they will be able to give additional information about the function of these structures as well as their location. Some authors have contended that the ancient Chinese scholars referred to the blood vessels when they spoke of meridian trajectories.(12) These recent studies and further studies on the Bonghan ducts may provide evidence of that if they are able to show the structures within blood vessels.
An additional proposed theory is the Liquid Crystalline Collagen Matrix System proposed by Dr. Mae-Won Ho. Dr. Ho proposes that the acupuncture system and the direct current bioelectrical body field are both located in part in the continuum of the liquid crystalline collagen fibers that constitute the majority of connective tissue.(13) Bound water layers on the collagen fibers provide proton conduction pathways for rapid intercommunication throughout the body, enabling the organism to function as an integrative circuit.
The classical tenants of Chinese medical theory instruct that when one places a needle in the correct point(s) Qi and Blood will be regulated; areas of stagnation can be relieved and the Organs function normally. A variety of studies have been done to try and explain how acupuncture works. Probably the earliest most well known is the Gate Theory proposed by Melzack and Wall in 1965 to explain acupunctures effect on decreasing pain. The basic proposal was that acupuncture stimulated the rapidly transmitting nerve fibers (A-delta, beta) and this signal reached the substantia gelatinosa of the dorsal horn synapsing on interneurons and thus "closing the gate" on any signals being transmitted by the slower C-fibers. These faster signals would also provide regional analgesia via Lissauer's tracts as the signals were relayed cranially and caudally to the adjacent spinal segments. This theory accounted for some of the effects of acupuncture but not all. In 1968 Melzack and Casey modified the theory allowing for central control triggers activated by input to higher centers that would also modulate activity in the dorsal horn. Part of the mechanism of acupuncture is known to be dependent on a functional nervous system. If an acupoint is injected with a local anesthetic, analgesia will not result from stimulation of that point, nor if acupuncture is performed in a paralyzed limb of a para- or hemiplegic.(4) Propagated sensations are blocked by procaine, opiates, naloxone and sometimes pressure, indicating the role of the nervous system and neurotransmitters in acupuncture.
Various neurotransmitters have been associated with acupuncture stimulation. Naloxone has been shown to reverse the effects of acupuncture analgesia in some studies, indicating an opiate mechanism. Acupuncture will stimulate the release of growth hormone through stimulation of opiate pathways. It stimulates the release of prolactin, oxytocin and luteinizing hormone, and can modulate thyroid function.(4) Pomeranz produced much evidence that serotonin (5-hydroxytriptamine, 5-HT) was important in mediating the effects of acupuncture. Following acupuncture stimulation the concentration of 5-HT s increased 30-40% in the systemic circulation; the concentration of 5-HT and a metabolite in the brain and spinal cord of rates indicated increased synthesis and utilization of 5-HT; and drugs that block 5-HT receptors also block acupuncture analgesia.
Noradrenaline, a monoamine with site dependent roles in acupuncture analgesia. In certain regions of the brain it antagonizes acupuncture analgesia, but in the spinal cord it is essential for analgesia. It exerts an inhibitory effect as it binds to receptors of nociceptive fibers in the dorsal grey matter--gamma neurons and preganglionic sympathetic fibers. This effect is blocked by naloxone.
Other neurotransmitters have also been shown to be affected by or to affect acupuncture analgesia(4):
Acetylcholine release is stimulated from the hypothalamus with acupuncture; may in turn release endorphins; Ach can dilate blood vessels in spastic or ischemic muscles.
Atropine, a cholinergic blocker, inhibits acupuncture analgesia.
Eserine, a parasympathomimetic, increases acupuncture analgesia.
GABA concentrations in the spinal cord are increased by high frequency electrostimulation thus inhibiting nociceptive information transmission in the spinal cord.
Substance P stimulates nociceptive neurons in the spinal cord and decreases effectiveness of acupuncture analgesia; it also increases the release of histamine from mast cells; however in the periaquaductal gray it has a potent anti-nociceptive effect, presumably by releasing enkephalin.
The relationship between acupuncture and the neuroendocrine system is obvious, if not completely understood at this time. Studies on specific points have shown increase in RBC and WBC with moxibustion at BL 17; increased GI motility in dogs with stimulation of ST 36 and BL 27; increase in white blood cell concentrations as well as specific lines of WBC, such as T-helper cells. Experimental studies showed effects of P6-stimulation on gastric myoelectrical activity, vagal modulation and cerebellar vestibular activities in f-MRI.(14) A f-MRI study in mice showed varying effects in the brain by electroacupuncture (EAP) depending on the point stimulated (15). After 5 minutes of EAP stimulation of LI 4, Hegu, consistent neural activation in the hypothalamus, PAG and MnR was seen. When GB 24, Riyue, was stimulated there was neural activity in the somatosensory areas and the hypothalamus but not in the PAG. Hegu is an acupoint known to be effective for pain control; Riyue, has a classical function of regulating visceral organ function. There is an interesting paper from 2005 that has looked at the various f-MRI and PET studies that have been done and their effectiveness (16). The abstract from the authors states that "these studies show that specific and largely predictable areas of brain activation and deactivation occur when considering the traditional Chinese functions attributable to certain specific acupuncture points".
References are available upon request.