Neurobiology of Pain
Pain, whether it is in its acute or chronic stage, or emotional or physical, is one of the most common conditions treated with acupuncture. In fact, I believe that every acupuncturist and practitioner of Chinese medicine must treat pain on some level daily. Millions of people suffer every year, either from acute or chronic pain, and it is estimated that pain costs the USA $560–635 billion annually, half of which ($261–300 billion) is spent on health care costs (see www.painmed.org). Pain is one of the main reasons why patients are going to see their doctor—accounting for two out of every three reasons to visit.
There are many different techniques of acupuncture that have been developed over the years to treat pain using many different thought processes. We have advanced scalp acupuncture systems, auricular systems including battlefield acupuncture and P-Stim medical devices, Japanese style, sports acupuncture, orthopedic acupuncture, osteopuncture, and of course Neuropuncture.
I believe that in order to treat a problem you must first fully understand it. Pain is actually a specific area of our body’s somatic sensory nervous system. The term “pain” is used to describe a wide range of unpleasant sensory and emotional experiences associated with actual or potential tissue damage. Remember that the terms “actual” and “potential” illustrate how pain is a subjective experience. Recent neuroscience research has illustrated that acute and chronic pain have neuroanatomical differences. Pain has been labeled a “neural signature” because it involves several areas of the brain, not just one specific region, depending on several variables. To best understand the neurobiology of pain I have broken it down into systems or neural pathways. These systems, or “orders,” follow the transmission of pain from its origin.
Transmission of pain
Descartes first illustrated the neurobiology of pain in 1664 (see Figure 3.1). It was a rudimentary drawing of a simple pain pathway—the result of a flame burning the bottom of a man’s foot. It depicts a line traced from the flame touching the man’s foot, all the way up his leg into his “spine,” then up to his head or “brain.” The head/brain is where people believed the spirit got involved and gave the emotional component of pain. Interestingly enough, like our Chinese medicine predecessors utilizing the terms Jing Luo and Mai to describe pathways of transmission, Descartes wasn’t that far off. The transmission of pain can be understood as following three main “orders” or systems of neurons, or simply three main neural-anatomical pathways. Let’s investigate these pathways.
The first nerve order begins at the location of the actual or potential tissue injury. The human pain experience begins at this local site as a result of chemicals that are released transmitting information electrically to the CNS. Locally, there are tiny free nerve endings known as nociceptors. Nociceptors innervate everything from muscle, skin, and hair, to tendons, ligaments, bones, and viscera, and transmit information to the spinal cord.
Figure 3.1 Descartes’ pain pathways
Nociceptors include the A-delta, A-gamma, A-beta, and C-fibers. These tiny free nerve endings become stimulated via chemicals released locally as a result of tissue damage or an acupuncture needle being inserted into the skin. They transmit electrical signals of sharp pain, pressure, and variables in temperature. Thus, these fibers, which extend throughout our entire body, transmit everything—direct trauma, chemical damage, and temperature damage or changes—to our brain for interpretation and response. This system is known as PANS (see Chapter 2).
The local inflammatory mediators that are released include neuropeptides that have neural sources and non-neural sources. The non-neural sources include acetylcholine, adenosine triphosphate (ATP), glutamate, cytokines, some opioids, and serotonin. The neural sources include Substance P, CGRP (see Chapter 2), neurokinin, choles kinase, somatostatin, glutamate, some opioids, and ATP. These neurochemical mediators all have an effect on specific receptors and they also affect the transducer channels such as the Na+, Ca++, and K+ channels. It is believed that these channels are directly affected by Electro-Acupuncture. Some of the chemicals are excitatory in nature and some are inhibitory. Together they are responsible for the transmission of pain.
This transmission continues directly to the spinal cord. The nociception fibers all enter the spinal cord at the substantia gelatinosa region of the lateral dorsal horn. This junction is where the transmission now enters the second order neuron of the pain neuroanatomical system. This junction is essential to pain transmission but also to acupuncture’s powerful neurophysiological mechanisms. It is here that one of acupuncture’s distal effects on the viscera and pain management can be clearly understood and explained (see Chapter 4). The different ways of stimulating the needle are what determine the message, which is transmitted as an electrical signal into a chemical one, then back into an electrical signal to the CNS.
It is at the lateral dorsal horn in the substantia gelatinosa that the “gate theory” applies. Since the C-fibers are unmyelinated and the A-gamma/beta fibers are myelinated and transmit faster, the signal transmitted by the A-gamma/beta fibers will block, or “gate,” the area of transmission of the C-fibers. After converging, these tracts will penetrate specific regions of the brain but all together will affect the “pain signature” accordingly.
After crossing over to the opposite side, the spinal nerves all ascend to the brain. During its ascent to the brain the signal passes each spinal segment. There are several tracts that are involved. The stimulus, which then determines the tract, will determine the areas of the brain that will be stimulated or activated. Figure 3.2 illustrates the tract and the regions of the brain where the signal terminates its pathway. (Although this is a nice, neat, simple-looking diagram, it is in fact fairly complicated.)
Figure 3.2 Ascending pathways along the spinal tracts
The transmission then continues into the brain and is now considered to be what neuroscientists call the “pain neural signature” or “pain matrix.” Sometimes this anatomical activity is also referred to as the “pain neural substrate.” It is a large, interconnected, complicated neural network that is still undergoing intense research. There are cortical and subcortical regions of the brain involved. The cortical areas include the somatosensory regions, anterior cingulate cortex, prefrontal cortex, and insula. The subcortical structures are the thalamus, amygdala, hippocampus, and basal ganglia. Collectively they form the pain matrix, which gives us the emotional and sensational experience and memory of pain.
For classification purposes I break pain down simply into nociceptive pain or neuropathic pain, and pain with a peripheral or central location. Of course, for chronic and acute pain there are additional differences, with some neural anatomical variations, as we will discuss later in the chapter.
Nociceptive pain includes acute inflammation and myofascial pain while, separately, neuropathic pain includes neuropathies and radiculopathy. Neuropathic pain is separate because it involves different neurobiochemical factors as well as neuropathology.
•Nociceptive pain: In short, this includes PANS and the signaling of actual or potential tissue damage to the CNS—transmission from the peripheral local site to the spinal cord up to the brain. It also includes any inflammatory condition, which will activate the PANS but through specific inflammatory mediators. It also includes pro-inflammatory cytokines such as IL-1-alpha, IL-1-beta, IL-6 and TNF-alpha, chemokines, reactive oxygen species, vasoactive amines, lipids, ATP, acid, and other factors released by infiltrating leukocytes, vascular endothelial cells, or tissue resident mast cells. In addition, it includes muscle and joint pain.
•Neuropathic pain: This is a result of sensory deficits and abnormalities in the nervous system. It includes hyperalgesia and allodynia.
•Peripheral pain: This is due to an injury that originates outside the CNS, along the peripheral (e.g. arms, legs, ankles, hands, wrists, knees, etc.).
•Central pain: This is pain that becomes centralized in the CNS. It usually applies to spinal lesions, CNS tumors, and chronic pain.
I use additional combination classifications that are determined by the location of the source of the pain (i.e. a central or peripheral source). For example, peripheral neuropathic pain is pain in the peripheral of the body (pertaining to the extremities) that is also neuropathic. Pain over a period of time begins to change the brain, and just as the brain has the ability to mold and adapt to positive stimulation like meditation, in patients who suffer from chronic pain the structure of the brain changes but not in a good way. Changes occur that can be seen with fMRI and MRI images due to the consistent firing of neurons, which create, complicate, and increase the painful experience.
Next we will look at pain intensity and the time needed for diagnosis and treatment. It is important to remember that there are many well-recognized pain disorders that are not easy to classify and this can cause confusion in understanding their underlying etiology. Intensity helps us to understand the patient’s experience of the pain signals. The time factor involves the practitioner understanding the area of the nervous system to address and focusing treatment in that direction.
Measuring the intensity of pain
I mainly use the Subjective Unit of Discomfort (SUD) in my clinic (see Figure 3.4). This method of clinical measurement requires the patient to grade their pain level at its highest and then at a constant. Level 10 represents pain so strong that they want to go to the ER and get an injection, and 1 represents pain that is barely noticeable.
•Mild: SUD 1–3
•Moderate: SUD 4–8
•Severe: SUD 9–10
Figure 3.4 The Subjective Unit of Discomfort Scale
Another method is the Visual Analog Scale (VAS), which measures pain from no pain to extreme pain (see Figure 3.5). This can be better than the SUD scale because sometimes patients try to remember the last number they told you and want to adjust to that number. With a VAS, that scenario is eliminated and a more accurate reading is achieved. VAS is commonly used in research trials.
Figure 3.5 The Visual Analog Scale
Chronic pain
The last thing to discuss in this chapter is chronic pain complicated by chemical dependency. Recently there has been a focused movement by the US government to reduce the number of opioid prescriptions. Opioid addiction, opioid abuse, and opioid death have reached epidemic proportions. It can be a tricky process treating chronic pain and chemical dependency. You cannot just remove the addictive pain medication because the patient will be in severe pain and you cannot continue to increase the medication for risk of tolerance and then sudden death, or higher rates of addiction. It is a lot for the nervous system to handle. One of the most important things to remember, and to control, is the patient’s anxiety level. During opioid cessation, it is common for patients to experience mild to severe levels at times, or a constant low level of feeling uneasy.
Auricular Electro-Acupuncture (EA) is very effective for opioid withdrawal. I use the National Acupuncture Detoxification Association (NADA) protocol in the dominant ear and then connect the Shenmen point in the NADA protocol to either the Tranquilizer or Lung point in the opposite ear and EA on 2 Hz millicurrent for 45 minutes. Remember to apply the Neuropuncture protocols at the end when necessary. So, for example, if the patient is experiencing hypertension, add the Neuropuncture hypertension protocol to the Neuropuncture anxiety protocol.
It is important to address the CNS when treating chronic pain. With chronic pain, the patient’s brain molds and adapts to the stress of the pain signaling and needs to be “rewired” in many cases. This includes an integrated team approach, utilizing nutrition and mind–body training (e.g. meditation, psychology, and counseling), and Neuropuncture protocols that address brain health and the repair of specific neurons and neurogenic systems. Therefore it is common to treat patients every other day for several months to correct and “rewire” their nervous systems. This involves local treatment for the initial injury site, as well as focusing treatment on brain chemistry and CNS neural pathways.
