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.