Book Notes: Pain Free 1-2-3
Part II Evaluating and Treating Common Types of Pain-Pain Pathways
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The Biochemistry of Pain
New research suggests that pain affects men and women differently. This likely occurs in part because of hormonal differences related to estrogen and testosterone. For example, male animals injected with estrogen appear to have a lower pain threshold while giving testosterone to female animals increases pain tolerance. How pain is transmitted in our bodies also contributes to the differences in how pain is felt by men and women. Both sexes have a natural ability to suppress pain but the mechanisms are different. For example, a pain receptor called kappa-opioid is more active in women. Because of this, some medications such as Stadol® are more effective in females.
The biochemistry and anatomy of pain is fairly complex, and it is not necessary that you understand these to use the information in this book.
When following pain pathways, it is good to start with the skin. Receptors on the skin trigger a series of electrical impulses that go to the spinal cord. Signals may be ignored, amplified, or changed in other ways before being sent to the brain. The back of the spine (called the dorsal horn) is important for receiving sensation and pain signals. In order to send the pain signal, the nerve cells have gate-like passages called “channels” that allow them to transmit the information. Blocking these channels allows us to also decrease pain.
Once the pain signal is sent from the spine to the brain it often goes to the thalamus and then to the cortex where it is interpreted as thought. This transmission process requires many chemicals called neurotransmitters, which transmit nerve impulses from one nerve cell to another via receptors specific for that neurotransmitter/chemical.
Some neurotransmitters/receptors seem to increase pain transmission. These include glutamate, NMDA, and substance P (for pain). Others, such as opiate/opioid receptors, turn on pain inhibiting circuits, decreasing pain. Neurotransmitters that appear to act as natural painkillers in the body include serotonin, norepinephrine, opioid-like chemicals, and endorphins. Endorphins are the natural painkillers that are released during jogging, resulting in a “runners high.” Disturbingly, nicotine may also affect endorphins, and a chemical (from an Ecuadorian frog) that resembles nicotine is highly toxic but a powerful analgesic (working on acetylcholine receptors). This raises the possibility that some people who are addicted to cigarettes may be self-anesthetizing themselves for pain. Although I’ve seen no studies done on this, and have not tried it in my practice out of concern over its addictiveness and health risks, it is possible that nicotine patches might also decrease pain.
Once we travel outside of the brain and nervous system, other hormones, which affect pain can be modified. For example, prostaglandins simulate nerves at the site of injury resulting in inflammation. Medications such as NSAIDs (e.g. Motrin®) and COX (Cyclooxygenase®) 2 inhibitors such as Celebrex® work by blocking the enzyme required to produce prostaglandin. Unfortunately, doing so also blocks the beneficial effects of prostaglandins (such as preventing stomach ulcers). Other chemicals used by the immune system called cytokines can also trigger inflammation. This allows another avenue to help decrease pain.
