Pain is not just a signal that certain tissues in the body have been damaged – it also alerts the sufferer to danger. The body has its own built in pain inhibition system.
Any event that causes a degree of damage to the tissues of the body – be it mechanical (from pressure or a wound), chemical (exposure to acid, for example) or thermal (extreme heat or cold) – brings about the release of large amounts of chemicals such as serotonin and histamine.
As well as producing reactions within the tissues, such as swelling and redness, these chemicals are detected by special sensory cells, called free nerve endings, which are found in the superficial layers of skin as well as in some of the internal organs. They are also known as nociceptors, because they react to noxious substances.
In response to the chemical changes within the tissues, the sensory cells send nerve impulses to relay stations in the spinal cord. From here, they are passed through further relays in the lower part of the brain in the brain stem and the thalamus, and so on to the higher levels of the brains. There, the information is analyzed and perceived as pain.
There are two types of pain, distinguished according to the speed with which the sensations are felt. The first, which is felt as soon as tissue damage is sensed, is sharp and stabbing and is known as acute pain. Its impulses travel extremely quickly to the brain along special nerve fibres, called A-fibres, that have myelin sheaths to speed the impulses along. The purpose of acute pain is to bring about an immediate, subconscious, reaction, to remove the body from the danger; A-fibre impulses cause a hand to be moved out of a flame, for example.
After some time, acute pain dies down and is replaced by the second type: the dull, throbbing, aching, persistent feeling that characterizes chronic pain. The impulses of chronic pain come from sensory receptors deeper in the tissues, and they travel 10 times more slowly than those of acute pain along unmyelinated nerve fibres called C-fibres.
How much pain is felt is determined by the quantity of endorphins (pain-relieving chemicals in the brain). Studies in the early 1980s found that acupuncture works in part by stimulating the release of endorphins. Now, a growing body of research suggests that acupuncture may have several mechanisms of action. Those include stimulating blood flow and tissue repair at the needle sites and sending nerve signals to the brain that regulates the perception of pain and reboot the autonomic nervous system, which governs unconscious functions such as heart beat, respiration and digestion. Neuroimaging studies show that it seems to calm areas of the brain that register pain and activate those involved in rest and recuperation. Doppler ultrasound shows that acupuncture increase blood flow in treated areas. Thermal imaging shows that it can make inflammation subside .
Fear, depression, anxiety, lack of exercise and concentrating on pain all reduce endorphin levels. The fewer endorphins there are, the more pain is felt.
- “Physiology”, by Peter Abrahams, ISBN 978-1-905704-64-4
- Finding Science in Acupuncture, WSJ.com. March 22, 2010 12:01 a.m. ET