Melzak and Wall proposed the ‘Gate Theory’ in 1965, and it remains the most respected theory of pain perception. They showed that pain impulses could be overridden and the perception of pain could be altered. Pain is an emotional experience encompassing neurobiological mechanisms. It is a very personal experience and highly influenced by circumstances and interpretation. Pain occurs when specialized nerve receptors are stimulated and messages are passed from the nervous system to the brain (dorsal horn) where they are interpreted as pain.
Basically the brain can only cope with a certain amount of information at once. For unknown reasons probably linked to survival, sensation impulses from the nerves have priority over pain signals. When a TENS unit is used you feel it strongly and the pain gets put to the ‘back of your mind’. You perceive the pain as less intense.
TENS stimulates endorphin release (the body’s natural painkillers) to help reduce pain. Endorphins (or 'endogenous opiates') were first discovered in 1975. They are natural opiate-like substances, similar to morphine, but not as potent and without the side effects. Endorphins are manufactured by our bodies in response to pain and act as a natural pain killer. Endorphins are released by the descending nerve fibres or nerves which travel down the spinal cord from the brain.
When we feel pain, the descending nerve fibres release endorphins at the spinal cord, where they meet the sensory nerves carrying the 'pain messages' from our body. This local release of endorphins by the nerves inhibits some or all of the pain messages going up to the brain. For many women, endorphins will also positively alter the memory of their birth experience and in some cases induce an amnesic effect (forgetting the pain). Endorphins can therefore empower women and provide a positive memory and experience associated with labour and birth.
There are two kinds of nerve fibres.
The dorsal horn is responsible for passing on information which can be interpreted as pain. This area is referred to as the 'gate' as it prevents the brain from receiving too much information too quickly. Impulses from the Labour TENS machine pass along nerve fibres to the dorsal horn and receive priority sending out a chemical blocker to 'close the gate' to pain. Labour TENS sensations are felt in the brain effectively over riding pain messages.