The gate control theory of pain states that the transmission of pain signals can be modulated at the spinal cord level by non-painful inputs as well as descending signals from the brain. Non-painful inputs typically close the gate to painful inputs, reducing pain. Pain sensation is mediated by pain receptors or nociceptors, which are essentially nerve endings of first-order neurons in the pain pathway.
The axons of these neurons form small diameter nerve fibers of two types, fast fibers A-delta, responsible for the initial sharp pain perceived at the time of injury, and slow C-fibers, responsible for a dull, longer-lasting pain. First-order neurons travel by way of spinal nerves to the spinal cord, where they synapse with second-order neurons, which, together with third-order neurons, carry pain signals to the brain. However, according to the gate theory of pain, perception of pain is not simply due to stimulation of nociceptors.
There is a so-called nerve gate, located in the dorsal horn of the spinal cord, that controls the passage of pain signals to the brain. This nerve gate consists essentially of interneurons that inhibit second-order neurons, thereby stopping or reducing signal transmission. When nociceptors are activated, They stimulate second-order neurons and, at the same time, inhibit the inhibitory interneurons, thus opening the gate, allowing transmission of pain signals. Now, if the area is also stimulated by other non-noxious stimuli such as touch, pressure, or changes of temperature, a different type of nerve fiber, large diameter Aβ, is activated, and this reactivates the inhibitory neurons stopping the transmission of pain signals. This mechanism underlies the pain-relieving effect of skin rubbing or heat or cold packs.
It is also the basis of pain treatment procedures such as transcutaneous nerve stimulation, which delivers a small electrical current to activate non-nosusceptive receptors in the skin. The brain can also send signals to the spinal cord to modulate the pain gate. Examples of such signals are endorphins, morphine-like substances released by the brain in response to pain. In addition to lowering pain perception by the brain, endorphins reduce transmission of pain signals between the first and second order neurons by two mechanisms, preventing the release of a key neurotransmitter, substance P, and inhibiting action potentials in postsynaptic neurons. Production of endorphins may be induced by a number of factors, including pleasurable activities, excitement, meditation, laughter, and vigorous exercise.
Endorphins action underlies the pain-relieving effect of physical exercise, positive state of mind, and explains, for example, why athletes may not feel pain from an injury until the exciting game is over.