Gate Control Theory of Pain, Animation

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 2 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 A-beta, 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-nociceptive 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 2 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.