This hypothesis proposes that the neural mechanism in the subnucleus caudalis ( Spinal dorsal horn) acts as a gate to decrease or increase the transmission of nociceptive nerve impulses from the periphery to the brain. This gate defines an area that is the terminus of a large number of primary afferent fibers, multiple interneurons and descending fibers from higher CNS centers.
Melzack and Wall (1965) proposed a theory of pain that has stimulated considerable interest and debate and has certainly been a vast improvement on the early theories of pain. According to his theory, pain stimulation is carried by small, slow fibers that enter the dorsal horn of the spinal cord; then other cells transmit the impulses from the spinal cord up to the brain. These fibers are called T-cells. The T-cells can be located in a specific area of the spinal cord, known as the substantial gelatinosa. These fibers can have an impact on the smaller fibers that carry the pain stimulation. In some cases they can inhibit the communication of stimulation, while in other cases they can allow stimulation to be communicated into the central nervous system. For example, large fibers can prohibit the impulses from the small fibers from ever communicating with the brain. In this way, the large fibers create a hypothetical "gate" that can open or close the system to pain stimulation. According to the theory, the gate can sometimes be overwhelmed by a large number of small activated fibers. In other words, the greater the level of pain stimulation, the less adequate the gate in blocking the communication of this information. There are 3 factors which influence the 'opening and closing' of the gate: (#1) The amount of activity in the pain fibers. Activity in these fibers tends to open the gate. The stronger the noxious stimulation, the more active the pain fibers. (#2) The amount of activity in other peripheral fibers—that is, those fibers that carry information about harmless stimuli or mild irritation, such as touching, rubbing, or lightly scratching the skin. These are large-diameter fibers called A-beta fibers. Activity in A-beta fibers tends to close the gate, inhibiting the perception of pain when noxious stimulation exists. This would explain why gently massaging or applying heat to sore muscles decreases the pain. (#3) Messages that descend from the brain. Neurons in the brainstem and cortex have efferent pathways to the spinal cord, and the impulses they send can open or close the gate. The effects of some brain processes, such as those in anxiety or excitement, probably have a general impact, opening or closing the gate for all inputs from any areas of the body. But the impact of other brain processes may be very specific, applying to only some inputs from certain parts of the body. The idea that brain impulses influence the gating mechanism helps to explain why people who are hypnotized or distracted by competing environmental stimuli may not notice the pain of an injury.