Gate Control Theory (Melzack and Wall, 1965)
Melzack has proposed a theory of pain that has stimulated considerable interest and debate and has
certainly been a vasy 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[6]:
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.
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.
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 peopie who are hypnotized or distracted by
competing environmental stimuli may not notice the pain of an injury.
The beauty of theory is that it provides a physiological basis for the complex phenomenon of pain. It
does this by investigating the complex structure of the nervous system, which is comprised of the
following two major divisions:[7]
Central nervous system (the spinal cord and the brain)
Peripheral nervous system (nerves outside of the brain and spinal cord, including branching nerves in
the torso and extremities, as well as nerves in the lumbar spine region)