LOCUS OF CONTROL ORIENTATION AND LEVEL - Drake University

pain as it reaches the brain. This pain experience can be altered by a number of
factors'"Me1zack & Wall, p. 176).
The most recent gate-control theory of pain called the Mark II and
developed by Melzack and Wall (1987) explains how the pain experience can be
altered. It presented the probability that there is an inhibitory "gate" in the
brain stem which inhibits pain. This brain-stern inhibitory circuit was said to be
a system including the midbrain, medulla, and spinal cord. This gate-control
theory explained that:
Activation of cells in the midbrain's periaqueductal
gray mattes by electrical stimulation, opiate
analgesic drugs, or possibly psychologic factors in
turn stimulates structures in the medulla. These
medullary structures then project to the inhibitory
spinal pain transmission fibers. Pain itself may
activate this system, so there is a natural control
mechanism limiting the severity of the pain
experiences (Melzack & Wall, 1987, p. 176).
Beyond the periaqueductal gray matter, there are several other levels at
which the pain message can be altered, including the substantia gelatinosa, the
preventricular gray matter and the intralaminar nuclei of the thdamus. Each of
these area contain a neurotransmitter called enkephalin which can modify the
perception of pain by intercepting the message before it reached the cortex
(Guyton, 1991).