Sensation and Perception 117sensation is of the first kind—sharp, fast, and bright. But later the bruised tissue simply
aches, letting you know to take it easy on that thumb.
PAIN: GATE-CONTROL THEORY One explanation for how the sensation of pain works
is called gate-control theory, first proposed by Ronald Melzack and Patrick Wall (1965)
and later refined and expanded (Melzack & Wall, 1996). In this theory, the pain signals
must pass through a “gate” located in the spinal cord. The activity of the gate can be
closed by nonpain signals coming into the spinal cord from the body and by signals
coming from the brain. The gate is not a physical structure but instead represents the
relative balance in neural activity of cells in the spinal cord that receive information
from the body and then send information to the brain. Additional research has revealed
that the activity of relay centers in the brain can also be influenced, and the exact loca-
tions and mechanisms are still being investigated. The video Gate-Control Theory pro-
vides a simulation of how pain signals travel along the spinal cord.
CCWatch the Video Gate-Control Theory of PainStimulation of the pain receptor cells releases a neuromodulator called substance P
(for “pain,” naturally). Substance P released into the spinal cord activates other neurons
that send their messages through spinal gates (opened by the pain signal). From the spi-
nal cord, the message goes to the brain, activating cells in the thalamus, somatosensory
cortex, areas of the frontal lobes, and the limbic system. The brain then interprets the pain
information and sends signals that either open the spinal gates farther, causing a greater
experience of pain, or close them, dampening the pain. Of course, this decision by the
brain is influenced by the psychological aspects of the pain-causing stimulus. Anxiety,
fear, and helplessness intensify pain, whereas laughter, distraction, and a sense of control
can diminish it. (This is why people might bruise themselves and not know it if they
were concentrating on something else.) Pain can also be affected by competing signals
from other skin senses, which is why rubbing a sore spot can reduce the feeling of pain.
Those same psychological aspects can also influence the release of the endorphins,
the body’s natural version of morphine. to Learning Objective 2.3. Endorphins
can inhibit the transmission of pain signals in the brain, and in the spinal cord they can
inhibit the release of substance P.