Early theories of pain took a mechanistic view, proposing that pain results from stimulus activating receptors and traveling along nerves to the brain. Later theories sought to explain psychological factors affecting pain perception. The gate control theory integrated ideas, proposing a neural gate in the spinal cord modulates pain signals reaching the brain based on activity in pain fibers, other peripheral fibers, and descending messages from the brain. Evidence supports the theory, showing brain stimulation can induce analgesia by activating pathways closing the gate.

1. THEORIES OF PAIN
Lesson # 2

2. EARLY THEORIES
• In the early 1900s, the dominant theories of pain took a
very mechanistic view of pain perception.
• They proposed that if an appropriate stimulus activates a
receptor, the signal travels to the spinal cord and then the
brain, and sensation results
• These early theories did not adequately explain pain
perception (Gatchel et al., 2007; Melzack & Wall, 1982).
• They do not attempt to explain why the experience of pain
is affected by psychological factors, such as the person’s
ideas about the meaning of pain, beliefs about the
likelihood of pain, and attention to (or distraction from)
noxious events.

3. SPECIFICITY THEORY
• Argued that the body has a separate sensory
system for perceiving pain—just as it does for
hearing and vision.
• This system was thought to contain its own
special receptors for detecting pain stimuli,
its own peripheral nerves and pathway to the
brain, and its own area of the brain for
processing pain signals.
• But this structure is not correct.

4. PATTERN THEORY
• Proposed that there is no separate system for
perceiving pain, and the receptors for pain are shared
with other senses, such as of touch.
• According to this view, people feel pain when certain
patterns of neural activity occur, such as when
appropriate types of activity reach excessively high
levels in the brain. These patterns occur only with
intense stimulation.
• Because strong and mild stimuli of the same sense
modality produce different patterns of neural activity,
being hit hard feels painful, but being caressed does
not.

5. GATE CONTROL THEORY
• Ronald Melzack and Patrick Wall (1965, 1982, 1993)
• integrated useful ideas from earlier theories - particularly
by describing a physiological mechanism by which
psychological factors can affect people’s experience of pain.
• At the heart of the gate-control theory is a neural ‘‘gate’’
that can be opened or closed in varying degrees, thereby
modulating incoming pain signals before they reach the
brain.
• The theory proposes that the gating mechanism is located
in the spinal cord—more specifically, in the substantia
gelatinosa of the dorsal horns, which are part of the gray
matter that runs the length of the core of the spinal cord.

6. GATE CONTROL THEORY
• Signals of noxious stimulation enter the gating
mechanism (substantia gelatinosa) of the spinal
cord from pain fibers (A-delta and C fibers).
• After these signals pass through the gating
mechanism, they activate transmission cells,
which send impulses to the brain.
• When the output of signals from the transmission
cells reaches a critical level, the person perceives
pain;
• the greater the output beyond this level, the
greater the pain intensity.

8. Three Factors that control the
opening and closing of the gate
• 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. Some peripheral fibers,
called A-beta fibers, carry information about harmless stimuli or mild
irritation, such as touching, rubbing, or lightly scratching the skin. 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.

9. Three Factors that control the
opening and closing of the gate
• The theory proposes that the gating mechanism
responds to the combined effects of these three
factors.
• 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.
• Melzack later proposed the idea of a neuromatrix, a
neural network in the brain that integrates information
from the senses, cognitive and emotional areas of the
brain, and stress regulation systems (Melzack & Katz,
2004).

10. Conditions that can open or close the Gate
(Based on material by Karol et al., cited in Turk, Meichenbaum,
& Genest, 1983)
Conditions That Open the Gate
• Physical conditions
Extent of the injury
Inappropriate activity level
• Emotional conditions
Anxiety or worry
Tension
Depression
• Mental conditions
Focusing on the pain
Boredom; little involvement in life
activities
Conditions That Close the Gate
• Physical conditions
Medication
Counterstimulation (e.g., heat or
massage)
• Emotional conditions
Positive emotions (e.g.,
happiness or optimism)
Relaxation
Rest
• Mental conditions
Intense concentration or
distraction
Involvement and interest in life
activities

11. Evidence
• One study, for instance, confirmed the theory’s prediction that impulses
from the brain can inhibit the perception of pain.
• David Reynolds (1969) - study with rats as subjects. He first implanted an
electrode in the midbrain portion of each rat’s brainstem, varying the
exact location from one rat to the next. Then he made sure they could
feel pain by applying a clamp to their tails—and all reacted. Several days
later, he provided continuous, mild electrical current through the
electrode and again applied the clamp to test whether the current would
block pain.
• Although most of the subjects did show a pain reaction, those with
electrodes in a particular region of the midbrain—the periaqueductal gray
area—did not.
• The electrical stimulation had produced a state of not being able to feel
pain, or analgesia, in these rats.
• Then Reynolds used these few rats for a dramatic demonstration: he
performed abdominal surgery on them while they were awake and with
only the analgesia produced through electrode stimulation.

12. • Subsequent studies by other researchers have
confirmed that stimulation to the periaqueductal
gray area can induce analgesia in animals and in
humans.
• Moreover, they have determined that morphine
works as a painkiller by activating the brainstem
to send impulses down the spinal cord (Chapman,
1984; Melzack & Wall, 1982; Winters, 1985).
• The gate-control theory clearly takes a
biopsychosocial perspective in explaining how
people perceive pain.

13. KEY TERMS/CONCEPTS
• Afferent neurons of the peripheral nervous system carry the signal
of injury to the spinal cord, which carries the signal to the brain.
The afferent nerve endings that respond to pain stimuli and signal
injury are called nociceptors. These fibers exist in every body tissue
except the brain.
• Pain signals are carried by afferent peripheral fibers of two types: A-
delta and C fibers. A-delta fibers (Fast nerve fibers) are coated with
myelin, a fatty substance that enables neurons to transmit impulses
very quickly. These fibers are associated with sharp, well-localized,
and distinct pain experiences.
• C fibers (slow nerve fibers) transmit impulses more slowly—
because they are not coated with myelin—and seem to be involved
in experiences of diffuse dull, burning, or aching pain sensations

14. KEY TERMS/CONCEPTS
• Pains originating from internal organs are
often perceived as coming from other parts of
the body, usually near the surface of the skin.
This is called referred pain (AMA, 2003;
Tortora & Derrickson, 2009). Heart attack -
pain is referred to the shoulders, pectoral area
of the chest, and arms.