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Module 1.6d

Biological Influences
Pain is a physical event produced by your senses. But pain differs from some of your other
sensations. No one type of stimulus triggers pain the way that light triggers vision. And no
specialized receptors process pain signals the way that your retina receptors react to light
rays. Instead, sensory receptors called nociceptors — mostly in your skin, but also in your
muscles and organs — detect harmful temperatures, pressure, or chemicals (Figure 1.6-23).

Figure 1.6-23
The pain circuit
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Sensory receptors (nociceptors)
respond to potentially damaging
stimuli by sending an impulse to
the spinal cord, which passes
Copyright © Bedford, Freeman & Worth Publishers.
the message to the brain, which
interprets the signal as pain.

Projection
to brain

Cross section of
Pain the spinal cord
impulse

Cell body of
nociceptor

Nerve
cell

Tissue
injury

Your pain experience depends in part on the genes you inherited and on your physical
characteristics (Gatchel et al., 2007; Reimann et al., 2010). Women are more sensitive to pain
than men are (their senses of hearing and smell also tend to be more sensitive) (Ruau et al.,
2012; Wickelgren, 2009).
No pain theory can explain all findings from research on pain. One useful model,
gate-control theory, suggests that the spinal cord contains a neurological “gate” that con-
trols the transmission of pain messages to the brain (Melzack & Katz, 2013; Melzack & Wall,
1965, 1983).
Small spinal cord nerve fibers conduct most pain signals. An injury activates the small gate-control theory the theory
that the spinal cord contains a
fibers and opens the gate. The pain signals can then travel to your brain, and you feel pain. neurological “gate” that blocks
But large-fiber activity (stimulated by massage, electrical stimulation, or acupuncture) can pain signals or allows them to
close the pain gate by blocking pain signals. Brain-to-spinal-cord messages can also close pass on to the brain. The “gate”
the gate. Thus, chronic pain can be treated both by gate-closing stimulation, such as mas- is opened by the activity of pain
sage, and by mental activity, such as distraction (Wall, 2000). signals traveling up small nerve
fibers, and is closed by activity
We also benefit from our own natural painkillers, endorphins, which are released in in larger fibers or by information
response to severe pain or vigorous exercise. People who carry a gene that boosts the avail- coming from the brain.
ability of endorphins are less bothered by pain, and their brain is less responsive to pain

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