442
UNIT THREE
Pain Nerve Pathways
The nerve fibers that conduct impulses away from pain
receptors are of two main types: acute pain fibers and
chronic pain F
bers.
The
acute pain f
bers
(also known as A-delta F bers) are
thin, myelinated nerve F
bers. They conduct nerve impulses
rapidly, at velocities up to 30 meters per second. These
impulses are associated with the sensation of sharp pain,
which typically seems to originate in a local area of skin.
This type of pain seldom continues after the pain-producing
stimulus stops.
The
chronic pain Fibers
(C fibers) are thin, unmyeli-
nated nerve fibers. They conduct impulses more slowly
than acute pain F
bers, at velocities up to 2 meters per sec-
ond. These impulses cause the dull, aching pain sensation
that may be widespread and difF
cult to pinpoint. Such pain
may continue for some time after the original stimulus
ceases. Although acute pain is usually sensed as coming
from the surface, chronic pain is felt in deeper tissues as
well as in the skin. Visceral pain impulses are usually car-
ried on C F
bers.
Commonly, an event that stimulates pain receptors will
trigger impulses on both types of pain F
bers. This causes
a dual sensation—a sharp, pricking pain, then a dull, ach-
ing one. The aching pain is usually more intense and may
worsen over time. Chronic pain that resists relief and control
can be debilitating.
stomach, or small intestine may seem to be coming from
the upper central (epigastric) region of the abdomen. Pain
from the urogenital tract may be referred to the lower central
(hypogastric) region of the abdomen or to the sides between
the ribs and the hip
(f
g. 12.2)
.
Referred pain may derive from
common nerve pathways
that sensory impulses coming both from skin areas and from
internal organs use. Pain impulses from the heart seem to be
conducted over the same nerve pathways as those from the
skin of the left shoulder and the inside of the left upper limb,
as shown in
f gure 12.3
. During a heart attack, the cerebral
cortex may incorrectly interpret the source of the impulses
as the shoulder and the medial surface of the left upper limb,
rather than the heart.
Pain originating in the parietal layers of thoracic and
abdominal membranes—parietal pleura, parietal pericardium,
or parietal peritoneum—is usually not referred; instead, such
pain is felt directly over the area being stimulated.
Neuropathic pain is an overreaction to a stimulus that would ordi-
narily cause pain or a pain response to a normally innocuous stimu-
lus. Reflex sympathetic dystrophy is a form of neuropathic pain
that causes an intense burning sensation in a hand or foot, even if
the extremity is paralyzed or has been amputated. During the Civil
War, it was called “causalgia.” Union Army Surgeon S. Weir Mitchell
described causalgia as “the most terrible of all tortures.”
Small
intestine
Appendix
Ureter
Lung and diaphragm
Heart
Stomach
Pancreas
Ovary
(female)
Colon
Kidney
Urinary bladder
Liver and
gallbladder
Liver and
gallbladder
FIGURE 12.2
Surface regions to which visceral pain may be referred.
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