Nervous System III
Pain receptors adapt very little, if at all. Once such a
receptor is activated, even by a single stimulus, it may con-
tinue to send impulses into the CNS for some time.
Visceral Pain
As a rule, pain receptors are the only receptors in viscera
whose stimulation produces sensations. Pain receptors in
these organs respond differently to stimulation than those
associated with surface tissues. For example, localized
damage to intestinal tissue during surgical procedures may
not elicit any pain sensations, even in a conscious person.
However, when visceral tissues are subjected to more wide-
spread stimulation, as when intestinal tissues are stretched
or when the smooth muscles in the intestinal walls undergo
spasms, a strong pain sensation may follow. Once again, the
resulting pain results from stimulation of mechanoreceptors
and from decreased blood fl ow accompanied by lower tissue
oxygen levels and accumulation of pain-stimulating chemicals.
Visceral pain may feel as if it is coming from some part
of the body other than the part being stimulated—a phenom-
enon called
referred pain.
For example, pain originating in
the heart may be referred to the left shoulder or the medial
surface of the left upper limb. Pain from the lower esophagus,
Increasing blood fl ow through the sore tissue may relieve the
resulting pain, and this is why heat is sometimes applied to
reduce muscle soreness. The heat dilates blood vessels and
thus promotes blood fl ow, which helps reduce the concentra-
tion of the pain-stimulating substances. In some conditions,
accumulating chemicals lower the thresholds of pain receptors,
making infl amed tissues more sensitive to heat or pressure.
Section of
nerve fiber
Sensory nerve
Sensory nerve
Tactile (Meissner’s)
(touch receptor)
Lamellated (Pacinian)
(pressure receptor)
Connective tissue
Free nerve
Touch and pressure receptors include (
) free ends of sensory nerve F
bers, (
) tactile corpuscle (with 225× micrograph), and
) lamellated corpuscle (with 50× micrograph).
The importance of the ability to feel pain is seen in people who can-
not do so. Children who have hereditary sensory and autonomic
neuropathy cannot feel pain, and as a result they inadvertently injure
and mutilate themselves. More common is peripheral neuropathy, in
which the hands and/or feet become numb due to decreasing num-
bers of tactile corpuscles. Skin biopsies are used to track the telltale loss
of the corpuscles, but biopsies are painful, time-consuming, and give
inconsistent results. An experimental approach uses re±
ectance confo-
cal microscopy to image the number of corpuscles. In one pilot study,
people with normal pain sensation had twelve corpuscles per square
millimeter of skin, whereas people with peripheral neuropathy had 2.8.
The most common causes of peripheral neuropathy are diabetes mel-
litus, cancer treatment, vitamin deF
ciency, and HIV infection.
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