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.
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-
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.
Touch and pressure receptors include (
) free ends of sensory nerve F
) 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±
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.