Sensory Physiology
205
When incoming nociceptive afferents activate interneu-
rons, it may lead to the phenomenon of
referred pain,
in which
the sensation of pain is experienced at a site other than the
injured or diseased tissue. For example, during a heart attack,
a person often experiences pain in the left arm. Referred pain
occurs because both visceral and somatic afferents often con-
verge on the same neurons in the spinal cord (
Figure 7–18a
).
Excitation of the somatic afferent fi bers is the more usual source
of afferent discharge, so we “refer” the location of receptor acti-
vation to the somatic source even though, in the case of visceral
pain, the perception is incorrect.
Figure 7–18b
shows the typi-
cal distribution of referred pain from visceral organs.
Pain differs signifi cantly from the other somatosensory
modalities. After transduction of the fi rst noxious stimuli into
Lung and
diaphragm
Heart
Stomach
Pancreas
Colon
Urinary bladder
Kidney
Liver and
gallbladder
Small
intestine
Ovaries
Appendix
Ureter
Liver and
gallbladder
Spinal
cord
Dorsal root
ganglion
Paravertebral
ganglion
Sensory
pathway
to brain
Sensory
nerve fiber
Skin
Heart
Pain
receptor
(b)
(a)
Figure 7–18
Referred pain. (a) Convergence of visceral and somatic afferent neurons onto ascending pathways. (b) Regions of the body surface where we
typically perceive referred pain from visceral organs.
previous page 233 Vander's Human Physiology The Mechanisms of Body Function read online next page 235 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off