Download The Interoceptive, or Visceral, Sensations

CHAPTER
34
The Interoceptive, or Visceral,
Sensations
nteroceptive sensations are general visceral sensations that arise from the internal organs. The special visceral sensations
(smell and taste) are discussed with the cranial nerves.
General visceral afferent fibers are found in cranial
nerves VII, IX, and X and in the thoracolumbar and
sacral autonomic nerves. Visceral afferent fibers run
with autonomic efferent fibers to the viscera. Cell
bodies are in the dorsal root and associated cranial
ganglia; impulses enter the central nervous system
through the posterior roots and ascend to higher
centers through pathways close to those that carry
general somatic afferent impulses.
Visceral afferent fibers are involved with unconscious visceral and autonomic reflexes and also likely
convey visceral sensations such as hunger, nausea,
sexual excitement, vesical distention, and visceral
pain. Afferent impulses from the viscera may reach
consciousness by a variety of routes. Some travel in
somatic nerves and some with efferent autonomic
nerves. Some synapse in the dorsal horn, and axons
of the next-order neurons cross to the opposite
spinothalamic tract, where the fibers that carry visceral pain lie medial to those that carry superficial
pain and temperature sensations. Others may travel
in the ipsilateral spinothalamic tract. Many ascend
for a great distance in Lissauer tract before synapsing, and some ascend by long intersegmental fibers
in the white matter at the border of the dorsal horn,
reaching the hypothalamus and thalamus without
decussating. As a consequence of the multiple pathways and redundancy, localization of visceral pain is
not precise. The gyms rectus, rather than the parietal
cortex, may be the end station for visceral afferent
sensation.
In the history, symptoms related to visceral function and conveyed by visceral afferent fibers include
I
such things as gastric fullness and early satiety, gastric
discomfort, intestinal spasm, a pressure sensation in
the chest, a sensation of fullness in the bladder or rectum, a desire for micturition, a sense of engorgement
from the genitalia, or pain in the internal organs.
The viscera are generally insensitive to the usual
stimuli that cause pain, but spasm, inflammation,
trauma, pressure, distention, or tension on the viscera
may produce severe pain, some of which results from
involvement of the surrounding tissues. Pain endings
are found in the parietal pleura over the thoracic wall
and the diaphragm, although probably none are present in the visceral pleura or the lungs. The parietal
peritoneum is sensitive, especially to distention, but
the visceral peritoneum is probably not sensitive.
Visceral pain is often vaguely localized or diffuse and likely to be described by the patient as
deep-seated. In addition to the pain experienced in
the viscus itself, there may be pain referred to other
areas, and the area where the referred pain is felt may
be hyperalgesic to stimulation. At times, there may
also be tenderness and muscle spasm in the same area.
Wide dynamic range (WDR) neurons in the dorsal
horn respond both to ordinary somatic sensory input
and to noxious stimuli. They respond progressively
as stimulus intensity increases. Nociceptive visceral afferents activate the same WDR neurons that
respond to somatic sensation. The convergence on
somatic and visceral sensation on the same neuronal
population may be one explanation for referred pain.
The zones of referred pain and hyperalgesia found in
disease of the various viscera are rather poorly localized and vary widely. Referred pain may be felt in the
dermatome or skin segment directly over the involved
organ as a result of corresponding segmental innervation in the area of cutaneous distribution of the
spinal nerves that correspond to the segmental spinal
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SECTION F | THE SENSORY SYSTEM
cord level that supplies the viscus, or the pain may
be quite distant from the diseased area, as a result of
shifting of the viscus during embryonic development.
Appendiceal pain is felt directly over the appendix;
the pain of angina pectoris may radiate down the
left arm; and renal pain is referred to the groin. The
phrenic nerve (C3-C5) is sensory as well as motor to
the diaphragm and to the contiguous structures—the
extrapleural and extraperitoneal connective tissues in
the vicinity of the gallbladder and liver. As a consequence, in disease of the gallbladder, liver, or central
portion of the diaphragm, there may be pain and
hyperesthesia not only in the viscus involved but also
on the side of the neck and shoulder in the C3-C5
cutaneous distribution or in the area supplied by the
posterior roots of those nerves whose anterior roots
supply the diaphragm. Other areas of referred visceral
pain include midthoracic levels for stomach, duodenum, pancreas, liver, and spleen; upper thoracic levels
for the heart; upper and midthoracic levels for the
lungs; and low thoracic and upper lumbar levels for
the kidney. With some exceptions, the referred pain
appears on the same side of the body in which the
diseased organ is located.
The anatomy of the pain pathways influences the
techniques for surgical management of chronic visceral
pain. Because the visceral afferent fibers lie medial in
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the spinothalamic tracts, a cordotomy to control visceral pain must be carried out with a deeper incision
than one for the relief of somatic pain. Also, because
the afferent impulses from the viscera ascend for a
greater distance before decussating, it must be done at
a higher level. Because visceral pain may be carried in
both crossed and uncrossed pathways, a cordotomy to
control visceral pain may have to be bilateral.
Visceral sensation, although clinically important, cannot be adequately evaluated by the routine
neurologic examination. There are special techniques
that may give some information, such as tests for the
appreciation of the sensations of distention, pain,
heat, and cold in the bladder during cystometric
examination.
BIBLIOGRAPHY
Gilman S. Clinical Examination of the Nervous System. New York:
McGraw-Hill, 2000.
Gilman S, Newman SW. Manter and Gatz’s Essentials of Clinical
Neuroanatomy and Neurophysiology. 10th ed. Philadelphia: FA
Davis, 2003.
Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science.
4th ed. New York: McGraw-Hill, 2000.
Williams PL. Gray’s Anatomy: the Anatomical Basis of Medicine and
Surgery. 38th ed. New York: Churchill Livingstone, 1995.
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