Interoceptive 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’s 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 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 of 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 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, 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 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.
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