The Physiology Of Nausea And Vomiting

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Nausea is a highly subjective and peculiarly unpleasant sensation, quite distinct from other feelings. It is "felt"

Encyclopedia of the Human Brain Volume 3

Copyright 2002, Elsevier Science (USA).

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in the throat or stomach or as a "sinking" sensation in the epigastrium; the symptoms may include malaise, drowsiness, anxiety, and reduced vigilance. It is invariably accompanied by autonomic changes including vasomotor disturbances, causing vasocon-striction-pallor and pupil dilation, tachycardia, salivation and sweating, and a relaxation of the lower part of esophagus and abdominal muscles. The latter tends to increase tension of the gastric and esophageal muscles, which may directly contribute to the sensation of nausea. In the acute stage it is perceived as an unpleasant and temporary effect that precludes other mental and physical activity. It frequently precedes vomiting (although either may occur alone) and usually is relieved by an emetic episode, which the subject or patient may welcome. The presence of chronic nausea severely reduces the quality of life.

Following the relaxation of the lower part of the esophagus and abdominal muscles, a contraction of the upper small intestine occurs closely followed by contraction of the pyloric sphincter and the pyloric portion of the stomach. These changes will empty the contents of the upper jejunum, doudenum, and pyloric portion of the stomach into the fundus and body of the stomach, which are relaxed. Also relaxed are the cardiac sphincter, esophagus, and esophageal sphincter. In this manner the system is prepared for retching and vomiting, which are reflex in origin and serve to remove the contents of the upper gastrointestinal tract.

This involves a series of highly coordinated changes in gastrointestinal motility, respiratory movements, and posture. Emesis, which is initiated by a deep and sharp inspiration, is immediately followed by reflex closure of the glottis and raising of the soft palate, which prevent the passage of vomitus into the lungs and nasal cavity. The abdominal muscles then contract in the rhythmic manner of "retching" movements, which compress the stomach between the contracted diaphragm and abdominal organs. The inevitable increase in intragastric pressure causes evacuation of the stomach contents through the relaxed esophagus; definite antiperistalsis in the stomach itself is rarely observed. These events and pressure changes are shown in Fig. 1. This profile of activities can vary somewhat between species and also in the human infant. In the latter, the abdominal muscles or diaphragm apparently does not play a role in, for example, the regurgitation of an oversized meal; the reverse peristalsis occurs by contraction of the stomach muscle alone.

The feelings of nausea followed by retching or vomiting are the expression of a precisely controlled reflex, albeit of an extraordinarily complex nature. Almost irrespective of the causes of nausea and emesis, the autonomic and somatic patterns of motor activities are the same. Also, because the presence of the emetic reflex occurs in so many species, it would have been predicted that the actual function of nausea and

Emesis Mechanism

Figure 1 The major mechanomotor components of retching and vomiting. Abbreviations: EMG, electromyogram; UOS, upper esophageal sphincter. Andrews, P. L. R., and Davis, C. J. (1995). The physiology of emesis induced by anticancer therapy. In Serotonin and the Scientific Basis of Antiemetic Therapy (D. J. M. Reynolds, P. L. R. Andrews, and C. J. Davis, Eds.), pp. 25-49. Oxford Clinical Communications, Oxford. Reproduced by permission of Oxford Clinical Communications.

Figure 1 The major mechanomotor components of retching and vomiting. Abbreviations: EMG, electromyogram; UOS, upper esophageal sphincter. Andrews, P. L. R., and Davis, C. J. (1995). The physiology of emesis induced by anticancer therapy. In Serotonin and the Scientific Basis of Antiemetic Therapy (D. J. M. Reynolds, P. L. R. Andrews, and C. J. Davis, Eds.), pp. 25-49. Oxford Clinical Communications, Oxford. Reproduced by permission of Oxford Clinical Communications.

vomiting should be well-established. But this has not occurred. The physiological "value" of nausea and vomiting is usually described as a "defense mechanism" to protect the host against a toxic challenge. Thus, the activation of smell or taste receptors causing nausea could warn the organism of a noxious or poisonous substance in order to avoid its ingestion. Similarly, if toxins are actually ingested, the chemor-eceptors in the stomach would warn of the presence of danger at an early stage and emesis would promptly expel the threat. However, such explanations remain unconvincing.

First, there are numerous toxins that do not cause nausea or emesis and simply kill the host; the defense mechanism is inadequate at best. More important, the defense theory fails to provide any reasonable explanation for the induction of nausea or emesis caused by nontoxins and, therefore, all other emetic stimuli, e.g., changes in blood pressure, irritation of the throat, pain, psychogenic stimuli, increased intracranial pressure, and motion. It is concluded that we have no satisfactory explanation for the phylogenetic persistence of the emetic reflex: its value appears extraordinarily limited. The absence of an emetic reflex in rats, mice, and other species confirms its nonessential role.

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