2. VOMITING
Vomiting is the means by which the upper gastrointestinal tract rids itself of its
contents when almost any part of the upper tract becomes excessively
irritated, overdistended, or even overexcitable.
Excessive distention or irritation of the duodenum provides an especially strong
stimulus for vomiting.
The sensory signals that initiate vomiting originate mainly from the pharynx,
esophagus, stomach, and upper portions of the small intestines.
The nerve impulses are transmitted, by both vagal and sympathetic afferent
nerve fibers to multiple distributed nuclei in the brain stem, especially the
area postrema, that all together are called the “vomiting center.”
motor impulses that cause the actual vomiting are transmitted from the vomiting
center by way of the fifth, seventh, ninth, tenth, and twelfth cranial nerves to
the upper gastrointestinal tract, through vagal and sympathetic nerves to the
lower tract, and through spinal nerves to the diaphragm and abdominal muscles.
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7. Antiperistalsis, the Prelude to Vomiting.
In the early stages of excessive gastrointestinal irritation or overdistention,
antiperistalsis begins to occur often many minutes before vomiting appears.
Antiperistalsis means peristalsis up the digestive tract rather than downward.
This may begin as far down in the intestinal tract as the ileum, and the
antiperistaltic wave travels backward up the intestine at a rate of 2 to 3 cm/sec;
this process can actually push a large share of the lower small intestine contents all
the way back to the duodenum and stomach within 3 to 5 minutes.
Then, as these upper portions of the gastrointestinal tract, especially the
duodenum, become overly distended, this distention becomes the exciting factor
that initiates the actual vomiting act.
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9. At the onset of vomiting, strong intrinsic contractions occur in both the
duodenum and the stomach, along with partial relaxation of the
esophageal-stomach sphincter, thus allowing vomitus to begin moving
from the stomach into the esophagus.
a specific vomiting act involving the abdominal muscles takes over and
expels the vomitus to the exterior.
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12. Vomiting Act.
Once the vomiting center has been sufficiently stimulated and the vomiting act
instituted, the first effects are the following;
1) a deep breath
2) raising of the hyoid bone and larynx to pull the upper esophageal
sphincter open
3) closing of the glottis to prevent vomitus flow into the lungs
4) lifting of the soft palate to close the posterior nares.
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16. Thus, the vomiting act results from a squeezing action of the muscles of the
abdomen associated with simultaneous contraction of the stomach wall and
opening of the esophageal sphincters so that the gastric contents can be
expelled.
Next comes a strong downward contraction of the diaphragm along with
simultaneous contraction of all the abdominal wall muscles.
This squeezes the stomach between the diaphragm and the abdominal
muscles, building the intragastric pressure to a high level.
Finally, the lower esophageal sphincter relaxes completely, allowing
expulsion of the gastric contents upward through the esophagus.
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18. Chemoreceptor “Trigger Zone” in the Brain Medulla for
Initiation of Vomiting by Drugs or by Motion Sickness.
Aside from the vomiting initiated by irritative stimuli in the gastrointestinal tract,
vomiting can also be caused by nervous signals arising in areas of the brain.
This mechanism is particularly true for a small area called the chemoreceptor
trigger zone for vomiting, located in the area postrema on the lateral walls of
the fourth ventricle.
Electrical stimulation of this area can initiate vomiting, but more importantly,
administration of certain drugs, including apomorphine, morphine, and some
digitalis derivatives, can directly stimulate this chemoreceptor trigger zone and
initiate vomiting.
Destruction of this area blocks this type of vomiting but does not block vomiting
resulting from irritative stimuli in the gastrointestinal tract itself
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22. Furthermore, it is well known that rapidly changing direction or rhythm of
motion of the body can cause certain people to vomit.
The mechanism for this phenomenon is the following:
The motion stimulates receptors in the vestibular labyrinth of the inner ear,
and from here impulses are transmitted mainly by way of the brain stem
vestibular nuclei into the cerebellum, then to the chemoreceptor trigger
zone, and finally to the vomiting center to cause vomiting
