The document summarizes the mechanism of nausea and vomiting. It discusses that nausea and vomiting are protective reflexes against toxins. The vomiting center in the medulla integrates afferent signals and coordinates the vomiting reflex. The vomiting center can be stimulated through irritation in the gastrointestinal tract, signals from the chemoreceptor trigger zone, stimulation of the vestibular system, or from higher brain centers. When stimulated, the vomiting center initiates the motor pathways that lead to nausea, salivation, and the expulsion of gastric contents through vomiting.

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Table of Contents
Background ................................................................................................................................................. 2
Etiology ........................................................................................................................................................ 2
Pathophysiology ......................................................................................................................................2-5
Management ...........................................................................................................................................5-6
Conclusion .................................................................................................................................................. 6
Refernces .................................................................................................................................................... 6

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—
Background
Nausea and vomiting are basic human protective reflexes against the absorption of toxins, as
well as responses to certain stimuli.9 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. Nausea which is often a prodrome of vomiting,
and is the conscious recognition of subconscious excitation in an area of the medulla closely
associated with or part of the vomiting center.1 And the aim of this paper, is to understand what
nausea and vomiting is? And how does it occur? Which is based on several references, both
medical books and articles.
Etiology
The main etiologies for nausea and vomiting include iatrogenic, that is almost any
medication that can cause nausea and vomiting, such as, chemotherapeutic agents. Toxic
infectious causes, which is mainly caused by ingestion of a toxin. The other causes contain
gastrointestinal disorders, central nervous system or psychiatric conditions, and some conditions
caused by metabolism such as Pregnancy, which is the most common endocrinologic cause of
nausea and vomiting. 2
Pathophysiology
There are many conditions out there that cause nausea and vomiting, but we know that there
are at least three kinds of nausea and vomiting, the first of which has been attributed to
anaesthetics such as ether, the second to reflex responses, the last to opioids. 8
Nausea and vomiting occurs via different mechanisms and pathways. The afferent and
efferent reflexes involved in nausea and vomiting are integrated with two distinct centers at
brainstem level, which are the vomiting center, where it is proposed that, it is multiple distributed
nuclei located in the medulla, that contain histamine (H1), acetylcholine (ACh) and
5hydroxytryptamine2 (5HT2) receptors, however, it is now thought that an anatomically discrete
vomiting center is unlikely to exist, as proposed nearly 60 years ago by Wang and Borison.
Rather, a number of loosely organized neuronal areas within the medulla probably interact to
coordinate the emetic reflex, and the neurons coordinating the complex series of events that
occur during emesis have been termed the “central pattern generator”.6 And the second center is
chemoreceptor trigger zone (CTZ), which is located bilaterally on the floor of the fourth
ventricle in area postrema near the obex, where this part has no blood-brain barrier and may be
accessible to humoral stimuli in either blood or cerebrospinal fluid, so various drugs, toxins and
metabolites can access it, and has dopamine (D2), 5HT3 and Neurokinin-1 receptors. The
vomiting center receives afferent stimulation via the CTZ, the cerebral cortex, and the vestibular
system, and peripheral stimulation via the vagal and sympathetic nerves which, in turn, leads to
emesis.

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Once the vomiting center stimulated, the vomiting reflex occurs which is divided into two
phases: (1) Pre-ejection phase, which is characterized by a sensation of nausea associated with
cold, sweating, pupil dilatation, salivation and tachycardia mediated by sympathetic and
parasympathetic nerves, (2) Ejection phase, where it comprises of retching and vomiting with
expulsion of gastric contents.
Nausea Which is the prodrome of vomiting, is excitation of part of the medulla that is
closely associated with vomiting center, which can be caused by (1) irritative impulses coming
from the gastrointestinal tract, (2) impulses that originate in the lower brain associated with
motion sickness, or (3) impulses from the cerebral cortex to initiate vomiting. But it doesn’t
mean that Nausea and Vomiting are coupled together, because vomiting can occasionally occur
without the prodromal sensation of nausea, indicating that only certain portions of the vomiting
center area associated with the sensation of nausea. Vomiting usually starts with salivation and
the sensation of nausea, and the vomiting reflex occurs only when through stimulation of the
vomiting center.
There are several ways that vomiting center is stimulated, the First way is when the stomach
and the upper portions of the small intestine faces irritation, excessive distention or
overexcitation, it causes antiperistalisis -the prelude of vomiting-, often appears many minutes
before vomiting, which is 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 and an impulse is formed by
stimulation of H1 and ACh receptors, leading to an
afferent stimulus that terminates in the brain stem by
both vagal and sympathetic afferent nerve fibers,
primarily in the nucleus tractus solitarius, and
subsequently activates the vomiting center, which is
shown in figure (1).
The emetic stimuli in gut are detected by two types
of vagal afferent fibres, mechanoreceptors and
chemoreceptors, and the abdominal vagal afferents
appear to have the greatest relevance for
chemotherapy-induced nausea and vomiting, and
post-operative nausea and vomiting.
Second way is arising nervous signals in
chemoreceptor trigger zone. Electrical stimulation
and administration of certain drugs, including
apomorphine, morphine, and some digitalis
derivatives, can directly stimulate this
chemoreceptor trigger zone and in return stimulating
Vomiting center, as shown in figure(1). 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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Third way, is through stimulation of Ach or H1 receptors in the vestibular labyrinth of the
inner ear, when direction or rhythm of motion of the body is changed, which then transmitted to
vestibular nuclei that mediates the nausea and vomiting of motion sickness, from there, it is
transmitted into the cerebellum, then to the chemoreceptor trigger zone, and finally to the
vomiting center to cause vomiting.
Fourth way is stimulation of the Vomiting
Center from higher centers of the brain. When a
stimulation is formed from the diencephalon,
limbic system and cerebral cortex, then
transmitted to the cortical center, after that to
vomiting center to cause vomiting. Because
emetic responses to emotionally charged
stimuli such as nauseating smells, sickening
sights and pain occur. In addition to the
pathways already described, when there is
raised intracranial pressure cerebral histamine
receptors may be stimulated and meningeal
mechanoreceptors stimulate the vomiting
center. Also vestibular cardiac afferent induce
nausea and vomiting as in MI, and pharyngeal
stimulation may induce nausea and vomiting,
too. Figure (2) demonstrates the most of the
pathways for the stimulation of the vomiting
center, which we talked about before.9
Once the vomiting center has been
sufficiently stimulated and the vomiting act
instituted, the vomiting act takes place, where
motor impulses that cause the actual vomiting
are transmitted from the vomiting center by
way of the 5th, 7th, 9th, 10th, and 12th 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, which cause the following effects that are (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, and (4) lifting of the soft palate to close the posterior
nares. Also Serotonin (5-HT) released from enterochromaffin cells in the small intestine that
initiate impulses via 5-HT 3 receptors trigger vomiting, where reverse peristalsis empties
material from the upper part of the small intestine into the stomach, and it causes distention of
the stomach, which means further stimulation of the vomiting center. Next comes a strong
downward contraction of the diaphragm along with simultaneous contraction of all the
abdominal wall muscles, along with partial relaxation of the esophageal-stomach sphincter, thus
allowing vomitus to begin moving from the stomach into the esophagus. 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, then the vomitus is expelled to the exterior
easily. 1, 4,6,8 The vomiting act is simply shown in Figure (3). 7

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Management
A wide variety of antiemetic agents are available for the prevention and treatment of nausea and
vomiting. These agents can be classified according to the therapeutic index of their usefulness as
high index, such as, 5-HT3 antagonists or low index, such as, cannabinoids. The table below
shows some common antiemetic agents.
Class of medication Example Common uses
Antihistamines cyclizine Possible adjunct for cytotoxic
chemotherapy, prophylaxis
and treatment of motion
sickness
Butyrophenones droperidol Anticipatory and acute
chemotherapeutic nausea and
vomiting, postoperative
nausea and vomiting
Cannabinoids dronabinol Refractory chemotherapy-
related nausea and vomiting
Corticosteroids dexamethasone Adjunct for chemotherapy
related symptoms
5-HT3 Antagonists dolasetron Post chemotherapy nausea and
vomiting, severe nausea and
vomiting

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There are also alternative treatments such as, Acupuncture, Acupressue, Ginger (powdered root),
and Pyridoxine for conditions like, chemotherapy-induced nausea and vomiting, post-operative
nausea and vomiting and early pregnancy nausea and vomiting.2,7
Conclusion
After over 150 years of research, still the mechanism of nausea and vomiting is not very clear?.
there are four factors that help us answer this ‘million’ dollar question: the First factor is
complexity of the problem, due to the variables which are so many that it becomes difficult to
identify the mechanism to assess the effects of an intervention, as it requires a considerable
number of patients in well controlled trials. Second is lack of animal model to study the
physiology and pharmacology of the mechanism of nausea and vomiting. Many species, such as,
rat and rabbit do not vomit irrespective of stimulus. Though, monkeys and dogs respond to the
same range of emetic stimuli as man with cytotoxic drugs and radiation, they do not suffer from
pregnancy and motion sickness and post-operative and post-anaesthetic emesis. The last factor is
inadequate quantification of the phenomena, irrespective of the huge number of clinical trials,
the phenomena have been poorly quantified. 8
References
1. Guyton AC, Hall JE. Gastrointestinal Physiology. Textbook of Medical Physiology 11th
ed. P823.
2. Scorza K, Williams K, Phillips JD, Shaw J. Evaluation of Nausea and Vomiting.
American Family Physician, 2007. V76, N1, P76-7.
3. Einarson TR, Piwko C, Koren G. Prevalence of Nausea and Vomiting of pregnancy in the USA.
University of Toronto.
4. Barrett KE, Boitano S, Barman SM, Brooks HL. Gastrointestinal Physiology. Ganong’s
Review of Medical Physiology 24ed .P502.
5. Chand S. Nausea and vomiting in palliative care, 2014. The Pharmaceutical Journal.
P2,3.
6. Hesketh PJ. Chemotherapy-Induced Nausea and Vomiting. N Engl J Med
2008.358,2482-94.
7. Paulev PE, Calleja GZ. New Human Physiology 2nd ed. Gastrointestinal Function and
Disorders, Chap22.
8. Islam S, Jain PN. post-operative nausea and vomiting (PONV) : a review article. Indian J.
Anaesth, 2004.48(4):253-258.
9. Garrett K, Tsuruta K, Walker S, Jackson S, Sweat M. Managing Nausea and Vomiting.
Critical Care Nurse, 2003. V23, N1.