Vomiting Reflex Physiology
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Direct answer
Vomiting is a reflex coordinated by a central pattern generator in the dorsolateral medulla — the nucleus tractus solitarius and adjacent reticular formation — receiving input from the chemoreceptor trigger zone in the area postrema, the vestibular nuclei, the vagal and glossopharyngeal afferents of the gut, and higher centres. The chemoreceptor trigger zone lies at the caudal fourth ventricle floor outside the blood-brain barrier, which is why circulating emetogenic drugs (apomorphine, digoxin, opioids, chemotherapy) reach it directly. The act itself comprises nausea, retching, then a deep inspiration with glottic closure and forceful abdominal contraction expelling gastric content, preceded by reverse peristalsis from the mid-small intestine.
What you must remember
- Central machinery: nucleus tractus solitarius and reticular formation form the vomiting centre pattern generator; the chemoreceptor trigger zone (CTZ) in the area postrema is a sensory input, not the generator itself.
- Why the CTZ sees everything: it sits outside the blood-brain barrier in the area postrema of the fourth ventricle floor, sampling blood and cerebrospinal fluid for drugs and toxins — the anatomic fact that explains drug-induced vomiting.
- Receptor map (the pharmacology bridge): dopamine D2 at the CTZ (metoclopramide, domperidone), serotonin 5-HT3 on vagal afferents and the CTZ (ondansetron for chemotherapy), muscarinic and H1 at the vestibular nuclei (scopolamine and promethazine for motion sickness), neurokinin NK1 centrally (aprepitant), and cannabinoid CB1 as an adjunct.
- Inputs catalogued: CTZ (drugs, uraemia, diabetic ketoacidosis, radiation), vestibular (motion, labyrinthitis), vagal-glossopharyngeal visceral afferents (gastric distension, mucosal irritation, obstruction), and cortical (fear, smells, raised intracranial tension — projectile vomiting without nausea).
- The sequence: salivation and sweating, deep inspiration, glottic closure, abdominal and diaphragmatic contraction against a fixed thorax, gastric content expelled; reverse peristalsis starts in the proximal jejunum, sweeping content back to the stomach first.
- Metabolic consequence: loss of gastric hydrogen and chloride with volume contraction produces the classic hypochloraemic, hypokalaemic metabolic alkalosis with paradoxical acidic urine (pyloric stenosis stem).
- Retching without vomit: rhythmic respiratory movements against a closed glottis without expulsion — and dry attempts in early pregnancy illustrate a cortical-hormonal input on the same machinery.
How to work through a vomiting patient
A two-month-old with progressive non-bilious projectile vomiting: the physiology is gastric outlet obstruction, so the lost fluid is pure gastric juice — hydrogen and chloride leave, the kidney initially excretes sodium bicarbonate to maintain pH, and the result is hypochloraemic hypokalaemic metabolic alkalosis with paradoxical aciduria (the kidney sacrifices hydrogen to defend volume as aldosterone surges). Correct with saline plus potassium chloride — chloride is the key that turns off renal bicarbonate loss — before definitive pyloromyotomy.
Contrast the chemotherapy patient: cisplatin triggers enterochromaffin cell serotonin release in the gut, stimulating vagal 5-HT3 afferents and the CTZ — hence ondansetron plus dexamethasone as standard prophylaxis, with aprepitant added for highly emetogenic regimens. Motion sickness is the vestibular arm: a sensory mismatch between vestibular and visual input reaches the vomiting machinery through muscarinic and histaminic synapses, which is why scopolamine and promethazine work while ondansetron — aimed at 5-HT3 — is nearly useless for it. A patient with raised intracranial pressure vomiting without preceding nausea localises centrally: the medullary centres are directly irritated, a red flag examiners embed in case stems.
Where students slip
The long-standing error is calling the area postrema "the vomiting centre"; it is the chemoreceptor trigger zone, an input region outside the blood-brain barrier, while the true pattern generator is the nucleus tractus solitarius-reticular formation complex — mixing them up loses the whole logic of why drugs (which do not cross into most of the brain's emetic circuits) still emetise. The second slip is expecting ondansetron to work on everything: match receptor to cause — 5-HT3 for chemotherapy, D2 for drug and metabolic causes, H1-muscarinic for vestibular causes, NK1 for delayed chemotherapy nausea. In the Indian ward setting, anticipate the metabolic viva on paradoxical aciduria: explain volume-driven aldosterone, not the alkalosis itself, as the reason the urine turns acidic.
Frequently asked questions
Why can circulating drugs cause vomiting without crossing the blood-brain barrier?
The chemoreceptor trigger zone in the area postrema lies outside the blood-brain barrier and samples systemic blood and CSF, exposing its D2, 5-HT3 and other receptors to circulating emetogens like apomorphine, digoxin and opioids.
Which neurotransmitter receptors mediate chemotherapy-induced vomiting?
Serotonin 5-HT3 receptors on vagal afferents and the CTZ acutely, with substance P at NK1 receptors driving the delayed phase — hence the ondansetron plus aprepitant combination in emetogenic protocols.
Why does motion sickness respond to antimuscarinics and antihistamines but not ondansetron?
The vestibular input reaches the vomiting centre through acetylcholine (muscarinic) and histamine (H1) synapses without a serotonergic link, so scopolamine or promethazine blocks it while 5-HT3 antagonism does not.
What metabolic derangement does prolonged vomiting cause?
Hypochloraemic, hypokalaemic metabolic alkalosis from loss of gastric hydrogen and chloride, with volume depletion driving aldosterone-mediated potassium and hydrogen loss in urine — paradoxical acidic urine.
What distinguishes vomiting from raised intracranial tension?
It is often sudden and projectile without preceding nausea, because the medullary vomiting centres are compressed directly rather than stimulated through the usual afferent inputs.