Neural Integration of Vomiting

Notes for MD Anaesthesiology Examination

1. Overview

2. The "Vomiting Centre" - Anatomical Basis

• Nucleus tractus solitarius (NTS) - the primary integrating relay; receives all visceral afferent signals; coordinates salivatory, vasomotor, and respiratory centres
• Dorsal motor nucleus of the vagus - supplies parasympathetic efferents to the gut
• Area postrema - the chemoreceptor trigger zone (CTZ); sits on the floor/lateral walls of the 4th ventricle

3. Afferent Inputs to the Vomiting Centre

3.1 Gastrointestinal Tract (Vagal & Sympathetic Afferents)

• Sensory signals originate from pharynx, oesophagus, stomach, and proximal small intestine (especially duodenum, which is the most potent trigger)
• Transmitted via vagal (CN X) afferents and splanchnic sympathetic afferents to the NTS and VC
• GI mucosal irritation, distension, chemotherapy, radiation, or acute gastroenteritis causes enterochromaffin cells to release serotonin (5-HT)
• Serotonin activates 5-HT3 receptors on vagal/splanchnic afferent nerve terminals → sensory signals to the NTS/VC and CTZ → emesis
• GI tract afferents also carry input from mechanoreceptors (stretch) and chemoreceptors

3.2 Chemoreceptor Trigger Zone (CTZ / Area Postrema)

• Located in the area postrema on the lateral walls of the 4th ventricle (floor in some descriptions)
• Critically, it lies outside the blood-brain barrier (a circumventricular organ), making it directly accessible to blood-borne and CSF-borne emetogenic substances
• Receptor-rich zone:
  • Dopamine D2 receptors (major target of metoclopramide, droperidol, haloperidol, phenothiazines)
  • Opioid receptors (mu > delta) - why morphine, opioids cause PONV
  • Serotonin 5-HT3 receptors (target of ondansetron, granisetron, etc.)
  • NK1 receptors (substance P) (target of aprepitant, fosaprepitant)
• Drugs that directly stimulate the CTZ: apomorphine, morphine, digitalis, dopamine agonists, cancer chemotherapy agents
• The CTZ signals the NTS/VC to trigger the efferent vomiting response
• Destruction of the area postrema abolishes drug-induced vomiting but NOT vomiting from direct GI irritation

3.3 Vestibular System (Motion Sickness Pathway)

• Motion stimulates receptors in the vestibular labyrinth of the inner ear
• Impulses travel via CN VIII → brainstem vestibular nuclei → cerebellum → CTZ → VC
• The vestibular system is rich in muscarinic M1/M2 and histamine H1 receptors
• This is the dominant pathway in motion sickness and accounts for post-operative vestibular emesis from head movements
• Drugs acting here: scopolamine (M1 antagonist), promethazine (H1 antagonist), meclizine, dimenhydrinate, cyclizine
• These drugs are effective for motion sickness but are ineffective against substances acting directly on the CTZ

3.4 Higher CNS (Cortical) Inputs

• The cerebral cortex and limbic system provide afferent input to the VC
• Responsible for:
  • Psychogenic vomiting (anxiety, fear, unpleasant sights/smells)
  • Anticipatory nausea and vomiting (conditioned response before chemotherapy)
  • Vomiting from increased intracranial pressure
  • Vomiting triggered by pain
• The role of higher centres explains why anxiolytic premedication (benzodiazepines) can reduce PONV

4. Efferent Pathways - The Vomiting Act

Sequential Events of the Vomiting Act:

1. Deep inspiration (to increase intrathoracic and intra-abdominal pressure)
2. Raising of hyoid bone + larynx → opens the upper oesophageal sphincter
3. Closure of the glottis (prevents aspiration into lungs)
4. Elevation of soft palate → closes posterior nares (prevents nasal regurgitation)
5. Retroperistalsis begins in the small intestine (antiperistaltic wave at 2-3 cm/sec travelling upward, sweeping contents back to the duodenum/stomach within 3-5 minutes)
6. Relaxation of lower oesophageal sphincter (LOS)
7. Simultaneous powerful contraction of diaphragm + abdominal muscles → squeezes stomach → raises intragastric pressure → forceful expulsion of gastric contents

Retching vs Vomiting: In retching, the upper oesophageal sphincter remains closed, so despite LOS opening and gastric contraction, contents return to the stomach. Actual vomiting requires upper oesophageal sphincter opening.

5. Prodrome - Nausea

• Nausea = conscious recognition of subconscious excitation of the medullary area closely associated with or part of the VC
• Produced by: (1) irritative GI impulses, (2) lower brainstem signals (motion sickness), (3) cerebrocortical signals
• Accompanied by increased vagal tone: hypersalivation, pallor, sweating, tachycardia or bradycardia
• Note: Vomiting can occur without nausea if only certain sub-portions of the VC are stimulated (explains projectile vomiting in raised ICP)
• In anaesthesia: sudden nausea at the onset of hypotension (particularly after spinal/epidural) is a classic warning sign of cardiovascular compromise

6. Neurotransmitters and Receptor Map

7. Anaesthetic Relevance - PONV Pathways

Key triggers in anaesthesia activating these pathways:

Apfel Score (PONV Risk Stratification):

1. Female sex
2. Non-smoker
3. History of PONV / motion sickness
4. Postoperative opioid use

8. Antiemetic Classification by Neural Target

Combination antiemesis (e.g. ondansetron + dexamethasone + droperidol) works by targeting multiple independent receptor pathways simultaneously - the cornerstone of PONV management in high-risk patients.

9. Clinical Pearls for the Exam

1. The CTZ lies outside the BBB - hence it can be activated by blood-borne toxins, drugs, and uraemic/hepatic metabolites without breaching the BBB. This is why renal or hepatic failure causes nausea/vomiting.
2. Vagotomy or high spinal anaesthesia abolishes GI afferent input but does not abolish vomiting from blood-borne stimuli (CTZ is intact).
3. Destruction of area postrema = abolishes drug-induced and uraemic vomiting, but NOT direct GI irritation-induced vomiting.
4. Projectile vomiting without nausea = raised ICP stimulating the VC directly via cortical pathways, bypassing the "nausea" sub-portions of the VC.
5. Increased vagal tone (bradycardia) commonly precedes or accompanies emesis in the PACU - always check BP and oxygenation before attributing it solely to PONV.
6. 5-HT3 antagonists are ineffective for motion sickness because motion sickness is mediated via the vestibular (H1/M1) pathway, not the vagal/serotonin pathway.
7. NK1 antagonists (aprepitant) are particularly effective for delayed CINV (>24h post-chemotherapy) because substance P drives the delayed phase.
8. Scopolamine anticholinergics are ineffective against CTZ-mediated vomiting (they act on M1 in the VC and vestibular nuclei, not on D2 or 5-HT3 in the CTZ).
9. Droperidol black box warning (QT prolongation) applies to high doses (5-15mg); antiemetic doses (0.625-1.25mg) are generally safe but require baseline ECG in at-risk patients.

10. Summary Diagram

• Guyton & Hall Textbook of Medical Physiology, 14e - Chapter 67
• Katzung's Basic and Clinical Pharmacology, 16e - Chapter 62 (Antiemetic Agents)
• Morgan & Mikhail's Clinical Anaesthesiology, 7e - Chapter 17 (PONV)
• Lippincott Illustrated Reviews Pharmacology - Chapter 42
• Costanzo Physiology, 7e - Chapter 8 (GI Physiology)