Question 10(p.2)

Describe the physiology of vomiting. 

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College Answer

For a good answer, candidates were expected to take the following approach and 
content of information. 
Triggers or initiators of vomiting include:

  • Excessive distension or irritation of the upper GI tract, in particular the 
  • duodenum
  • Stimulation of the chemoreceptor trigger zone (CTZ)
    • Directly by certain drugs eg apomorphine, morphin
    • Rhythmic motion of the body stimulating the vestibular labyrinth of the inner ear 
  • Cerebral excitation of vomiting by stimuli such as disquieting scenes, odors

Neuronal pathways:

  • Stimuli of the GI tract conveyed by vagal and sympathetic afferents to the bilateral vomiting centre within the medulla.
  • Efferent arc from the vomiting centre via the 5th,7th,9th,10th and 12th cranial nerves, and spinal nerves to the abdominal wall muscles (and the diaphragm). 

Vomiting act:

  • Antiperistalsis as the prelude to vomiting
  • At the onset of vomiting, strong intrinsic contractions occur in both the duodenum and the stomach
  • Partial relaxation of the lower oesophageal sphincter (LOS)
  • Deep breath
  • Raising of the hyoid/larynx to open the upper oesophageal sphincter
  • Glottic closure
  • Lifting of the soft palate to close the posterior nares
  • Strong down ward contraction of diaphragm and simultaneous contraction of all the abdominal wall muscles
  • Complete relaxation of the LOS

Most candidates answered this question well. Antiemetic drugs and their mechanism 
of action gained no marks. 


This is a rare and welcome set of college comments which very clearly outlines what an ideal answer would have been, making any alternative model answer pointless. Still, one is offered here, purely to be contrary:

Neurology of nausea and vomiting:

  • Stimuli: toxinaemia, sensory stimuli, visceral afferents, psychological stimuli
  • Sensors: Chemoreceptor trigger zone, sense organs (eg. visual, olfactory and gustatory), vestibular labyrinth, mechanoreceptors in the GIT.
  • Afferents: 
    • Vagus nerve (to the NTS): main receptors involved are 5-HT3, D2, H1 and muscarinic.
    • Vestibular apparatus: main receptors involved are H1 and muscarinic.
    • Ill-defined central pathways (to the CTZ): main receptors involved are 5-HT3, D2, H1 and muscarinic.
  • Central processors:
    • Nucleus of the solitary tract (NTS) integrates vagal and vestibular inputs
    • Chemoreceptor trigger zone (CTZ) integrates inputs from the higher CNS, vestibular apparatus, and directly senses blood-borne toxins
    • "Central pattern generator", the exact position of which is unknown, but thought to be in the reticular area dorsomedial to the retrofacial nucleus, integrates the inputs from NTS, CTZ and higher cortical areas
  • Efferents: 
    • Descend from the central pattern generator to:
      • Vagus nerve
      • Hypoglossal nerve
      • Glossopharyngeal nerve
    • Ascend from the central pattern generator to:
      • hypothalamus and insula, which coordinate the sympathetic responses
      • Motor cortex, which coordinates the somatic motor response

Mechanics of vomiting:

  • Phase 1: giant retrograde peristaltic contraction, with closed lower oesophageal sphincter and relaxed abdominal muscles
  • Phase 2: increased intraabdominal pressure due to abdominal wall and chest muscle contraction, with closed lower oesophageal sphincter
  • Phase 3: abdominal wall contraction, pyloric contraction, with open lower oesophageal sphincter, coordinated with the upper airway to prevent aspiration


Horn, Charles C. "The physiology of vomiting." Nausea and Vomiting. Springer, Cham, 2017. 15-25.

Parkes J.D. (1986) A Neurologist’s View of Nausea and Vomiting. In: Davis C.J., Lake-Bakaar G.V., Grahame-Smith D.G. (eds) Nausea and Vomiting: Mechanisms and Treatment. Advances in Applied Neurological Sciences, vol 3. Springer, Berlin, Heidelberg.

Singh, Prashant, Sonia S. Yoon, and Braden Kuo. "Nausea: a review of pathophysiology and therapeutics." Therapeutic advances in gastroenterology 9.1 (2016): 98-112.

Stern, R. M. "The psychophysiology of nausea." Acta Biologica Hungarica 53.4 (2002): 589-599.

Horn, Charles C. "Why is the neurobiology of nausea and vomiting so important?." Appetite 50.2-3 (2008): 430-434.

Napadow, Vitaly, et al. "The brain circuitry underlying the temporal evolution of nausea in humans." Cerebral cortex 23.4 (2013): 806-813.

Donnerer, Josef. "The emetic reflex arc." Antiemetic therapy (2003): 1.

Willems, J. L., and R. A. Lefebvre. "Peripheral nervous pathways involved in nausea and vomiting." Nausea and vomiting: Mechanisms and treatment. Springer, Berlin, Heidelberg, 1986. 56-64.

Koga, Tomoshige, and Hiroyuki Fukuda. "Descending pathway from the central pattern generator of vomiting." Neuroreport 8.11 (1997): 2587-2590.

Lang, Ivan M. "Digestive tract motor correlates of vomiting and nausea." Canadian journal of physiology and pharmacology 68.2 (1990): 242-253.