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 
 Stimulation of the chemoreceptor trigger zone (CTZ)
o Directly by certain drugs eg apomorphine, morphin
o 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.