Describe the physiology of vomiting.
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
Cerebral excitation of vomiting by stimuli such as disquieting scenes, odors
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).
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)
Raising of the hyoid/larynx to open the upper oesophageal sphincter
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.