Educational objective: The vomiting reflex is initiated by the chemoreceptor trigger zone (ie, area postrema) and nucleus tractus solitarius within the dorsal medulla at the caudal end of the fourth ventricle. The area postrema does not have a well-developed blood brain barrier, allowing it to be activated directly by emetogenic substances (eg, drugs, toxins) or indirectly by ascending afferents from the bowel wall (eg, chemotherapy-induced release of serotonin).