= Solution
\b[<Vomiting> is a coordinated nervous motor act that can be initiated from the <digestive system>, circulating emetic substances, the <vestibular system>, or higher and lower <brain> pathways.] A cause need not lie in the <stomach>.
Local causes include irritant food or poisons, <gastritis>, <gastroenteritis>, <gastric outlet obstruction> and <intestinal obstruction>. Chemical irritation, <inflammation> or distension activates visceral sensory pathways, notably afferents of the <vagus nerve>, which relay to the <medulla oblongata>. In obstruction, retention and distension provide continuing stimulation; merely seeing food return does not imply excessive gastric contractile strength. <Peritonitis> and painful disease elsewhere in the abdomen can also recruit visceral afferent pathways.<Nausea and Vomiting by Merck Manual Professional Edition>[Etiology; table Some Causes of Nausea and Vomiting, gastrointestinal disorders]{ref}
Circulating emetic agents, including some drugs and metabolic products accumulating in <kidney failure>, act partly through the <area postrema>, a chemosensitive region with unusually direct access to substances in the <blood>. Gastrointestinal and blood-borne inputs converge through the <nucleus tractus solitarius> and connected medullary networks. It is more accurate to describe interacting groups of <neurons> than one isolated “vomiting centre”.<Central neurocircuitry associated with emesis>[Abstract, area postrema, abdominal vagal afferents, nucleus tractus solitarius and distributed pattern generation]{ref}
Other important groups are <motion sickness>, where information from the <vestibular system> conflicts with other sensory information; <migraine> and intracranial disease, including <raised intracranial pressure> from a <brain tumour> or <intracranial haemorrhage>; and higher-centre influences such as a repulsive sight, smell, fear or learned association. <Pregnancy-associated vomiting> belongs in the differential, with endocrine and central contributions rather than an explanation that it is simply mechanical pressure by the <uterus> upon the <stomach>. These causes engage different inputs to an overlapping final motor programme.<Nausea and Vomiting by Merck Manual Professional Edition>[Etiology; table Some Causes of Nausea and Vomiting, CNS and systemic causes]{ref}
The act itself requires several coordinated changes. The proximal <stomach> and <lower oesophageal sphincter> relax; retrograde activity can bring contents of the upper <intestine> into the <stomach>; the <diaphragm> and abdominal <muscles> generate a pressure gradient that expels the contents. During expulsion the upper passage opens while coordinated laryngeal closure helps protect the <airway>. <Retching> and actual expulsion are related stages, and neither should be confused with effortless <regurgitation>.<Physiology of the Digestive Tract Correlates of Vomiting>[Digestive tract correlates and neurophysiology; relaxation, retrograde giant contraction, retching and expulsion]{ref}
This mechanism explains the diversity of causes: \b[many sensory and chemical inputs can recruit the same coordinated output]. A pressure recording and observation of gastrointestinal movements during an emetic response can distinguish the patterned act from a simple spontaneous return of contents; they would not, without further evidence, identify the original cause in an individual.
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