Vomiting is a coordinated bodily process and protective reflex. The body uses this mechanism to expel harmful substances from the stomach. This action involves a sequence of events, directed by the brain and muscles for swift removal.
How the Brain Directs the Process
The vomiting center, located in the medulla oblongata of the brainstem, is the central command center for vomiting. It integrates signals from the body to coordinate emesis, orchestrating the muscular movements.
Adjacent to the vomiting center is the chemoreceptor trigger zone (CTZ), located in the area postrema. The CTZ detects toxins because it lies outside the blood-brain barrier, allowing direct access to circulating substances.
Upon detecting harmful chemicals, the CTZ relays these signals to the vomiting center. The CTZ has receptors that bind to emetic agents. This informs the vomiting center about potential threats before coordinating the response.
What Triggers the Vomiting Reflex
Many stimuli activate the vomiting center, leading to the reflex. GI tract irritation, like food poisoning, is a common trigger. Receptors in intestinal lining detect abnormal contractions or harmful chemicals, sending signals to the brain.
Sensory inputs also trigger the reflex. Unpleasant sights, odors, or tastes send impulses from higher brain centers to the vomiting center. Intense pain or emotions like fear and anxiety also stimulate the reflex.
Motion sickness results from vestibular system activation in the inner ear. Signals from this system travel to the central nervous system, activating the chemoreceptor trigger zone. This explains why balance disturbances lead to nausea and vomiting.
Chemicals and toxins, including medications or alcohol, directly stimulate the CTZ. Its location outside the blood-brain barrier makes it susceptible to these substances. Increased intracranial pressure also induces vomiting, sometimes without nausea, by affecting the brainstem.
The Body’s Expulsion Sequence
Vomiting unfolds in an involuntary sequence once the brain’s command center is activated. This often begins with prodromal symptoms like nausea, an unpleasant sensation preceding expulsion. Increased salivation, sweating, and a pale appearance may also occur.
Following the prodromal stage, retching, or “dry heaves,” occurs. This involves involuntary contractions of the abdominal muscles, diaphragm, and chest wall against a closed glottis. No stomach contents are expelled during retching, but these contractions create negative pressure in the chest and increased abdominal pressure.
The expulsion phase involves lower esophageal sphincter relaxation, allowing stomach contents to move upward. The diaphragm contracts downward, and the abdominal muscles contract. This coordinated action generates pressure, propelling gastric contents out through the mouth.
During expulsion, the glottis closes to protect the airway, preventing stomach contents from entering the lungs. The soft palate rises, sealing nasal passages, preventing regurgitation through the nose. The force for expulsion comes from the abdominal muscles and diaphragm, as the stomach does not contract.