Vomiting is the process by which gastric contents come out of the mouth due to reverse peristalsis in the small intestine and an organized sequence of muscular activity secondary to a well-coordinated nervous circuit both of which eventually result in expulsion of the vomitus.
How does vomiting occur?
Vomiting occurs when the vomiting centres (present in the lateral reticular formation of the medulla) are stimulated. The following areas can stimulate the vomiting centre:
- The Chemoreceptor Trigger Zone: This area is located in the floor of the 4th ventricle of the brain and constitutes the area postrema, a circumventricular organ which is devoid of the blood brain barrier due to which substances such as drugs and toxins carried in the blood can directly reach it. This explains why some drugs cause or stop vomiting. This zone contains dopamine D2, serotonin 5-HT3, opioid, acetylcholine and substance P receptors whose activations results in different pathways, the end result of all of which includes substance P.
- The Vagus Nerve: This is the 10th cranial nerve and gets activated whenever the pharynx is activated and the gag reflex results.
- The Vestibular System: This system controls balance and sends its input to the CNS via the vestibular nerve, playing a role in any disturbance of balance (such as motion sickness), eventually resulting in vomiting. This system has a rich supply of muscarinic receptors.
- Vagal and Enteric Nervous System: The nervous system of the gut, when stimulated, can induce vomiting via the vagal afferents or its rich sympathetic nervous supply. The gastrointestinal tract can be irritated by chemotherapy, radiation, certain drugs, severe distension and infection.
- The CNS: When the higher brain centres such as the cerebrum are affected by stress or certain psychiatric conditions, this can also result in vomiting.
So to sum up, whatever stimulates these five components will stimulate the vomiting centre, thus causing vomiting.
The Phases of Vomiting:
Vomiting consists of three phases;
The physiology behind vomiting:Vomiting is mediated by the parasympathetic & sympathetic nervous system along with the motor system, as described below:
- The parasympathetic nervous system causes excessive salivation to protect the tooth enamel from the acid contents of the stomach when are vomited out.
- The sympathetic nervous system causes sweating as well as increase in heart rate.
- The motor system causes a deep breath so that the vomitus is not aspirated. In addition, reverse peristalsis begins from the middle of the small intestine so that its contents are brought up to the stomach, passing the pyloric sphincter, which lies relaxed at this point due to the parasympathetic activity. Contrary to popular belief, the oesophagus or stomach do not contract and there is not negative peristalsis in these organs. Then how is the vomitus expelled? That happens due to two processes: inspiration against a closed glottis and contraction of the abdominal musculature. The former lowers the pressure inside the thoracic cavity and the latter increases abdominal pressure, thus propelling the contents of the small intestine to move easily from a region of high pressure to a region of low pressure. The stomach is contracted only at the angular notch whereas the lower oesophageal sphincter relaxes when the stomach contents have to be propelled out. To put this in to order, inspiration lowers the thoracic pressure while the lower oesophageal sphincter is contracted and the abdominal muscles are contracting forcefully, thus building up pressure within the abdominal cavity. The intestines are undergoing negative peristalsis and as soon as their contents reach the lower oesophageal sphincter, it opens and pressure is suddenly released, propelling the gastric contents out of the mouth through the relaxed oesophagus.
Vomiting: The molecular level:
Vomiting is mediated by neurotransmitters such as histamine, dopamine, serotin, neurokinin and vasopressin, with the former three being the main controllers. That is why dopamine, serotonin and histamine antagonists are given for the suppression of vomiting.