Vomiting is a forceful expulsion of the contents of the stomach and sometimes the gut. The whole mechanism is guided and controlled by the brain and its vomiting centre.

The vomiting centre of the brain

The fourth ventricle of the brain hosts the vomiting centre. The floor of the fourth ventricle contains an area called the chemoreceptor trigger zone (CTZ). It is also called the area postrema. When the CTZ is stimulated, vomiting may occur.

The CTZ contains receptors for dopamine, serotonin, opioids, acetylcholine and the neurotransmitter substance P. When stimulated, each of these receptors gives rise to pathways leading to vomiting and nausea. Present in high concentrations in the vomiting centre, Substance P seems to be involved in the final common pathways that give rise to vomiting.

The CTZ lies outside the blood-brain barrier (BBB). Normally, the BBB controls substances that affect the brain. Medications and chemicals in the blood have selective access to the brain when protected by the BBB. As the CTZ, however, lies outside the BBB, drugs and medications are capable of stimulating this area to trigger vomiting. Medications that control or are used to treat vomiting may inhibit this area to prevent vomiting.

Inputs to the vomiting centre in the brain

The CTZ in the brain is stimulated by various inputs from different parts of the body and this leads to vomiting. Some of the inputs to the CTZ include:
 

• Inputs from the vestibular system of the inner ear. These travel via the eighth cranial nerve or the vestibulocochlear nerve and are involved in motion sickness causing nausea and vomiting. There is an abundance of the H1 type of histamine receptors in this system that can be suppressed by the H1 type of antihistaminics to control vomiting induced by motion sickness.
• The tenth cranial nerve or the vagus nerve carries signals to the CTZ when the back of the throat or pharynx is irritated or stimulated. This is called the gag reflex.
• The nervous system around the gut or the enteric nervous system also transmits signals to the brain via the vagus nerve. It is via this system that radiation therapy, chemotherapy and gastroenteritis activate the 5-HT3 receptors leading to vomiting.
• Dopamine receptors are activated by stress and several psychiatric conditions, leading to vomiting.

The process of vomiting

The process of vomiting involves several phases and steps. These include:

• Stimulation of the CTZ leading to activation of the motor, parasympathetic and sympathetic nervous system
• Stimulation of the parasympathetic nervous system leading to increased salivation
• Deep breathing preceding the actual vomiting to protect the lungs from aspiration
• Heaving or retching before the actual vomiting
• Relaxation of the pyloric sphincter that guards the lower end of the stomach to bring up content from the gut
• The pressure within the abdomen rises and the pressure within the chest or thorax is lowered. The abdominal muscles contract to expel the contents of the stomach
• Activation of the sympathetic nervous system leads to sweating, palpitation and rapid heart rate