Chemoreceptor Trigger Zone

The fifty-eight year old lady mentioned in the case history is diagnosed with skin cancer. She has been referred for a chemotherapy session and is informed by the oncologist of the side effects; nausea and vomiting. Vomiting also referred to as emesis in medical terms ‘is the forceful expulsion of the contents of the gastrointestinal system out through the mouth’ (Virtual medical centre, 2005). It is the symptoms of many different conditions and the side effects of certain drugs. Vomiting is the response of an activated emetic reflex by the integrative vomiting centre. The initiation of an emetic response can be prevented by the use anti-emetics drugs. These drugs are often useful for patients that are undergoing chemotherapy. The drugs act
as a receptor antagonist blocking the activation of the receptors that stimulate the emetic response (Rang, H.P., 2011).
The emetic reflex is controlled by two of the medullary centres in the brain; the chemoreceptor trigger zone (CTZ) and the integrative vomiting centre (IVC) (Virtual medical centre, 2005).
The chemoreceptor trigger zone is situated on the floor of the fourth ventricle of the brain within the area postrema (Bowen, R., 1996). It consists of a blood-brain barrier that is able to detect chemical stimuli within the blood and the cerebrospinal fluid. The chemoreceptors which are situated within the chemoreceptor trigger zone are stimulated by many drugs and toxic substances; referred to as emetics (Sherwood, L., 2013). When emetics cause a stimulation of the chemoreceptor trigger zone, impulses are relayed across the to the integrative vomiting centre, causing an initiation of the emetic reflex. Therefore it is not merely the chemoreceptor trigger zone that stimulates the vomiting itself, but the integrative vomiting centre that results in the act of emesis (Rang, H.P., et al,
2011).
The integrative vomiting centre is situated within the medulla and is involved in the action of the emetic reflex. It receives afferent impulses via the chemoreceptor trigger zone, the cerebral cortex, and abdominal organs. Once the integrative vomiting centre becomes activated by the afferent pathways, the efferent pathways are stimulated sending impulses back initiating the emetic response (Virtual medical centre, 2005). The efferent pathway of the integrative vomiting centre are controlled by both the somatic and autonomic nervous system (Donnerer, J., 2003). The impulses initiated by the integrative vomiting centre pass along the efferent fibres of the phrenic, spinal and visceral nerves triggering emesis by stimulating the diaphragm, abdominal muscles and the stomach (Becker, D.E., 2010).
One of the common ways of activating the integrative vomiting centre is via the afferent pathways within the abdominal organs. Any damage or irritation to the intestinal lining caused by conditions such as gastrointestinal distension and mucosal irritation, is detected by the mechanoreceptors situated on the intestinal walls (Calvey, T.N., Williams, N.E., 2008). These receptors then relay signal via the afferent pathways to the integrative vomiting centre, initiating vomiting. Chemoreceptors also initiate vomiting in a similar way to the mechanoreceptors, however these receptors are activated by the presence of chemical toxins (Virtual medical centre, 2005).
Vomiting can also be caused by factors such as sight, smell, taste and stress.
Any unpleasant sight, smell and taste can result in the activation of the higher centres in the brain. The higher centres of the brain then relay impulses to the integrative vomiting centre activating the emetic response. In addition infections of the labyrinth can lead to motion sickness. The vestibular nerve is stimulated by the infection, leading to the transmission of impulses that activate the chemoreceptor trigger zone (Virtual medical centre, 2005).
The emetic reflex mechanism is categorised into two phases: a retching phase and the expulsive phase. The retching phase occurs first, and involves the contraction of the abdominal muscles, chest wall and diaphragm. The sustained contraction of the diaphragm, chest wall and abdominal muscle leads to a build-up of increased pressure within the stomach. The release of this pressure is characterised as the expulsive phase, and this is what results in the expulsion of the contents of the stomach (Virtual medical centre,
2005).
Prior to the first chemotherapy session, the patient referred to in the case history was prescribed anti-emetic drugs, to take during the day of her first chemotherapy session. Anti-emetic are drugs used to treat against nausea and vomiting, they act by preventing the stimulatory effects of the integrative vomiting centre. The patient mentioned in the case history was prescribed a combination of two anti-emetics; an antagonist of the 5-HT3 receptor and glucocorticoid dexamethasone.
The chemoreceptor trigger zone consists of the following receptors; serotonin type 3 (5-HT3), dopamine type 2 (D2), acetylcholine (M1), histamine (H1), opioid, substance P/neurokinin (NK1) (Rang, H.P., 2011). The activation of these receptors, each trigger a different pathway for the initiation of the emetic reflex. Vomiting triggered by chemotherapy or radiotherapy is initiated by damage caused by the cells that line the intestines. When these cells become damaged they release a chemical messenger called serotonin, and these bind to and activate the 5-HT3 receptors located within visceral nerves. Once the 5-HT3 receptors become activated, impulses are sent from the intestines to the integrative vomiting centre where activation of the emetic reflex occurs (Virtual medical centre, 2011).