Chemoreceptor Trigger Zone - Evolutionary Significance

Evolutionary Significance

The CTZ is in the medulla oblongata, which is phylogenetically the oldest part of the nervous system. Early lifeforms developed a brainstem, or inner brain, and nothing more. This part of the brain is responsible for basic survival instincs and reactions, for example to make an organism turn its head and look where an auditory stimulus was heard. The brainstem is where the medulla is located, and therefore also the area postrea and the CTZ. Then later lifeforms developed another segment of the brain, which includes the limbic system. This area of the brain is responsible for producing emotion and emotional responses to external simuli, and also is significantly involved in memory and reward systems. Now, the most advanced lifeforms to date have developed the cerebral cortex, which is the most recent part of the nervous system. This area of the brain is responsible for critical thinking and reasoning, and is actively involved in decision making. It has been discovered that a major cause of increased intelligence in species including humans is the increase in cortical neurons in the brain. The emetic response was selected for, for protective purposes, and serves as a safeguard against poisoning of the body. This response gets toxins and drugs out of the body by summoning control over motor neurons which stimulate muscles in the chest and thoracic diaphragm to expel contents from the stomach. The hope here is that whatever poison or toxin that is mediating its effects in the stomach will be inhibited. This is why the CTZ is located in the brain stem, outside of the blood-brain barrier: because your body needs to get rid of toxins and drugs even if you are unconscious and cannot consciously produce the emetic response.