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Chemoreceptors are specialized nerve cells which are designed to respond to chemical stimuli. The body contains both direct and distant chemoreceptors, all of which play important roles in bodily function and daily life. These cells are also sometimes known as chemosensors, because they behave like sensors which “sniff” for specific chemicals of interest. Like other neurons, these cells are usually designed with customized locks which only fit the keys of specific chemicals, which makes them sensitive only to certain types of chemicals or chemical families.
An example of direct chemoreceptors are the cells located on the tongue. When people taste food, it is because these cells respond to the chemicals in the food, sending a signal to the brain to let the brain know about what's happening in the mouth. Specific regions of the mouth have areas which are targeted towards specific tastes, such as salty and sweet. This explains why foods can taste different as they are chewed and swallowed, and also why some foods have an aftertaste, as certain chemicals can take longer to stimulate the chemoreceptors.
Direct chemoreceptors are also found in the carotid body, a structure located at the branch of the carotid artery. These nerves detect levels of oxygen and carbon dioxide in the blood to determine when someone needs to breathe. The carotid body explains why people cannot suffocate themselves by holding their breath, because these cells will eventually trigger an involuntary breathing response due to the critical levels of chemicals they detect in the blood. These cells can also monitor temperature and pH balance.
Indirect chemoreceptors can be found in the nose, sensing smells and detecting hormones. As many people have observed when they have a cold, their receptors in the mouth and nose are both involved in the process of taste, as foods will not taste right when the nose is congested, or when the tongue is damaged.
All of these nerve cells send impulses back to the brain. Chemoreceptors can also be found in the brain itself, monitoring pH level and alerting the brain to the presence of chemicals which could indicate a problem. A special area of the brain called the chemoreceptor trigger zone receives input from those receptors which are sensitive to chemicals which can be dangerous. When these cells send a message to the brain that those chemicals have been ingested, the brain triggers a vomiting reflex so that the chemicals will be voided from the body before they can be metabolized.