CHEMORECEPTORS: CLASSIFICATION AND CHEMOSENSORY SYSTEMS - BIOLOGY - 2025

A chemoreceptor is a cellular sensor specialized in detecting and converting chemical signals - coming from both inside and outside the body - into biological signals that will be interpreted by the brain.

Chemoreceptors are responsible for our senses of smell and taste. These receptors take these chemical signals and transform them into a signal for the brain.

The perception of odors is mediated by chemoreceptors.

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Similarly, crucial biological functions, such as the heartbeat and respiration, are controlled by chemoreceptors that detect molecules related to these processes, such as the amount of carbon dioxide, oxygen, and the pH of the blood.

The ability to perceive chemical signals is ubiquitous in the animal kingdom. Particularly in humans, chemoreceptors are not as sensitive as in other mammals. In the course of evolution, we have lost the ability to perceive chemical stimuli related to smell and taste.

Some simpler non-metazoan organisms, such as bacteria and small protozoa, are capable of picking up chemical stimuli in their environment.

What is a receiver?

A receptor is a molecule that is anchored to the plasma membrane of our cells. They have the ability to recognize other molecules with very high specificity. By recognizing the indicated molecule - called the ligand - a series of reactions is triggered that will carry a specific message to the brain.

We have the ability to perceive our environment, since our cells have a significant number of receptors. We can smell and taste food thanks to chemoreceptors located in the sensory organs of the body.

Classification

Generally, chemoreceptors are classified into four categories: general, internal, contact, and olfactory chemical receptors. The latter are also known as distance chemoreceptors. We will describe each type below:

General chemical receptors

These receptors do not have the ability to discriminate and are considered relatively insensitive. When stimulated, they produce a series of responses of the protective type for the body.

For example, if we stimulate the skin of an animal with some aggressive chemical that could damage it, the response would be an immediate flight from the place and prevent the negative stimulus from continuing.

Internal chemoreceptors

As their name implies, they are responsible for responding to stimuli that occur inside the body.

For example, there are specific receptors to test the concentration of glucose in the blood, receptors within the digestive system of animals and receptors located in the carotid body that respond to the concentration of oxygen in the blood.

Contact chemoreceptors

Contact receptors respond to chemicals that are very close to the body. They are characterized by high thresholds and their ligands are molecules in solution.

According to the evidence, these seem to have been the first receptors to appear in evolutionary evolution, and they are the only chemoreceptors that the simplest animals present.

They are related to the feeding behavior of animals. For example, the best known with receptors associated with the sense of taste in vertebrates. They are located mainly in the oral area, since it is the region where food is received.

These receptors can discern between the apparent quality of the food, producing reactions of acceptance or rejection.

Olfactory or remote chemoreceptors

Smell receptors are the most sensitive to stimuli and can respond to substances that are at a distance.

In animals that live in aerial environments, the distinction between contact and distance receptors is easy to see. The chemicals that are transmitted through the air are those that manage to stimulate the olfactory receptors, while the chemicals dissolved in liquids stimulate the contact ones.

However, the boundary between the two receptors seems to be diffuse, since there are substances that stimulate the receptors at a distance and must be dissolved in a liquid phase.

The limits look even more indefinite in animals that live in aquatic ecosystems. In these cases, all the chemicals will be dissolved in an aqueous medium. However, receptor differentiation is still useful, as these organisms respond differentially to near and far stimuli.

Chemosensory systems

In most mammals there are three separate chemosensory systems, each dedicated to the detection of a particular group of chemicals.

Smell

The olfactory epithelium is made up of a dense layer of sensory neurons located in the nasal cavity. Here we find about a thousand different olfactory receptors that interact with the wide diversity of volatile substances present in the environment.

Taste

Taste buds

Non-volatile chemicals are perceived differently. The sense of perception of food consists of four or five taste qualities. These "qualities" are commonly called flavors, and include sweet, salty, sour, bitter, and umami. The latter is not very popular and is related to the taste of glutamate.

Sweet and umami flavors - corresponding to sugars and amino acids - are associated with nutritional aspects of food, while acidic flavors are associated with rejection behaviors, since most of the compounds with this flavor are toxic to mammals..

The cells responsible for perceiving these stimuli are found associated in the taste buds - in humans they are located on the tongue and at the back of the mouth. The taste buds contain 50 to 120 cells related to taste.

Vomeronasal organ

The vomeronasal organ is the third chemosensory system and is specialized in the detection of pheromones - however, not all pheromones are detected by this system.

The vomeronasal organ has qualities that are reminiscent of both the sense of taste and smell.

Anatomically, it is similar to smell, since it has the cells that express the receptors are neurons and they project directly to the brain. In contrast, the cells that possess the receptors on the tongue are not neurons.

However, the vomeronasal organ perceives nonvolatile chemicals through direct contact, in the same way that we perceive the taste of food through the taste system.

References

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