An olfactory receptor is a contact site on certain cells in the brains of humans and most animals that helps process and identify smells. Many insects have these sorts of receptors, too, though in most cases they are located on external sensors like antennae rather than being located in the brain; this is because, usually, insects sense smell not through a defined nose but rather through feelers that interact more directly with the environment. No matter their location, receptors tend to work in about the same way. They process smell as a chemical reaction, then convert that into a signal that can be read and understood by the brain or other processing area. These receptors are how people are able to identify whiffs of something in the air, and also play a role in smell memory, which is to say memories of people, places, or experiences associated with certain smells.
Smell can be a somewhat complex thing. In most scientific settings, smells are known as “odorants.” Odorants are essentially chemical “signatures” that are shed by most substances and creatures; they can also be created, usually as a byproduct of processes like people sweating, flowers blooming, or things burning. Odorants attach themselves to atmospheric and environmental molecules. They are interpreted into smells that organisms recognize through specific olfactory receptors.
How They Work
Receptors are usually understood to be part of the neural system in humans and most vertebrate animals. They sit on the very surface of certain cells in the olfactory center of the brain and basically wait for a triggering chemical to indicate a smell nearby. In insects, this system happens on the antennae rather than in the brain, but the process is usually the same, at least from a chemical perspective. Receptors tend to be specific, which means that there are different receptors for different smells.
One way to visualize this is to imagine odorants as keys and receptors as individual locks. Any given odorant will only bond to its corresponding receptors, and this is how smells are identified. When a receptor bonds to an odorant, it is said to be “activated.” It is sometimes possible for one smell to activate many receptors, usually if it is a complex or layered smell.
In people and most animals, olfactory receptors are bundled into small groups at the back of the nasal cavity. There are usually millions of cells when counted individually, and a very high number of grouped bundles, too. Together, these groups form what’s known as the “olfactory epithelium,” which acts as a sort of coating or boundary.
The epithelium is covered in hair-like cilia that collect inhaled odorant molecules and send them to the receptors. Mucus that lines the nose and sinuses also helps to trap these odorants, enabling them to be processed.
Importance of Smell
Many biologists have said that the sense of smell is one of the most essential tools for the survival of any given organism. Indeed, nearly all living things, from simple invertebrates to humans, have at least some use for, and indeed need for, olfactory receptors. Fish, for instance, have approximately 100 varieties of these sorts of sites, whereas humans have around 10,000. In fact, researchers believe that nearly 3% of the human genetic code is devoted to their construction.
Most people associate smells with immediate sensations, particularly pleasure and repulsion. These sensations are important, but the sense of smell also has a crucial subconscious and evolutionary role, too. It enables humans and animals to discern factors concerning everything from the fitness and health of a potential mate to identifying poisonous substances and edible food. It can also help identify danger, and some animals are said to be able to “smell fear” — usually because of the ways they are able to process adrenaline and increased sweat put off by others.
Smell Interpretation Process
The process from smell recognition in the olfactory center to processing and action in the brain can be very complicated. Most of the time, when receptors first “lock” with a smell, they make a permanent record of the connection that is stored in the memory center of the brain. When an animal ingests a food substance that is rotten or causes illness, for example, the animal retains that smell in its memory bank. The same thing happens with smells that are good; this is partly why smelling something like freshly baked cookies can make a person think of childhood times spend with a grandma or mom in the kitchen.
Smell also plays a role in taste. In most cases, the taste buds and taste receptors also work really closely with smell receptors, and together these two components allow the complete experience of any food product. People who have olfactory problems or disorders — or who have a blocked nose from something like a bad cold — often find that they taste foods differently, and this is why.