Sense of smell

**Olfaction**, or **smell**, is one of the five main senses. It helps people and animals detect scents in the air. It allows us to notice pleasant smells, like flowers or fresh food. It also allows us to sense danger, like smoke or something rotten. Smell plays an important role in how we experience the world. The study of smell is called olfaction. Scientists are still learning about how it works and why people smell things differently.

The sense of smell begins in the nose. When we breathe in, tiny scent particles in the air enter the nose. They reach a special area called the olfactory epithelium. This area is high up inside the nose. It contains millions of olfactory receptor cells. Each receptor cell has tiny hairs called cilia that pick up the scent molecules. When a scent molecule attaches to a receptor, it sends an electrical signal through the cell. These signals travel along the olfactory nerve to a part of the brain called the olfactory bulb. The olfactory bulb organizes the signals and sends them to other parts of the brain, especially the olfactory cortex, which helps identify the smell. It also sends signals to areas linked to memory and emotions, like the amygdala and hippocampus. This is why some smells can bring back strong memories or feelings. Each smell has a unique combination of molecules, and different receptor cells are activated depending on the scent. This system allows us to recognize thousands of different smells, even in small amounts.

The main function of smell is to help us detect and recognize different scents in our environment. This sense can alert us to danger, such as smoke from a fire, gas leaks, or spoiled food. It helps keep us safe by warning us when something may be harmful. Smell also plays an important role in enjoying food. It works closely with the sense of taste to create flavor. When we eat, the smell of the food travels through the nose and helps us notice different flavors. That is why food tastes bland when we have a cold or stuffy nose. Smell helps us connect with memories and emotions. A scent we know, like a favorite perfume or the smell of fresh bread, can bring back strong memories or feelings. This is because the brain areas that handle smell are close to those that control memory and emotions. In animals, smell is used to find food, sense danger, and communicate. Some animals can track scents over long distances or recognize other animals by their smell.

The sense of smell can vary from person to person. Some people have a very strong sense of smell, while others may have a weak sense or none at all. This difference can be caused by genetics, age, health conditions, or even injuries. Genetics play a big role in how we smell things. Some people are born with more olfactory receptors or more sensitive noses, while others may not be able to smell certain scents at all. For example, some people can smell a strong odor in certain vegetables, while others cannot smell it. Smell often gets weaker with age. As people grow older, the number of working smell receptors in the nose can decrease. This means older adults may not smell things as clearly as they used to, which can also affect how much they enjoy food. Health problems can also affect smell. Colds, sinus infections, and some diseases like COVID-19 can temporarily or permanently reduce the ability to smell. Head injuries or damage to the brain can also affect how the brain processes smell signals. Experience and culture matter too. People who grow up around certain foods or scents may be better at recognizing them. In some cultures, strong-smelling spices are common, while in others, mild scents are more usual.

Different organisms use smell in many ways, and their sense of smell can be much stronger or weaker than in humans. For example, dogs have an extremely powerful sense of smell. They have many more smell receptors in their noses than humans do, which helps them track scents, find missing people, or find drugs and diseases. Insects like moths and ants also use smell to survive. Insects use their antennae to detect smells. Their antennae are covered with tiny hair-like structures that contain olfactory receptors. These receptors can pick up chemical signals, such as the scent of food, mates, or danger. Moths can smell other moths from far away to find a mate. Ants use scent trails to find food and lead others back to it. They talk to each other using chemicals called pheromones that other ants can smell. Fish and other aquatic animals can also smell, even underwater. They detect chemicals dissolved in water using special receptors. This helps them find food, avoid predators, and recognize their environment. Birds generally have a weaker sense of smell, but there are exceptions. For example, vultures use their sense of smell to find dead animals to eat. Kiwi birds in New Zealand also use smell to find insects in the ground at night. Plants and bacteria do not smell in the way animals do, but they can release and detect chemicals that act like smells. These chemicals help them interact with each other or with animals.

## Olfactory reception cells 
The olfactory reception (OR) cells are neurons (nerve cells). Many tiny hair-like cilia stick out of these cells into the mucus covering the surface of the epithelium. The surface of these cilia is covered with olfactory receptors, a kind of protein.  

There are about 1000 different genes which code for the ORs, though only about a third are functional. The rest are pseudogenes. The OR genes are the largest gene family. An odor molecule dissolves into the mucus of the olfactory epithelium and then binds to an OR. Various odor molecules bind to various ORs. The basis of the sense of smell is that different groups of scent molecules bind to different receptor cells and so fire different groups of neurons. Inside the olfactory region of the brain, the firing of neurons produces the perceived smell. 

When the OR is activated, changes start in the cells. Positive ions come in and  negative ions go out of the cells. This causes the neuron to fire an impulse (generate an action potential).

## References 
## Other websites 
*Smells and Odours - How Smell Works
*Olfaction
*The importance of smell, and pheromones, to Humans and other Animals
*Structure-odor relations: a modern perspective (PDF)
*Olfactory network dynamics and the coding of multidimensional signals  (PDF)