Neuroscience For Kids

Smell - The Nose Knows

The smells of a rose, perfume, freshly baked bread and cookies...these smells are all made possible because of your nose and brain. The sense of smell, called olfaction, involves the detection and perception of chemicals floating in the air. Chemical molecules enter the nose and dissolve in mucous within a membrane called the olfactory epithelium. In humans, the olfactory epithelium is located about 7 cm up and into the nose from the nostrils.

The Olfactory System

Hair cells are the receptors in the olfactory epithelium that respond to particular chemicals. These cells have small hairs called cilia on one side and an axon on the other side. In humans, there are about 40 million olfactory receptors; in the German Shepherd dog, there are about 2 billion olfactory receptors.

No one knows what actually causes olfactory receptors to react - it could be a chemical molecule's shape or size or electrical charge. The electrical activity produced in these hair cells is transmitted to the olfactory bulb. The information is then passed on to mitral cells in the olfactory bulb.

The olfactory tract transmits the signals to the brain to areas such as the olfactory cortex, hippocampus, amygdala, and hypothalamus. Many of these brain areas are part of the limbic system. The limbic system is involved with emotional behavior and memory. That's why when you smell something, it often brings back memories associated with the object.

As you probably know, when you have a cold and your nose is stuffed up, you cannot smell very well. This is because the molecules that carry smell cannot reach the olfactory receptors.

Did you know?

About 2 million people in the United States have NO sense of smell. This disorder is called anosmia. A serious head injury can cause anosmia. Most likely this results in damage to the olfactory nerves as they enter the olfactory bulb. It is also possible that damage of the frontal lobes caused by a tumor or surgery can cause anosmia. Elderly people often have a reduced sense of smell.
People are very sensitive to the smell of green bell pepper. This smell can be detected when it is mixed with air at only 0.5 parts per trillion!
Natural gas, which many people use to heat their homes and to cook, is odorless. Because natural gas is dangerous to breathe and is explosive, it is important for people to be able to detect a gas leak. That is why gas companies add a smelly ("rotten egg smell") chemical called mercaptan to natural gas.
1 in 1,000 people are insensitive to butyl mercaptan, the stinky smell of skunks.

Try It!

A complete lesson plan on the sense of smell - teacher and student guides available.
Try some experiments to test your sense of smell.
Take a short, interactive quiz about the sense of smell.

For more information about the sense of smell, see:

Following Our Noses - from Time Magazine, March 23, 1998 - Other animals can communicate volumes through smell. Now it appears we can too.
Seeing, Hearing and Smelling the World
ChemoReception Web - Taste and Smell
Chemoreception - Monell Chemical Senses Center