Smell and the Brain
Why do certain smells evoke specific memories or feelings? One theory involves the way your brain processes odors and memories. Our sense of smell 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, this is located about 7 cm up into the nose from the nostrils.
Hair cells are the receptors in the olfactory epithelium that respond to certain chemicals. Those 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 contrast, a German Shepherd dog has about 2 billion olfactory receptors.
We don’t know exactly what causes these receptors to react, it could be a chemical molecules shape or size or electrical charge. The electrical activity produced in these hair cells is transmitted to the olfactory bulb and relayed to mitral cells located there. 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 associated with the limbic system, which in turn is involved with emotional behavior and memory. That’s why when you smell something, it often brings back memories associated with the object.
Smell can be used therapeutically to stimulate areas of the brain to enhance outcomes of physical or mental tasking. Different smells can stimulate focus, relieve stress and anxiety, and general state of arousal (i.e., state of energy and feeling awake). For more on aromatherapy check out http://umm.edu/health/medical/altmed/treatment/aromatherapy.
- About 2 million people in the US have no sense of smell. This disorder is called anosmia and can be caused by a head injury. Most likely this results in damage to the olfactory nerves as they enter the olfactory bulb. It is also possible that damage to 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.
- People can detect at least one trillion distinct scents.
- Smell is the oldest sense.
- Women have a better sense of smell than men.
- Scent cells are renewed every 30 to 60 days