To humans, the sense of smell has a direct link to emotions. Who among us has not, at one time or another, had a memory and its associated feelings come rushing at us when catching a whiff of a perfume, theater popcorn or one of thousands of smells that we associate with events from our past?

Smell is the first sense we use when we are born, and 1 out of every 50 genes in our genome relates in some way to smell, to not only trigger emotions, but also the flight-or-fight response such as the smell of smoke in a building.

At the back of the nose is the olfactory epithelium, a patch of tissue that contains the olfactory receptor cells — special neurons that detect smells, the taste buds of the nose. A layer of mucus on the olfactory epithelium captures odor molecules that have passed through the nose’s filtration system.

If a molecule is one of the 10,000 or so that the nose recognizes, the receptor cell sends a message to the brain. The human olfactory epithelium is 20 times smaller than that of a dog, but research over the past decade reveals that we are better smellers than previously thought.

The brain recognizes odors through 40 million different types of receptor cells. A given molecule will trigger a particular combination of cells to cause a unique response that the brain interprets as, “This is a strawberry.”

Olfactory receptor cells are the only neurons in the body that are replaced regularly, every six to eight weeks, so the sense of smell does not get “worn out,” although the rate of replacement slows with age.

Once an odor molecule triggers the receptor cells, the signal travels along a bundle of nerves to the brain, where they land first in the amygdala, then the thalamus and on to the neocortex. This is in sharp contrast to sight and sound, for which signals travel to relay centers in the middle of the cerebral hemisphere for distribution to various processing regions.

Although we all have the same basic olfactory physiology, not all odors smell the same to everyone. One of the most famous is “asparagus urine,” which is detectable by only one-fourth of us. The other three-fourths smell nothing.

Another one is androstenone, which smells like vanilla to some and like sweaty urine to others. Pigs produce this hormone and transmit it to pork. Thus, pig farmers castrate boars so they will not produce it and “spoil” the pork.

Anosmia is the inability to smell certain smells, such as garlic or cloves, and some people cannot smell anything.

Research has confirmed that performance among humans and other animals strongly depends on the odors tested because different species have different odor receptors. With the selection of appropriate odors, humans actually outperform rodents and dogs in detecting some odors.

Humans can distinguish at least 10,000 different scents and can follow outdoor scent trails. Furthermore, because of our larger brain, the olfactory environment affects behaviors and emotions as well as recalling distinct memories, and gives us more processing power.

We once thought only “lower” animals used odor as a communications medium, but now we understand that humans use it to process information about familial relationships, stress, anxiety and reproductive status, although we are not always conscious of it.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.