Thanks to modern food safety standards, most of what’s in the grocery store is safe to consume. But keep that sliced turkey breast a little too long and your nose will tell you something is off. Smell and taste are critical senses, helping us detect hazardous substances we might inhale or ingest before they can harm us.
Our sense organs are the brain’s windows to the external world. The closely linked taste (gustation) and smell (olfaction) senses help us navigate the chemical world. Just as hearing is the perception of sound and sight is the perception of light, smell and taste are your perceptions of tiny molecules in the air and in food. Information we receive as different types of energy and molecules combines into the seamless experience of our surroundings. That perception would be lacking and even dangerous if we could not taste and smell.
From Molecules to Taste
Our ability to taste depends on the molecules set free when we chew or drink. These molecules are detected by gustatory cells in taste buds on the tongue and along the roof and back of the mouth. Each taste bud has sensory cells that respond to one of at least five basic taste qualities: sweet, sour, salty, bitter, and umami. All tastes are detected across the tongue and are not limited to specific regions. When taste receptor cells are stimulated, they send signals through three cranial nerves to taste regions in the brainstem — the facial, glossopharyngeal, and vagus nerves. These impulses get routed through the thalamus, which relays sensory information to other brain regions. The impulses travel to the gustatory cortex in the frontal lobe and the insula where specific taste perceptions are identified.
From Molecules to Smell
Like taste, the sense of smell depends on detecting molecules. Odors are small molecules that can become airborne. They enter the nose on air currents and bind to specialized cells. These olfactory neurons reside on a small patch of mucus membrane high inside the nasal cavity.
The tips of olfactory cells are equipped with several hair-like structures, called cilia, that are receptive to different odor molecules. For example, red wine contains a bouquet of different smells, and each part — cherries, vanilla, leather — is a distinct smell. Each scent stimulates a unique combination of olfactory cells, creating a distinct activity pattern for ‘cherries’ or ‘leather.’ This signature pattern of activity gets transmitted to the olfactory bulb along the long, extended arms of neurons called axons. Axons from olfactory neurons travel to two olfactory bulbs, one for each nostril. Next, the information encoding the smell of cherries reaches the primary olfactory cortex, located on the anterior surface of the temporal lobe. Olfactory information then passes to nearby brain areas, where odor and taste information are mixed. Together, these senses create the perception of flavor: the smell of cherries combines with tartness (bitter) and sweetness from the wine to complete your sipping experience. Recent research suggests that people can identify odors as quickly as 110 milliseconds after their first sniff.
The size of the olfactory bulbs and the way neurons are organized can change over time. The olfactory bulbs in rodents and primates, including humans, are one of the few brain regions able to generate new neurons throughout life.
Taste and Smell in Aging
We lose some of our sensitivity to taste and smell as we age. The cells that process tastes and smells are exposed to the outside environment. Usually, since these cells are exposed and therefore vulnerable to damage, taste receptor cells regularly regenerate. However, as we age, damaged receptors and sensory neurons might not be replaced by new ones. Researchers are examining stem cells and how they transform into the neurons that mediate smell or taste, to better understand these senses. They may find stem cell-based therapies to help restore taste or smell.
Combining Taste and Smell
You may notice the relationship between taste and smell when a cold stuffs up your nose and everything tastes bland. It seems like taste no longer works. The real problem is that during a cold, you’re detecting only taste, rather than taste and smell combined. Taste and smell information appear to converge in several central brain regions. There are also neurons in the inferior frontal lobe that respond selectively to specific taste and smell combinations. Taste sense itself is rather crude, distinguishing only five basic taste qualities. Our sense of smell adds great complexity to the flavors we perceive. Studies have found exposing people to matching combinations of familiar tastes and smells enhances their taste perceptions. Sugar tastes very sweet combined with the smell of strawberries and less sweet when paired with the smell of peanut butter or no odor.
Our chemical receptors, along with the brain networks that process taste and smell, give us access to the wonderful array of scents and flavors in our world.
This article was adapted from the 8th edition of Brain Facts by Marissa Fessenden.