The Science of Mouth and Nose – How We Taste and Smell

Taste

The physiology behind taste is quite complex, going far beyond one’s taste buds. A combination of naturally occurring chemical substances coming into contact with nerve receptors is what transmits the perception of taste to one’s brain. This, of course, happens inside one’s mouth and involves primarily the tongue. The bumps on one’s tongue are taste papillae, and there are three different types, each characterized by its own shape.

The human tongue has approximately 200 to 400 fungiform papillae, which are primarily found on the tip and at the edges of the tongue. Fungiform papillae are also heat receptors, telling a human if something is too hot or too cold. The tongue’s touch sensation is found in the fungiform papillae as well. Fungiform papillae are only visible microscopically, and one fungiform papilla contains anywhere from three to five taste buds.

Toward the back of the throat is where the tongue’s circumvallate papillae are found. While the average human only has approximately seven to 12 circumvallate papillae, each papilla has several thousand taste buds in it. This is why the papillae on the back of the tongue are much larger, visible to the naked eye. As a person chews and swallows, glands surrounding the circumvallate papillae wash taste-identifying substances into the taste sensory cells.

Also visible to the naked eye are the tongue’s foliate papillae. The foliate papillae are located near the rear of the human tongue on its edges. The foliate papillae look like little rows of folds in the tongue. Each human tongue has approximately 20 foliate papillae. The reason each papilla is so big is because each one protects hundreds of taste buds. This is also why taste often seems stronger on the sides of the tongue instead of on the top.

Taste buds are the human taste organ and get their “bud” name because they are shaped like a flower bud. Each human taste bud has anywhere from 10 to 50 sensory cells, which are called taste hairs and are connected to nerve fibers. As a person chews, proteins bind taste chemicals to the taste hairs for taste processing. An average adult has 2,000 to 4,000 total taste buds, and taste hairs renew themselves once a week.

Alongside the taste buds and papillae in the tongue, humans also have the ability to detect and transmit taste in the back of their throats via the epiglottis, which is a flap of elastic cartilage. Humans can also detect taste through their nasal cavities. The top portion of the human esophagus can detect taste, and when humans are in the infantile and toddler stages, tastes are perceived through the hard palate, the middle of the tongue, and mucous membranes in the cheeks and lips.

Humans can detect four primary taste sensations: sweet, sour, salty, and bitter. Sugars in a food source produce the sweet sensation, and G-protein receptors in human taste buds detect sweetness. A food’s acidity determines its sourness, and sourness is detected by taste bud cells recognizing PKD2L1 protein. Sodium ions produce the salty taste, as do certain lithium and potassium ions. It is these ions passing through the taste buds that produce a salty taste. Bitter taste is generally unpleasant, and early man considered bitter taste to be a sign of a toxic food source.

Smell

How a human smells is also quite complex. The human sense of smell is called olfaction because the scent information is passed through a human’s olfactory tract to be processed by the brain’s olfactory cortex. Diffused molecules create odors, and it is the chemical signatures of these odors that the human brain uses to identify the specific smell. It is believed that smell is the oldest of all of the senses, and it is also the sense that is most associated with memory.

The human nose is filled with olfactory receptor neurons, which are the smell cells. Olfactory receptor neurons are found on the olfactory epithelium, which is approximately a 1-inch-by-2-inch tissue strip inside and toward the back of the nose. In fact, the olfactory epithelium sits approximately 3 inches above the nostrils, which are naturally where the odors enter into the nose. Supporter cells excrete mucus, which makes it easier for the olfactory receptor neurons to trap odor molecules.

Olfactory receptor neurons can detect numerous odor molecules. When the odor molecules are inhaled into the nasal cavity, the mucus traps the molecules on the olfactory receptor neurons. The olfactory receptor neurons are connected to the olfactory bulb – the two-part slender and bulbous olfactory cortex of the brain – by axons, which are nerve tendrils. There are approximately 100 neurons used by the human body to process smell.

Should a human’s olfactory system become compromised, the person might suffer from anosmia, or the inability to process smell. Anosmia occurs when any portion of the olfactory system is traumatized. One might suffer from temporary anosmia should they contract a cold that causes inflammation and blockage of nasal passages. Permanent anosmia might be caused by an event disabling the olfactory cortex of the brain. Humans hypersensitive to smell suffer from the opposite of anosmia, hyposmia.

Taste and Smell Working Together

Both taste and smell are linked through the human body’s chemosensation, its chemical sensing system. The human body’s chemical sensing system includes the mouth and nose as well as the eyes and throat. In all of these areas, thousands of free nerve endings identify sensations such as taste, odor, hot, cold, or even something that stings, such as ammonia evaporation that irritates the eyes. It is also through chemosensation that taste and smell actually work together.

A person might remember as a child holding their nose closed while they ate in an effort to chew and swallow a taste they did not like. The child found this method of ingesting the undesirable food successful because taste and smell work together to bring a human the full sensation of flavor. When a human chews their food, they actually push air through the nasal passages without realizing it. The result is smelling the food alongside tasting it for an even greater ability to process flavor.

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