SIW - Gustation and Olfaction (Taste and Smell) Lesson

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Gustation and Olfaction (Taste and Smell) 

Gustation and olfaction are the senses commonly known as taste and smell. Both smell and taste use chemoreceptors, which essentially means they are both sensing the chemical environment. In order for us to taste or smell, chemical stimuli must enter into our nasal cavity or the mouth, be transduced by chemoreceptors into an action potential, and be processed by the brain.

Gustation

The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae. There are several structurally distinct papillae. Filiform papillae are located across the tongue and are tactile, providing friction that helps the tongue move substances. These papillae contain no taste cells. In contrast, the fungiform papillae located mainly on the anterior two-thirds of the tongue each contain one to eight taste buds. These papillae also have receptors for pressure and temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior margin of the tongue.  

Review the structure of the tongue below.

The structure of the tongue is illustrated.

The primary tastes detected by humans are sweet, sour, bitter, salty, and umami. The first four tastes need little explanation. The identification of umami (also known as savoriness) as a fundamental taste occurred fairly recently. It was identified in 1908 by Japanese scientist Kikunae Ikeda, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The adaptive value of being able to distinguish umami is that savory substances tend to be high in protein.

Each taste has only one corresponding type of receptor. Thus, each receptor is specific to its taste stimulus (tastant). Transduction of the five tastes happens through a variety of mechanisms depending upon the chemical composition of the tastant. 

The five tastes are salty, sour, bitter, sweet, and umami. Each taste has only one corresponding type of receptor.

  • A salty tastant (containing NaCl) provides sodium ions (Na+) that can enter into taste neurons and excite them directly.
  • Sour tastants are acids. When a molecule of acid binds to a taste receptor, it triggers a change in the ion channel by increasing hydrogen ion (H+) concentrations, and, thus, depolarizing them.
  • Sweet, bitter, and umami tastants require a protein receptor. When a molecule of these tastants binds to their respective protein receptor, they will trigger an action potential. You may have seen a picture of the "tongue map," which divides the tongue into sections where each type of taste receptor can be found on the tongue when learning about senses in elementary school. This map is actually incorrect. Recent evidence suggests that taste receptors are uniformly distributed across the tongue.

Are you repulsed by broccoli and black coffee and wonder how others are able to ingest these items? Sometimes a person's tastes are controlled by their genes.  

Learn more about this in the video below.

Olfaction

The primary organ of smell is the nose. Odorants (odor molecules) enter the nose and dissolve in the mucous membrane of the olfactory epithelium at the back of the nasal cavity. An olfactory receptor responds when it binds with certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors distributed among around 350 different receptor types that respond to different odors. This allows the average human to sense about 10,000 different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds (dogs selectively bred for their sense of smell) have about 4 billion. Olfactory stimulation is very primitive and the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.

Working Together

There are specialized receptors in the nose that coordinate with the activation of the taste receptors. Although humans commonly distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A person's perception of flavor is reduced if he or she has congested nasal passages.

The senses of smell and taste combine at the back of the throat. When you taste something before you smell it, the smell lingers internally up to the nose causing you to smell it.

Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing.

Taste and Smell

Detecting a taste (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules.

Although more emphasis is typically placed on the senses of vision and hearing, losing the sensations of taste or smell can have a dramatic impact on a person's life.  

Watch the video below to learn more:

Complete the learning module below to review what you have learned in this module.

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