Tastes and Odors
Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans are sweet, sour, bitter, salty, and umami . The first four tastes need little explanation. The identification of umami as a fundamental taste occurred fairly recently. It was identified in 1908 by Japanese scientist Kikunae Ikeda while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the savory taste of certain foods. The adaptive value of being able to distinguish umami is that savory substances tend to be high in protein.
Uniform Distribution of Taste Receptors
Humans detect taste using receptors called taste buds. Each of these receptors is specially adapted to determine one type of taste sensation. Recent evidence suggests that taste receptors are uniformly distributed across the tongue; thus, the traditional tongue map is no longer valid.
All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. If a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow us to sense about 10,000 different odors. Compare that to mice, for example, which have about 1,300 olfactory receptor types and, therefore, probably sense many more odors.
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 smell and taste use chemoreceptors, which essentially means they are both sensing the chemical environment. This chemoreception in regards to taste, occurs via the presence of specialized taste receptors within the mouth that are referred to as taste cells and are bundled together to form taste buds. These taste buds, located in papillae which are found across the tongue, are specific for the five modalities: salt, sweet, sour, bitter and umami. These receptors are activated when their specific stimulus (i.e. sweet or salt molecules) is present and signals to the brain.
In addition to the activation of the taste receptors, there are similar receptors within the nose that coordinates with activation of the taste receptors. When you eat something, you can tell the difference between sweet and bitter. It is the sense of smell that is used to distinguish the difference. 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.
Uniform distribution of taste receptors (the myth of the tongue map)
Humans detect taste using receptors called taste buds. Each of these receptors is specially adapted to determine one type of taste sensation. Recent evidence suggests that taste receptors are uniformly distributed across the tongue; thus, this traditional tongue map is no longer valid.