Making sense of tongue senses
In historical terms, research into the area of taste is relatively young, and since the late 1990’s, a new wave of research has been underway. Not surprisingly, this is being closely monitored by food, drink and medicinal interests. The geographical landscape of the tongue is changing, and a GPS will not help you negotiate the new terrain.
Specifically, two myths are gradually being dispelled. There are five basic tastes identified so far (instead of the largely accepted 4), and the entire tongue can sense all of these tastes more or less equally.
Before the recent focus, it was held that separate areas of the tongue were responsible for each taste. Popularly referred to as the "tongue map," the antiquated concept most likely found its origin in a German text mistranslated into English in the early 1900’s. As recently as 1996, tongue map diagrams still appeared in college neuroscience textbooks.
Just before the turn of the century, however, molecular biologists had learned enough to make emphatic changes to the textbooks. It is now known that thousands of taste buds inhabit the tongue, each containing 50 to 100 taste cells, each of which has two poles. One end is coated with taste receptors projecting from the tongue's surface and one end is inside the tongue, connecting the tongue to the brain via nerves.
Most scientists now agree that there's a fifth distinct taste, called umami, identified by a Japanese scientist named Kikunae Ikeda in the early 1900s. This is the taste of glutamate. It is common in Japanese foods, and in bacon and monosodium glutamate (MSG), which Ikeda isolated and patented.
There is also considerable debate about the existence of a sixth taste receptor for fat, so all bets are off for drawing a tongue map that can be considered accurate for very long..
Even more recently (2006), it was discovered that each taste cell is covered with receptors for only one taste. Therefore, a sweet taste cell can only be turned on by a sweet molecule, because it contains only sweet receptors. No longer is it thought that all taste cells can sense all tastes.
To add more confusion, it was also discovered that some cells, such as those with sweet taste receptors, have but one type of receptor, while others, such as those with bitter receptors, have many more. Extrapolation allows us to believe this is due to survival genetics. There are many dangerous substances we need to avoid, but too many sweets predisposes us to disease.
Tasting and Smelling
On TongueTown, it might seem odd that we need to discuss the olfactory sense (smell) with the gustatory sense (taste), but taste and smell are so tightly entwined that we can’t discuss one without the other. Rather than go into detail here, an excellent discussion can be found on this page of the University of Nebraska Medical Center’s web site.