How Do We Taste?
The least understood sense that humans possess is probably the sense of
taste. Taste is something that has been misrepresented in textbooks. Many
of us remember the "tongue map" that said that sweet was on the front of
the tongue, bitter on the back, salty on the front at the sides, and sour
on the back on the sides. I still remember my mother telling me to put a
bitter medicine on the front of my tongue because my bitter taste buds were
in the back of my mouth. Like a lot of the old ideas, this one is totally
incorrect.
In the past several years, there has been a great growth in understanding about how we taste, but many mysteries remain. Taste buds, onion-shaped groupings of between 50 and 100 cells, have a finger like projection called microvilli that poke through an opening at the top. Chemicals from the food we eat called tastants dissolve in saliva and contact these openings and interact with proteins on the surface of the cells. These interactions cause electrical changes in the taste cells which send signals that ultimately end up in signals going to the brain.
The real question is how do we tell the difference between sweet, salty, bitter, sour, etc. The answer is not where on the tongue the material lands, but rather how each substance reacts chemically. Salt, for example, dissolves in the mouth allowing sodium ions to enter the taste buds through the microvilli. When the sodium ions accumulate, a chemical change releases calcium ions that enter the cells which in turn release chemical signals called vesicles. Nerve cells receive these messages and send a signal to the brain. Acids, on the other hand, generate hydrogen ions which block potassium ions on the microvilli and a different signal (sour) is sent to the brain. Sweet things do not enter the taste cells at all, but cling to the surface, releasing enzymes which generate a different signal to the nerve cells to take to the brain.
What we have sketched in the previous paragraph is a grossly oversimplified and incomplete explanation of what allows us to have different tastes. (For a more complete explanation, see Scientific American, March, 2001, page 32.) The point is that tasting things is a highly sophisticated, carefully designed process that not only involves a great deal of sophisticated chemistry, but also an incredible operation by the brain. How the brain interprets these different signals is a long way from being understood. It appears to involve patterns of electrical responses. David's comment in Psalm 139:14, "I will praise thee Lord, for I am fearfully and wonderfully made. Marvelous are thy works, and that my soul knoweth right well," seems to be appropriate as he praises God for the complexity and beautifully functional design of his body.
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