Let’s talk about Taste
Have you ever seen the tongue map?
Figure 1. Distribution of taste sensitivity
Figure 1. Illustrate the different location of tongue responsible for different taste sensitivity (ref: http://www.diwinetaste.com/dwt/en2005026.php).
We have been taught that the front of the tongue is responsible for sweet sensation, salty is on the side of the tongue, sour is more sensitive in the inner position of the tongue and the bitter taste sensation is largely distributed at the back. Lastly, the area shown in pink is the bind spot that cannot distinguish any taste sensation at all.
However, IT IS WRONG! We have all been cheated!
Let’s start from the beginning…
The taste sensation is actually results from the interaction between taste receptors and specific chemical substance. Our taste receptors can distinguish 5 different taste sensations.
We all agree that sweet, bitter, sour and salty are the first four tastes that our taste receptors can distinguish. And now please spend 5 seconds to have a guess on the fifth taste.
5…4…3…2…1… time’s up
I think some of you may think that the fifth taste is spicy. However, spicy cannot be determined by our taste receptors but rather it activates the temperature-sensitive ion channel.
Actually, the fifth taste is the sensation of delicious, called “Umami”.
Table 1. Different taste receptors and taste sensation-induced chemical
Table 1. shows that the interaction between different taste receptors and chemicals are responsible for different taste sensation. (Ref: Yarmolinsky et al., 2009)
Simply taste sensation pathway
Acidic foods contain H+ ions and it blocks the potassium ion channel located in the taste receptor cells and prevent leakage of the hyperpolarizing potassium. As a result, depolarization occurs in the receptor cell and results in sour sensation.
The injection of food containing salt causes the entry of Na+ ion into the ion channel of receptor cell. As a result, depolarization occurs.
Sweet and Umami
The sugar molecule (both natural and artificial) binds to the sweet receptors. This binding process activates the second messenger pathway the G-protein coupled receptors. This then evaluates the cyclic AMP levels and causes the blockage of potassium channels. Depolarization then occurs.
Similarly, G-protein coupled second messenger pathway is used to cause Umami sensation. Moreover, Umami sensation can only be stimulated by L-glutamate and L-asparate.
Bitter sensation can be stimulated by a large variety of substances. The bitter stimulated substances bind to the T2R/T2B taste receptor and activate the G-protein coupled second messenger pathway. This causes decrease in cyclic AMP and an increase IP3. Increase in IP3 stimulates the release of intracellular Ca2+ ions. Depolarization then occurs.
Back to our first question, why the tongue map is wrong?
Each taste receptors cells are not just specific to one type of taste sensation, but they can also provide multi information to different taste.
Only two statements illustrated by the tongue map are correct,
1) Bitter taste is more sensitive at the back of the tongue
2) There is absence of taste receptor cells at the center of the tongue
(However, your brain will collect and analysis information from all taste neurons to give you a correct taste response)
Some of you may worry that…if all my taste receptor cells are damaged then I am not able to taste anything.
Luckily, our taste receptor cells will be replaced every two weeks
Hector Lau is an Msc candidate in Biotechnology at the University of Melbourne.
Clapp, T. R., Stone, L. M., Margolskee, R. F. and Kinnamon, S. C. 2001, ‘Immunocytochemical evidence for co-expression of Type III IP3 receptor with signaling components of bitter taste transduction’, BMC Neuroscience, vol. 2, pp. 1-9.
Widmaier, E. P., Raff, H., Strang, K. T. (eds) 2008, ‘Sensory Physiology’, Vander’s Human Physiology, 11th edition, Michelle Watnick, New York, pp. 191-231.
Yarmolinsky, D. A., Zuker, C. S. and Ryba, J. P. 2009, ‘Common Sense about taste: From Mammals to Insects’, Cell, vol. 139, pp. 234-244.
Zhang, Y. and Hoon, M. A. 2003, ‘Coding of Sweet, Bitter, and Umami Tastes: Different Receptor Cells Sharing Similar Signaling Pathways,’ Cell, vol. 112, pp. 293-301.