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

Sour
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.

Salty
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
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.

 

 

 

References

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.


9 Responses to “Let’s talk about Taste”

  1. whlau says:

    Thanks Jeane. I think you may have special umami buds, which make every food taste good in your mouth… 😛

  2. Jeane Angelina says:

    Nice post! I heard this from my food lecturer back when I was undergrad about the 5th receptor! But, until now, I still can’t feel where is my umami buds? 🙁 but, anyway I don’t bother because every food tastes good inside my mouth

  3. Vivian Loh says:

    Great post!

    I have a sweet tooth and I LOVE YUMMY FOOD – now I secretly wish I can somehow bioengineer my tongue to have only the sweet/umami taste receptors 😛

  4. whlau says:

    Guys, thank for the comments.
    @katied I think the temperature sensitive channels are involved in regulating your body temperature, whereas taste receptor is just for taste sensation. Their functions are different. However, I think bitter and TPR1 use similar pathway (G-protein coupled pathway and involved IP3). I suggested you to read Dhaka, A., Viswanath, V. and Patapoutian, A. 2006, ‘TRP Ion Channels and Temperature Sensation’, Annual Review of Neuroscience, vol.29, pp. 135-161. This article gives a really good summary on different class of temperature sensitive channel and how they caused the themosensation effects. Thanks.

  5. setiawan says:

    Cool! I have to say I feel slightly deceivedby my primary school teachers who told me about human only having 4 taste sensations

  6. katied says:

    Good post! I was wondering, how are these “temperature sensitive channels” (ie. TRPV1 receptors) different from the other taste receptors? What makes a taste receptor … a taste receptor? If salt taste receptors are activated by Na+ influx that then cause APs; how are they any different from when capsacin activates TRPV1 receptors to cause an influx Na+/Ca2+ which causes AP? Thanks for your post!

  7. eroberts says:

    I remember looking at the tongue ‘map’ in high school psychology class and thinking it was all too simple. I also thought the fifth flavour was spicy as well… foiled again!

  8. juanur0 says:

    Great post, I thought fifth sense was spicy too! I’ve never heard about umami taste before, interesting…

  9. ccroft says:

    Pretty cool, I really liked the diagram of the receptors,that was really good. a good party piece to see if people can guess the 5th taste, Thanks for sharing!!!!