English translation of the original Chinese article. Publication dates and the extent of recovered text are preserved. Figures retain their original labels. Read the Chinese original.
Southern Medical University
Keywords: sensory biology, receptors, ion channels
Although I am not a foodie, the question that first drew me to neurobiology was a foodie's question: how do we perceive sour, sweet, bitter, salty and umami? Why not sour, sweet, bitter and spicy? Scientifically, spiciness is a pain sensation rather than a taste. The scientific definition of taste includes the six modalities mentioned above: sour, sweet, bitter, salty and umami.
Taste arises when food directly stimulates taste buds (Figure 1). Looking in a mirror, we can see many tiny papillae covering the tongue. Each papilla contains many taste buds. On average, a person has 2000–5000 taste buds, although extreme cases may have as few as 500 or as many as 20000. Each taste bud contains 50–100 taste-receptor cells, and different types of these cells allow us to distinguish different flavors. Different molecules act on different receptors in various taste-receptor cells, which transmit signals through corresponding neural pathways to the cerebral cortex.
How do we distinguish different tastes? Different receptors in taste-receptor cells are activated by different chemical molecules. For example, an appropriate amount of NaCl activates the sodium-ion channel ENaC, depolarizing the corresponding taste-receptor cells. Action potentials are transmitted through specific nerve impulses to the corresponding cortical regions. Umami arises when L-glutamate in food specifically binds to the T1R1/T1R3 heterodimer. Interestingly, taste and smell are closely related in humans. Without a sense of smell, apples and onions taste much the same: very bland.
Many scientific questions about taste remain unanswered. For example, an appropriate amount of NaCl tastes salty, but why excessive NaCl produces a bitter sensation is still unknown. Which receptor mediates sour taste is also still unknown.
Figure 1: Taste-receptor cells. A, tongue; B, different types of small papillae; C, a taste bud and its taste-receptor cells.
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