Press release: The Nobel Prize in Physiology or Medicine 2021 - NobelPrize.org
To
David Julius and Ardem Patapoutian
for their discoveries of receptors for temperature and touch
As an introduction, how many senses do we have? Five is the traditional number: sight, hearing, touch, smell, and taste. But we have more senses than that, and "touch" is not one sense but several: light touch, pressure, heat, cold, and pain. We also have less-obvious senses like proprioception, joint-orientation sensing, and senses of hunger and thirst.
It's a long list, so I'll sort them out by category:
- Chemical - smell, taste, hunger, thirst, various internal
- Optical - sight
- Temperature - heat, cold
- Mechanical - light touch, pressure, proprioception, orientation, hearing
- (other species) Magnetic
- (other species) Electrical
Chemical senses are well-understood. A molecule sticks on a receptor molecule on the sense cell's surface, and that produces a cascade of signaling that gets transmitted to the nervous system.
Optical receptors are also well-understood. A photon excites a receptor molecule, producing a cascade of signaling.
Orientation is done in various ways. Vertebrates have fluid-filled semicircular canals in their heads, and moving one's head moves that fluid. Some invertebrates have "statocysts", hollow balls with a "statolith" in them.
Hearing I listed as mechanical because it works by sensing amount of vibration of ear parts, like the vertebrate inner ear.
David Julius and Ardem Patapoutian first worked on finding the receptor that responds to capsaicin, what makes chili peppers feel burning hot. It was eventually named TRPV1. Menthol feels cool, and they discovered that it triggers a cold receptor, TRPM8. Others found additional receptor proteins for different temperatures.
Ardem Patapoutian then turned his attention to mechanical senses. They were understood in bacteria, but not in vertebrates. He and his collaborators found a cell line that would respond to being touched with a micropipette. The researchers then acted on the hypothesis that touch receptors act like other receptors, and he found 72 candidate genes for them. They silenced the genes, one by one, and looked for which silencing caused a lack of touch sensitivity. They found one, and named it Piezo1. They also found a second one, Piezo2. Their proteins were later shown to respond to pressure.