We have more than five senses. What you might think of as your sense of “touch” is actually a series of different sensory pathways that allow you to isolate different types of mechanical forces, detect changes in temperature, and feel pain. .
The Nobel Prize in Physiology or Medicine, announced this week, went to American physiologist David Julius and Lebanese-born researcher Ardem Patpoutian, for uncovering the mechanisms underlying these various sensations of touch. So how do these mechanisms work?
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Our brain constantly processes a large amount of tactile and thermal information from our environment. As your hand wraps around your coffee cup in the morning, you may feel that the coffee is too hot, too cold to drink, or too cold. You can feel the weight of the cup in your hand and the smoothness of its surface, and feel the position of your arm as you move to take a sip.
To make sense of all these stimuli, our bodies need to convert external environmental information into biological signals. This process starts from the nerve cell endings in our skin.
On the surface of these nerve cells are special molecules called “ion channels” that can open in response to an environmental stimulus, resulting in a localized electrical signal. This signal can then be amplified into an electrical impulse that is transmitted through nerve cells to our brain, where it is interpreted as a sensation.
Julius and Patapoutian made separate and equally important contributions to our understanding of exactly what types of ion channels can function as sensory receptors.
In 1997, Julius and his team identified the first known receptor for heat, examining how cells respond to capsaicin, the chemical that causes burning when eating hot peppers.
Their research identified an ion channel known as TRPV1 as a receptor activated by capsaicin. What’s more, they demonstrated that this receptor is also activated by high temperatures that we perceive as painful.
Subsequent research has identified other members of the same family of ion channels that are each activated by a different temperature range. Thus, to sense different temperatures, our bodies use different receptors to differentiate between painful or harmful heat or cold, and to sense moderate changes in temperature.
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More than two decades later, in 2010, Patapoutian finally identified one of the receptors that responds to mechanical forces, enabling our sense of touch. He and his team identified a receptor molecule that responds to pressure by using a fine probe to make tiny indentations in lab-cultured cells.
He named the ion channel PIEZO1 from the Greek word for pressure. They demonstrated that a second ion channel, PIEZO2, is also required for our nerve cells to touch. When the surface of a sensory nerve cell is indented, both of these receptor molecules change shape, initiating an electrical impulse.
What’s more, PIEZO2 receptors not only enable the sense of touch but also signal mechanical information from within our bodies. Thus they allow us to detect stretch in our limbs so that we can control our movement, and to indicate whether our lungs are fully inflated or our bladder is full.
Research is still ongoing to find out whether there are other mechanically active ion channels in our nerve cells that help us understand our environment in other ways.
So the next time you take a sip of hot coffee, or feel a cool breeze on your face, imagine those tiny receptor molecules in your nerve endings working hard to deliver those signals to your brain. so that you can enjoy the world around you.