Think About This Next Time You Enjoy a Burrito: Why You Don't Often Bite Your Tongue
"Control your tongue!" parents warn. You can control it when you think about it, but most of the time, our tongue performs its many functions automatically. It's good we don't have to control our tongues consciously while eating, or we might end up with extra protein in our meals and less ability to talk about it afterward.
When you think about the risks to a tongue at mealtime, it's like an action-adventure movie: the hero ducks under the city gates right before they slam shut. Hundreds of times a day, our tongue escapes danger in the blink of an eye. We only worry about it on the rare occasions when we are not careful, and bite our tongue: then we really notice.
Duke University scientists were curious about this automatic response, so they set about locating the brain centers that control chewing. Working with rats, they found a complex system at work:
Eating, like breathing and sleeping, seems to be a rather basic biological task. Yet chewing requires a complex interplay between the tongue and jaw, with the tongue positioning food between the teeth and then moving out of the way every time the jaw clamps down to grind it up.
If the act weren't coordinated precisely, the unlucky chewer would end up biting more tongue than burrito.
They used a "sophisticated tracing technique" to "map the underlying circuitry" that keeps mealtime relatively painless. There's more to chew on in their findings than eating; what they unveiled could apply to smiling or complex vocalizations, and even to disorders like nighttime tooth grinding.
It almost sounds like the researchers approached the question with design in mind:
"Chewing is an activity that you can consciously control, but if you stop paying attention these interconnected neurons in the brain actually do it all for you," said Edward Stanek IV, lead study author and graduate student at Duke University School of Medicine. "We were interested in understanding how this all works, and the first step was figuring out where these neurons reside."
Paul Nelson's father's proverb comes to mind: "If something works, it's not happening by accident." Sure enough, there are at least two interconnected sets of neurons behind this automated process: motoneurons and premotor neurons. That much was known, but how they were connected was not.
To trace the connections precisely, the scientists cleverly used modified rabies viruses that act by jumping backwards through neurons until the whole brain is infected. By labeling their special tracer viruses with red and green fluorescent tags and then inserting them into the chewing muscles of their unfortunate lab rats, they could watch the map light up.
Stanek injected these fluorescently labeled viruses into two muscles, the tongue-protruding genioglossus muscle and the jaw-closing masseter muscle. He found that a group of premotor neurons simultaneously connect to the motoneurons that regulate jaw opening and those that trigger tongue protrusion. Similarly, he found another group that connects to both motoneurons that regulate jaw closing and those responsible for tongue retraction. The results suggest a simple method for coordinating the movement of the tongue and jaw that usually keeps the tongue safe from injury.
It's not that simple, though. This is only part of the map; "it is important to keep in mind that individual neurons can have effects in multiple downstream areas." There's a lot more mapping to go:
"This is just a small step in understanding the control of these orofacial movements," Stanek said. "We only looked at two muscles and there are at least 10 other muscles active during chewing, drinking, and speech. There is still a lot of work to look at these other muscles, and only then can we get a complete picture of how these all work as a unit to coordinate this behavior," said Stanek.
A clearer statement of irreducible complexity could hardly be found: multiple parts coordinated for function. Disable any part, and the function breaks down.
These researchers didn't show any eagerness to try to explain how this system might have originated by unguided Darwinian processes. Bite your tongue! (Sorry, no more puns after this.) Instead, they wanted to understand how something works. That's a good approach in science. It gives biology a chance to demonstrate that it isn't happening by accident.