Similar wiring diagram may be used elsewhere in the brain.
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.
Duke University researchers have used a sophisticated tracing technique in mice to map the underlying brain circuitry that keeps mealtime relatively painless. The study, which appears June 3 in eLife, could lend insight into a variety of human behaviors, from nighttime teeth grinding to smiling or complex vocalizations.
“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.”
Previous mapping attempts have produced a relatively blurry picture of this chewing control center. Researchers know that the movement of the muscles in the jaw and tongue are governed by special neurons called motoneurons and that these are in turn controlled by another set of neurons called premotor neurons. But the exact nature of these connections — which premotor neurons connect to which motoneurons — has not been defined.
Senior study author Fan Wang, Ph.D., associate professor of neurobiology and a member of the Duke Institute for Brain Sciences, has been mapping neural circuits in mice for many years. Under her guidance, Stanek used a special form of the rabies virus to trace the origins of chewing movements.
The rabies virus works naturally by jumping backwards across neurons until it has infected the entire brain of its victim. For this study, Stanek used a genetically disabled version of rabies that could only jump from the muscles to the motoneurons, and then back to the premotor neurons. The virus also contained a green or red fluorescent tag, which enabled the researchers to see where it landed after it was done jumping.
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.
“Using shared premotor neurons to control multiple muscles may be a general feature of the motor system,” said Stanek. “For other studies on the rest of the brain, it is important to keep in mind that individual neurons can have effects in multiple downstream areas.”
The researchers are interested in using their technique to jump even further back in the mouse brain, eventually mapping the circuitry all the way up to the cortex. But first they plan to delve deeper into the connections between the premotor and motoneurons.
“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.
Notes about this neuroscience research
The research was supported by grants from the National Institutes of Health (NS077986 and DE019440).
Contact: Karl Bates – Duke University Source:Duke University press release Image Source: The image is credited to Fan Wang Lab, Duke University and is adapted from the press release. Original Research: Full open access research for “Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination” by Edward Stanek, Steven Cheng, Jun Takatoh, Bao-Xia Han, and Fan Wang in eLife. Published online April 30 2014 doi:10.7554/eLife.02511
Open Access Neuroscience Abstract
Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination
Feeding behaviors require intricately coordinated activation among the muscles of the jaw, tongue, and face, but the neural anatomical substrates underlying such coordination remain unclear. Here we investigate whether the premotor circuitry of jaw and tongue motoneurons contain elements for coordination. Using a modified monosynaptic rabies virus based transsynaptic tracing strategy, we systematically mapped premotor neurons for the jaw-closing masseter muscle and the tongue-protruding genioglossus muscle. The maps revealed that the two groups of premotor neurons are distributed in regions implicated in rhythmogenesis, descending motor control, and sensory feedback. Importantly, we discovered several premotor connection configurations that are ideally suited for coordinating bilaterally symmetric jaw movements, and for enabling co-activation of specific jaw, tongue, and facial muscles. Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination.
“Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination” by Edward Stanek, Steven Cheng, Jun Takatoh, Bao-Xia Han, and Fan Wang in eLife, April 30 2014 doi:10.7554/eLife.02511