According to foreign media reports, a new study found that roundworms respond to bright light to change the flow of material in their "mouths", revealing new ways for neurons to control muscle cells. For more than a decade, researchers have known that despite the lack of light-absorbing molecules needed for eyes and vision, the roundworm Caenorhabditis elegans can "detect" and avoid short wavelengths of light.
As a graduate student in Horwitz's lab, Nikhil Bhatla offers an explanation for this ability. He observed that light not only made roundworms squirm, but also prompted them to stop eating. This clue led him to conduct a series of studies showing that roundworms did not "see" light at all — they were "detecting" toxic chemicals produced by light, such as hydrogen peroxide. Soon after, researchers in Howitz's lab realized that roundworms not only taste the nasty chemicals produced by light, they can also spit them out.
Now, in a study published in eLife, a team led by graduate student Steve Sando reports that C. The mechanism of spitting out in elegans. Individual muscle cells are often thought to be the smallest units that neurons are able to control independently, but the researchers' findings call this hypothesis into question. In the case of spitting out substances, they determined that neurons can direct specialized subregions of individual muscle cells to produce multiple movements —expanding our understanding of how neurons control muscle cells to shape behavior.
Professor of Biology at the Massachusetts Institute of Technology, member of the McGovern Brain Institute and the Koch Institute for Comprehensive Cancer Research, Howard Hughes Institute of Medicine Fellow, and senior author of the study, H. H. Lee. Robert Horvitz said: "Steve has a remarkable discovery that the contraction of a small area of a particular muscle cell can be decoupled from the contraction of other parts of the same cell. In addition, Steve found that the muscle regions of this subcellularity can be controlled by neurons, dramatically changing behavior. "
Roundworms act like vacuum cleaners, squirming around to suck bacteria away. The "mouth" (also known as the "pharynx") of a bug is a muscular tube that captures food, chews it, and then transfers it into the intestines through a series of "sucking" contractions.
For more than a decade, researchers have known that roundworms can escape ultraviolet, violet or blue light. But Bhaatla found that this light also interrupted the constant twitching of the pharynx, because the taste produced by the light was so annoying that the worms paused eating. When he looked closely, Bhatla noticed that the roundworm's response was actually quite subtle. After the initial pause, the pharynx briefly began to twitch briefly again, then stopped completely—almost as if a bug was still chewing after tasting the unpleasant light. Sometimes, a bubble will come out of the mouth, like a hiccup.
After he joined the project, Sando found that the roundworms neither burped nor continued to chew. Instead, the "burst pump" is pushing the substance in the opposite direction, coming out of the mouth and into the local environment, rather than further back into the pharynx and intestines. In other words, "bad taste" causes the worm to spit out. Sando then spent years chasing his subjects under a microscope with glare and recording their movements in slow motion in order to determine the neural circuits and muscle movements required for this behavior.
"It was quite surprising to find roundworms 'spitting' because the mouth seemed to move as if it were when chewing," Sando said. "It turns out that you really need to zoom in and slow down to see what's going on because animals are so small and behavior happens so fast."
To analyze what happens in the pharynx to produce this "spitting" action, the researchers used a tiny laser beam that surgically removes individual nerve and muscle cells from the mouth and tells how this affects the behavior of roundworms. They also monitored the activity of cells in the mouth by labeling them with specially designed fluorescent "reporter" proteins.
They saw that as roundworms were feeding, three muscle cells called pm3, near the front of the pharynx, contracted and relaxed together in a synchronized pulse. But once the roundworm tastes the light, the subregion closest to the anterior part of the mouth in these individual cells locks into a contracted state, opening the front of the mouth and allowing the substance to be pushed out. This reverses the flow direction of the ingested substance and converts eating into spitting.
The team determined that this "decoupling" phenomenon is controlled by a single neuron in the back of the ascarf's mouth. This nerve cell is called M1, and it stimulates a local inflow of calcium at the front end of the pm3 muscle, possibly responsible for triggering subcellular contractions.
M1 is responsible for forwarding important information. It receives afferent signals from many different neurons and passes this information on to the muscles involved in "spitting." Sando and his team suspect that the intensity of the incoming signal could adjust the worm's response behavior to tasting light. For example, their findings suggest that an offensive taste can cause a strong response, while a slight unpleasant sensation causes the worm to spit out more gently by simply squirting out the contents.
In the future, Sando thinks the worm could serve as a model to study how neurons trigger sub-regional contractions of muscle cells and shape behavior — a phenomenon they suspect occurs in other animals, possibly humans.
"We basically found a new way for neurons to move muscles," Sando said. "Neurons coordinate the movement of muscles, which could be a new tool that enables them to exert a complex kind of control." It's quite exciting. ”