A group of nerve cells in the brain exhibits a remarkable ability to stop all forms of movement, as revealed by a recent study conducted on mice. This finding contributes significantly to our understanding of how the nervous system exerts control over our movements.
When a hunting dog detects the scent of a deer, it sometimes freezes completely. This phenomenon can also be observed in people who have to focus intensely on a complex task.
Now a recent discovery contributes to our understanding of what happens in the brain when we suddenly stop moving.
“We have found a group of nerve cells in the midbrain which, when stimulated, stop all movement. Not just walking; all forms of motor activity. They even make the mice stop breathing or breathe more slowly, and the heart rate slows down,” explains Professor Ole Kiehn, co-author of the study.
“There are different ways to stop motion. What’s so special about these nerve cells is that once they’re activated, they cause the motion to stop or freeze. Just like pausing a movie. The actors’ movement suddenly stops on the spot,” says Ole Kiehn.
When the researchers stopped activating the nerve cells, the mice would start the movement exactly where it stopped. Just like when you press “play” again.
“This ‘pause-and-play’ pattern is very unique; it’s unlike anything we’ve seen before. It does not resemble any other forms of movement or motor stopping that we or other researchers have studied. There, the movement does not necessarily start where it stopped, but perhaps starts again with a new pattern,” says Ph.D. Haizea Goñi-Erro, who is the first author of the study.
The nerve cells stimulated by the researchers are found in the midbrain in an area called the pedunculopontine nucleus (PPN), and they differ from other nerve cells and thereby express a specific molecular marker called Chx10. PPN is common to everyone vertebrates including humans. So although the study was conducted in mice, the researchers expect the phenomenon to also apply to humans.
Not related to fear
Some might suggest that the nerve cells are activated by fear. Most people are familiar with the phenomenon of “freezing” caused by extreme fear. But that is not the case.
“We have compared this type of engine stoppage with engine stoppage or freezing caused by fear, and they are not identical. We are very sure that the arrest of motion here is not related to fear. Instead, we think it has something to do with attention or vigilance, which is seen in certain situations,” says assistant professor Roberto Leiras, who is a co-author of the study.
The researchers believe that this is an expression of focused attention. However, they emphasize that the investigation has not revealed whether this is actually the case. This is something that requires more research to prove.
May be able to understand Parkinson’s symptoms
The new study may help us understand some of the mechanisms behind Parkinson’s disease.
“Motor stoppage or slow movement is one of the main symptoms of Parkinson’s disease. We speculate that these special nerve cells in the PPN are overactivated in Parkinson’s disease. This would inhibit movement. Therefore, the study, which has primarily focused on the basic mechanisms that control movement, can in the nervous system, eventually help us to understand the cause of some of the motor symptoms of Parkinson’s disease,” concludes Ole Kiehn.
Reference: “Pedunculopontine Chx10+ neurons control global motor arrest in mice” by Haizea Goñi-Erro, Raghavendra Selvan, Roberto Leiras and Ole Kiehn, 27 July 2023, Nature’s neuroscience.
The study is funded by the Novo Nordisk Foundation, the Lundbeck Foundation and the Swedish Research Council.