When we focus deeply, our bodies can go into a state of literal levitation. Our limbs freeze. The breathing stops. The heart rate slows down. The universe narrows until there is nothing but the task before us.
If we’re like the mice recently studied by researchers at the University of Copenhagen in Denmark, a selection of cells in a part of the brainstem called the pedunculopontine nucleus (PPN) could be responsible for this “freeze frame” effect.
Not only does the discovery help us map the location of tissues that contribute to the Zen-like qualities of the human mind, it could also help us better understand the course of symptoms in neurodegenerative diseases such as Parkinson’s disease, which may be could lead to better forms of therapy.
Animals like us have many reasons to stop suddenly. Freezing triggered by fear, for example, gives the prey a fighting chance to avoid detection.
Defensive global motor arrest is a relatively well-studied nervous system function, with pathways running between the lower brainstem and the amygdala, the “fear center,” and the periaqueductal gray in the midbrain.
But predators have their own reasons for playing statue, and it’s more out of a need for focus than fear. In fact, most animals have moments when they need to pause in a perfect still frame and take in the world.
To locate the brain cells responsible for this specific form of motor arrest, the researchers used mice engineered to have specialized ‘light-activated’ neurons in the PPN, a part of the brain already known to that it suppresses muscle tone when stimulated.
This small knot of neurons in the pons — a region of the brainstem implicated in conditions like sleep paralysis — is made up of three different classes of neurons known as glutamatergic, cholinergic, and GABAergic neurons, all layered in different ways.
Triggering of the glutamatergic neurons of the PPN tends to cause rodents to slow down their movements, which encourages them to be more exploratory. Curiously, not all studies have produced the same results, with some even observing that subjects’ muscles became completely blocked.
By confining activation of glutamatergic neurons to specific regions of the PPN, the scientists in this latest experiment localized the small clusters of neurons that caused the mice to pause, freeze, and then resume their previous activity.
“This ‘pause and play pattern’ is very unique; it’s unlike anything we’ve seen before,” says the study’s lead author, neuroscientist Haizea Goñi-Erro.
“It doesn’t resemble other forms of movement or motor standstill that we or other researchers have studied. There the movement does not necessarily start where it left off, but can start over with a new pattern.”
We humans also have a PPN, so it’s not too far-fetched to assume that it also contains a small population of nerve cells that coordinate our muscles in a “stop-and-think” moment and give us the mental space to think about it to remember where we left our keys or to help us hit the perfect putt in golf.
Of course, as with any neurological wiring, there can be short circuits in the functioning of the PPN. Given this finding, it is highly likely that the slowed or stopped movements in Parkinson’s patients could be the result of overactivation of these specific nerves.
“Therefore, the study, which primarily focuses on the fundamental mechanisms that control movement in the nervous system, could ultimately help us understand the origin of some motor symptoms in Parkinson’s disease,” says neurologist Ole Kiehn.
This research was published in natural neuroscience.