Researchers have identified a group of cells in the midbrain that act as a play/pause button for all types of body movement. When these brain cells are activated, mice pause all forms of movement and only resume when the activation stops. Researchers say that people may also have this very interesting and new phenomenon.
Most people know the feeling of being in the middle of something but suddenly hearing a sound that causes the whole body to stop moving so that all attention can be directed towards the sound. You may think that someone is knocking on the door or that someone is shouting.
The same is seen in animals, such as when a cat is hunting and suddenly stops completely and focuses all its concentration on a bird in the grass. When the bird has flown away, or when a person figures out the meaning of the sound being heard, movement continues as if someone had first pressed pause and then pressed play again.
Researchers have identified brain cells that may cause this type of global movement arrest.
“This is a very interesting discovery, and we think that the global movement arrest may condition for elevated awareness,” explains lead author Ole Kiehn, Professor, Department of Neuroscience, University of Copenhagen.
The research, which was carried out with Haizea Goñi-Erro, Raghavendra Selvan, Vittorio Caggiano and Roberto Leiras, has been published in Nature Neuroscience.
Made neurons light sensitive
The researchers used an optogenetic approach to make the neurons in mice light sensitive. This involved modifying the neurons genetically so they can be stimulated by light rather than electrically.
This means that the researchers can very precisely examine the function of neurons by activating them with small flashes of light from a light source inserted in the brain.
The neurons the researchers stimulated in the mice can be distinguished from other neurons by the fact that they express a specific molecular marker called Chx10. The neurons are in the midbrain in the pedunculopontine nucleus, an area also present in the midbrain of other animals and humans.
The researchers did not know what to expect when they stimulated the neurons deep inside the brains of mice. However, they found that stimulating the neurons with light produced stopped all ongoing movement. When the researchers stopped illuminating the neurons, the mice continued what they were doing. But the arrest of movement was very special. It had a pause-and-play pattern:
The stimulation instantaneously interrupts movement (pause). And when the stimulation was stopped the movement resumed at the exact time it was stopped (play).
“Stimulating these brain cells arrests all types of movement. This also applies to breathing, which either stops or slows, and the heartbeat, which slows,” says first author Haizea Goñi-Erro.
Not the same response as fear
Researchers already know other neurons that also arrest movement when they are activated – for example, some neurons stop the forward movement of the legs as we walk towards an obstacle, such as a door we want to open.
Other neurons are active when people and animals freeze if they are startled or sense danger.
“The motor pattern of freezing looks completely different from the pause-and-play pattern. So even though you might think they are the same, they are completely different phenomena,” explains Roberto Leiras.
The researchers think that the type of arrested movement they have discovered may be related to hyperfocusing: when a person or animal focuses all attention on a sound or a visual image and simultaneously stops movement.
May have a role in Parkinson’s
In addition to being extremely fascinating, the new discovery may prove to be important knowledge in understanding disease.
People with Parkinson’s disease often experience – especially late in the illness trajectory – that their body stops mid-movement, such as while walking.
According to Ole Kiehn, in this scenario the cells activated during the pause-and-play reaction could be overactivated in Parkinson’s, and this may contribute to the walking disturbances.
“We are now investigating this possibility in mice. Perhaps this can bring us closer to understanding which changes in the brain contribute to some of the motor symptoms of Parkinson’s disease,” he concludes.