How does the brain make the body move? Researchers are just beginning to understand how the brain plans and executes the body’s movements. A new study from Howard Hughes Medical Institute’s Janelia Research Campus investigated a little-understood aspect of movement: planning. Before moving a muscle, the brain is active for a fraction of a second. The researchers tracked this planning period to neurons in the anterior lateral motor cortex, providing evidence that activity in the brain’s left side controls movement on the body’s right side, and vice versa.
The movement-planning neurons presented a puzzle because these neurons do not directly receive sensory input nor directly stimulate body movement. The researchers knew that movement planning took place in the prefrontal cortex and that it was visible on EEG readouts, but they did not know how the signals were translated from cognition into movement. The research team investigated this brain section after discovering that mice have a movement-planning region similar to that of humans and other primates.
The researchers introduced a light-sensitive protein into mice using a technology called optogenetics, which allowed them to switch the proteins on and off with a laser pulse. The mice had learned a behavioral task in which they licked either to the left or to the right following a sensory cue and a 1.3 second movement planning period. After the mice were exposed to the cue, the researchers silenced the neurons on one side of the mice’s premotor cortex, disrupting the movement planning period.
Silencing the mice’s neurons impaired their ability to lick. When they silenced neurons on the right side of the brain, the mice demonstrated impairments in licking left. Likewise, when the left-side neurons were silenced, the mice demonstrated impairments in licking right.
The researchers were able to identify a small group of neurons called pyramidal tract neurons. The pyramidal tract neurons’ activity was associated only with future movements on the opposite side of a mouse’s body. When they followed up on this finding, the researchers discovered that stimulating these neurons could direct the mice’s licking response.
The findings explain why injuries like stroke typically disrupt the ability to plan movements on just one side of the body. The researchers explain that their next study will analyze how planning activity is generated and stored.
This research is published in the journal Nature.
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