Motor Cortex Involvement in Dextrous Movement

Reaching out and grabbing the coffee cup in front of you may seem like it’s happening on autopilot, but your caffeine-deprived brain is actually working hard. It’s collecting sensory information, sending it to your motor cortex, planning the upcoming movement, and then signaling your muscles to make it happen. In a study published yesterday in Nature, scientists examined the role of the feedback signals entering the motor cortex in order to untangle how and when they’re necessary to guide dexterous movements like grasping.

Some neural circuits can generate rhythmic, patterned output without sustained input. Just as a single nudge from a rider can send a horse into a trot, these ‘central pattern generators’ can help animals walk, swim, and fly without ongoing stimulation. But not the motor cortex, it turns out. “What we show is the motor cortex is fundamentally different from that,” says Britton Sauerbrei of Janelia Research Campus. “You can’t just give the cortex a little kick and have it take off and generate that pattern on its own.”

The team trained mice to perform a behavior that depends on the motor cortex: reaching for and grasping a food pellet. In some animals, the scientists turned off the thalamus—a switchboard in the brain that directs sensory information and other kinds of feedback to and from the cortex. When the researchers blocked the signals coming into the motor cortex before the mice began to reach, the animals didn’t initiate movement. And when incoming signals were blocked mid-reach, the mice stopped moving their paw closer to the pellet.

The rhythm of those signals also matters, the researchers showed. In another experiment, they stimulated neurons carrying signals from the thalamus to the cortex with different patterns of incoming signals. The frequency of the stimulation affected the motor cortex output, with fast pulses disrupting mice’s grasping skills.

The signals entering the motor cortex via the thalamus come from all over, and it’s not yet clear which ones are most important for directing movement. Inputs to the thalamus include sensory information about the position of the arm, visual information, motor commands from other brain regions, and predictions about the upcoming movement. The researchers are planning to use tools they developed to switch specific regions of the thalamus on and off in order to test which inputs are really driving the behavior.