Patients with Parkinson’s disease often have trouble with walking. Either they cannot take the first step, or they cannot stop moving when they reach their destination. The problem is not with the steps themselves but with starting and stopping the action—a pervasive difficulty that affects every aspect of daily life. Now research has finally pinpointed the neurons in the brain that initiate and end movements.

Rui Costa of the Champalimaud Neuroscience Program in Portugal and Xin Jin of the National Institutes of Health designed a task for mice that was the equivalent of taking eight steps. If the mice pressed a bar exactly eight times, no more, no less, they received a reward. Costa and Jin implanted tiny electrodes in each mouse’s brain to record the ac­tivity of neurons within the striatum, a structure deep in the brain known to be involved in motor commands. They found that some neurons became active right before the mouse started to press the bar and other neurons became active right before stopping.

To confirm that these neurons were indeed responsible for starts and stops, the researchers then genetically altered mice to lack the neurons, and subse­quently the mice could not learn the task. They were slow to begin pressing the bar, and they tended to randomly stop in the middle of the task. These mice did not have trouble with movements per se, Costa explains, but like people with Parkinson’s or Huntington’s disease, with starting and stopping a task properly. The work should help scientists under­stand precisely what goes wrong in the brains of patients and help them design better therapies, Costa says.