Abstract: The organization of behaviour as sequences of actions requires proper initiation and termination of each action sequence. The neural circuit that signals instructions to start and stop is now revealed. When executing a behaviour, appropriate initiation and termination of the action sequence is vital; this process is compromised in nigrostriatal disorders such as Parkinson's and Huntington's disease. The neural mechanisms that underlie the learning and execution of a fixed behavioural pattern are not well understood, but Xin Jin and Rui Costa reveal start/stop neuronal activation patterns in basal ganglia circuits that emerge during task training. Mice can learn to perform a self-paced sequence of a set number of repetitive actions — lever pressing for a sucrose reward — very precisely. Using this task, Jin and Costa found that many neurons in nigrostriatal circuits can selectively signal the self-initiation or self-termination of each executed sequence. Genetic alteration of these circuits disrupted the start/stop activity and impaired performance, providing evidence for a causal relationship between the specific neuronal activity and task learning.