How do the basal ganglia let one movement start and hold the others back? · Grey Matter

Their output nuclei keep the motor thalamus under constant inhibition, a brake on every possible action.

How do the basal ganglia let one movement start and hold the others back?

Their output nuclei keep the motor thalamus under constant inhibition, a brake on every possible action. When the cortex proposes an action, the striatum's direct pathway inhibits the output nuclei for that action alone, releasing the brake so the thalamus and motor cortex can run it, while the indirect pathway reinforces the brake on the competing actions. Dopamine favours the direct pathway, so when the dopamine neurons die in Parkinson's disease the brake dominates and movements become hard to start, slow and small.