Video narration
If dopamine is a movement switch, why does losing it make movement so slow? Think of movement like driving: you need power, but you also need to release the brake. It's a memory aid, not literal wiring. The direct pathway helps a movement proceed; the indirect pathway restrains movement. Dopamine from the substantia nigra pars compacta, a dopamine-producing brain region, acts on both. At D1 receptors, it boosts direct-pathway cells, adding power. At D2 receptors, it suppresses indirect-pathway cells, easing restraint. Dopamine does not just press the accelerator. It also eases the pathway that acts like a brake. In Parkinson disease, dopamine falls: the direct pathway gets less support, while the indirect pathway is less quieted and restrains movement more. That shift means less drive reaches the movement system. So dopamine loss slows movement because you lose power and the brake stays on.