A sensor that picks up nerve signals from the spinal cord could let people control a prosthetic arm by simply imagining the movement they want to make. In tests using the sensor, which has to be surgically implanted, people were able to control a virtual arm on a screen with a larger range of movement than many existing prosthetics. Prosthetic arms currently on the market are usually controlled by the user flexing muscles in their arm or chest. But there is a limit to how many commands can be mapped onto these muscle movements, which makes it difficult to do more intricate manoeuvres like pinching two fingers together. A team led by Dario Farina, then at University Medical Centre Göttingen, Germany, instead developed a technique that interprets signals directly from motor neurons in the spinal cord.