Full Idea
The motive power or the nerves themselves originate in the brain, which contains the imagination, which moves them in a thousand ways, as the common sense is moved by the external sense.
Gist of Idea
Nerves and movement originate in the brain, where imagination moves them
Source
René Descartes (Rules for the Direction of the Mind [1628], 12)
Book Reference
Descartes,René: 'Rules for the Direction of the Mind' [Newcomb Library 2023], p.36
A Reaction
This sounds a lot more physicalist than his later explicit dualism in Meditations. Even in that work the famous passage on the ship's pilot acknowledged tight integration of mind and brain.