Carl Zimmer — whose enjoyable book Soul Made Flesh tells the story of Willis and the age of intellectual ferment and social turmoil in which he lived—details Willis’s understanding of the body’s nerves:
When Willis and his friends looked at them through a microscope they saw solid cords with small pores, like sugar cane. Reaching back to his earliest days of alchemy, Willis found a new way to account for how this sort of nerve could make a body move. He envisioned a nervous juice flowing through the nerves and animal spirits riding it like ripples of light. The spirits did not move muscles by brute force but rather carried commands from the brain to the muscles, which responded with a miniscule explosion. Each explosion, Willis imagined, made a muscle inflate.
The New Atlantis article goes on to talk about neurology and neuroelectronics.