The vestibular system detects movement through hair cells in the inner ear.

“It's basically in your inner, you know, ear hairy cells. They got little psyia sticking up off the surfaces. And depending on which way you bend those, the cells will either be inhibited or excited. But then they talk to neurons with a neuron-like process and off you go. Now you've got an auditory signal. If you're sensing things bouncing around in your cookia, which is >> sympathetically the bouncing of your eard drum, which is in sympathetically the sound waves in the world. But in the case of the vestibular apparatus, evolution has built a system that detects the motion of say fluid going by those hairs.”