Since the long process of the incus articulates with the stapes, the stapes moves medially as well. The base of the stapes is attached to the oval window, and so the medial movement of the stapes means that the oval window is also moved medially.

What moves in response to stapes movement?

As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells, which are auditory receptor cells of the inner ear embedded in the basilar membrane.

What happens when the stapes vibrates?

The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate. This stimulates the sensory cells of the organ of Corti, atop the basilar membrane, to send nerve impulses to the brain.