= Inner ear
The <inner ear> contains fluid-filled sensory structures for hearing and balance. In the <cochlea>, motion transmitted from the middle ear creates a traveling disturbance along the <basilar membrane>. Movement of sensory <hair cells> bends their stereocilia, changing mechanically gated ion-channel opening and producing an electrical response.<How Do We Hear? by the National Institute on Deafness and Other Communication Disorders>[Steps 3–6: cochlear fluid, basilar membrane, hair bundles and auditory signaling]{ref}
The mechanical response varies along the <cochlea>, so position contributes to <frequency> selectivity. The vestibular part of the <inner ear> instead detects head motion and orientation. The distinction is anatomical and functional: detecting a <sound> <frequency> and detecting a rotation of the head use related mechanosensory principles in different structures. Mechanical input, hair-cell response and activity in the associated sensory <neurons> are successive measurable stages.
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