Structure of the Crista ampullaris: The gelatinous cupula (G) runs across the cavity of the ampulla like a swinging door. Hair cells (H) have long tufts of cilia (HT) at the distal borders. They are innervated at the bases by axons (N) whose cell bodies lie in the vestibular ganglion. These cilia are mainly stereocilia, lacking the microtubular structure of typical motile cilia. One cilium, however, the kinocilium, is motile (white in diagram below). Bending the cilia towards this kinocilium causes excitation; movement away from it causes inhibition. All the cells in the crista are oriented with the kinocilium towards the base of the semicircular canal, so that fluid movement in this direction causes excitation, an in the opposite direction, causes inhibition.

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The three semicircular canals (Superior, Lateral or Horizontal, and Posterior) are orientated mutually at right angles with respect to one another.

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Each works with the correspondingly oriented one in the contralateral ear, to detect rotational acceleration in a particular plane.

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