Relationship Between Semicircular Canals and Extraocular Muscles

Figure 1: When stimulated, each of the 6 angular acceleration detecting semicircular canals (3 on the right and 3 on the left) responds with a conjugate eye movement, with the vector(s) indicated below. PC=posterior canal; HC=horizontal (also known as lateral) canal; AC=anterior (also known as superior) canal. Figure 2: When the right AC is stimulated as in right AC benign paroxysmal positional vertigo (BPPV), excitatory slow phases that are upward and torsional (towards left ear) are generated. This is due to contraction of the right superior rectus (up and incycloduction OD) and left inferior oblique (up and excycloduction OS) muscles. Fast phases in the opposite direction - downbeat and torsional towards the right (ipsilateral) ear - are then seen clinically. Figure 3: When the right HC is stimulated as in a rapid righward head turn, the vestibulo-ocular reflex (VOR) results in the eyes moving conjugately to the left (to avoid retinal/foveal slippage off the intended target) via contraction of the right medial rectus and left lateral rectus muscles. Figure 4: When the right PC is stimulated as in right PC benign paroxysmal positional vertigo (BPPV), excitatory slow phases that are downward and torsional (towards left ear) are generated. This is due to contraction of the right superior oblique (down and incycloduction OD) and left inferior rectus (down and excycloduction OS) muscles. Fast phases in the opposite direction - upbeat and torsional towards the right (ipsilateral) ear - are then seen clinically. Figure 5: Figures 2, 3, and 4 are combined here so that the extraocular muscles stimulated with excitation of the right PC, HC, and/or AC can be seen simultaneously.