Synaptophysin immunostaining reveals olivocochlear (efferent) synaptic terminals under outer hair cells. In wild types (left panel), outer hair cells are contacted by 2-5 synaptic boutons (arrows). In α9-/- mice (right panel), olivocochlear terminals are most often less numerous (1-2 per outer hair cell) but larger (hypertrophied, small arrow), although occasionally hair cells are contacted by 2-3 boutons (large arrow). In SK2-/- mice (middle panel), terminals (arrows) progressively degenerate. All images are from mid-cochlea regions of adults (8-12 weeks old). Scale bar applies to all panels.