Mechanisms of movement in outer hair cells and a possible structural basis

Isolated outer hair cells were found to slowly shorten when subjected to a solution that would induce contraction in a muscle fibre. Two possible mechanisms underlying this behaviour emerge from ultrastructural and immunocytochemical investigations. Antibody labelling at the electron microscopic level demonstrates that actin is present not only in the stereocilia and in the cuticular plate but also along the wall of outer hair cells, between the plasma membrane and the subsurface fenestrated cisternae. The latter are interconnected by regularly spaced pillars, resembling those seen between the T-tubules and sarcoplasmic reticulum in muscle fibres. Contraction also results from the application of positively charged macromolecules to the bathing solution. This implies sensitivity of the membrane-associated complex (the cortex system) to an electrical current. A second contractile system may reside in the cytoplasm, where calmodulin is present in contracted hair cells. This protein is a calcium-binding control protein for contraction-like events in smooth muscle and non-muscle cells. The unique presence of the cortex system in outer hair cells, and its absence in inner hair cells, indicates a functional significance that relates to a motor function of outer hair cells in hearing.