[Active movements of the hair cells: a new mechanism in hearing]

The concept that the outer hair cells of the cochlea possess an active motor capacity confers new dimensions on the models of the physiology and patho-physiology of the inner ear. Results are presented which suggest the existence of motor properties due to actin in the outer hair cells of the guinea pig: Permeabilized isolated outer hair cells contract in a concentration-dependent manner in the presence of ATP and calcium. Movement of the cuticular plate, but not of the stereociliary bundle, can also be observed. Contraction is inhibited by cytochalasin and by bisphosphate; The movement depends on the free calcium concentration. The calcium-binding protein calmodulin can be demonstrated in the hair cell. The substance TFP, which inactivates calmodulin, inhibits the contractile response of the hair cell; The existence of a novel infracuticular actin network can be demonstrated directly by monoclonal antibodies against actin and by selective binding of rhodamine-phalloidin to actin filaments; Furthermore, the motility of the outer hair cells is not contractile in nature, but is also associated with polymerization of actin molecules. Thus the outer hair cells are evidently capable of movement. The ATP- and calcium-induced contractions of permeabilized isolated outer hair cells suggest a physiological actin dependent mechanism. Accordingly, outer hair cells must be peripheral effector cells. The hypothetical model includes an influence of OHC motility on the micromechanics of the basilar membrane and the stereociliary region of outer hair cells to allow adaptation to high sound pressure. Moreover, control of the damping characteristics of the basilar membrane appears reasonable.