COCHLEAR TRANSDUCTION

Those of us who study the sense of hearing appear more reluctant to change our interpretation of cochlear nerve activity than we are to alter our interpretation of cochlear transduction. The travelling wave theory first described by von Békésy in the 1920s served us for decades (1). The mechanical tuning it described was very broad, and it was suggested that some second filter (possibly lateral inhibition) between the basilar membrane and the cochlear hair cells sharpened up the tuning. His experiments predicted that, at detection threshold, the basilar membrane vibrated a distance far less than the diameter of a hydrogen molecule. It was also suggested that the more numerous cochlear outer hair cells (OHCs) were the transducers for low intensity acoustic stimuli, whereas the less numerous inner hair cells (IHCs) might serve as receptors for more intense acoustic stimuli (2).

 But Spoendlin’s (3) observation that approximately 95% of the cochlear nerve fibers receive their input exclusively from the IHCs suggested an alternative role for the OHC. Recent evidence has provided support for a radically different cochlear transduction model in response to low intensity acoustic stimuli. Even though Gold (4) had proposed as early as 1948 that the cochlea must add acoustic energy in order to respond to low level sounds, clear evidence for such a mechanism was not published until Kemp’s (5) report of sounds (termed otoacoustic emissions) originating from the healthy ear. The observations that both the cochlear hair cells and the basilar membrane were as sharply tuned as auditory nerve fibers (6, 7, 8), and that slight damage to the cells of the inner ear could alter the mechanical tuning of the basilar membrane (8) led to the current interpretation that the outer hair cells provide a micromechanical sharp tuning function, and that the inner hair cells act as the transducers which provide information to the cochlear nerve (9). This interpretation is quite different, and more complex, than that which we accepted just 2 decades ago.