<P> The most common kind of hearing impairment, sensorineural hearing loss, includes as one major cause the reduction of function in the organ of Corti . Specifically, the active amplification function of the outer hair cells is very sensitive to damage from exposure to trauma from overly - loud sounds or to certain ototoxic drugs . Once outer hair cells are damaged, they do not regenerate, and the result is a loss of sensitivity and an abnormally large growth of loudness (known as recruitment) in the part of the spectrum that the damaged cells serve . </P> <P> While hearing loss has always been considered irreversible in mammals, fish and birds routinely repair such damage . A 2013 study has shown that the use of particular drugs may reactivate genes normally expressed only during hair cell development . The research was carried out at Harvard Medical School, the Massachusetts Eye and Ear Infirmary, and the Keio University School of Medicine in Japan . </P> <Ul> <Li> <P> Transverse section of the cochlear duct of a fetal cat . </P> </Li> <Li> <P> Diagrammatic longitudinal section of the cochlea . </P> </Li> <Li> <P> Floor of ductus cochlearis . </P> </Li> <Li> <P> Limbus laminæ spiralis and membrana basilaris . </P> </Li> <Li> <P> Section through the spiral organ of Corti . Magnified . </P> </Li> </Ul> <Li> <P> Transverse section of the cochlear duct of a fetal cat . </P> </Li>

Which of the following stimulates the hair cells found within the spiral organ of corti