Final answer:
As sound levels increase, outer hair cells stiffen the basilar membrane in the organ of Corti, fine-tuning the hearing process by sharpening the frequency response.
Step-by-step explanation:
As sound levels increase in the spiral organ (of Corti), outer hair cells stiffen the basilar membrane. This is the correct answer. Outer hair cells function to fine-tune the precision of sound transduction by changing their length and, in turn, affecting the mechanical properties of the basilar membrane.
The organ of Corti, which contains the hair cells responsible for sound transduction, reacts to the movement of the basilar membrane when sound waves strike it. The inner hair cells primarily act as the auditory receptors, translating mechanical motions into electrical signals sent to the brain. Meanwhile, the outer hair cells modify the response of the inner hair cells by modifying how the basilar membrane moves in response to sound, thereby sharpening the frequency response and increasing the sensitivity of hearing.