How many inner hair cells are recruited for loud sounds?

• A. 4-7 IHCs
• B. 8-12+ IHCs
• C. 2-3 IHCs
• D. 6-9 IHCs

Answer: (B)

When loud sounds occur, more inner hair cells within the stimulated region of the cochlea are driven above their threshold and contribute to transduction. Each inner hair cell connects to many auditory nerve fibers, so recruiting additional IHCs increases the total neural input as intensity rises. This spread of activation across neighboring IHCs is how the auditory system encodes greater loudness. In humans, a typical estimate for the number of IHCs engaged at high sound levels is about eight to twelve or more within the region of the basilar membrane that’s being stimulated. Softer sounds activate fewer IHCs, while the larger recruitment seen with loud sounds reflects the broadening of the excitatory input across IHCs.