How are inner hair cells different from outer hair cells in the cochlea?

In the organ of Corti, inner and outer hair cells serve distinct structural and functional roles in the process of hearing:

Physical Characteristics and Arrangement

• Inner Hair Cells: These are larger, pear-shaped cells arranged in a single row. They are uniform in size and possess 40 to 60 stereocilia arranged in two or more irregularly parallel rows.12
• Outer Hair Cells: These are smaller, cylindrical cells typically arranged in three or more rows. They possess approximately 100 stereocilia arranged in a distinct W pattern.1

Functional Differences

• Signal Transmission: The inner hair cells are the primary sensory receptors; they receive approximately 95 percent of the afferent nerve fibres that transmit sound information to the brain.1
• Mechanical Feedback: Outer hair cells contain contractile proteins (actin and myosin) that allow them to contract rhythmically. They act as mechanical-electrical stimulators and feedback elements, believed to modify and enhance the discriminatory responses of the inner hair cells.2
• Innervation: Inner hair cells have a dedicated, direct connection to the cochlear nerve, whereas outer hair cells share nerve endings in a "party line" arrangement, where one fibre may supply many cells.1