Stereociliary

Stereociliary refers to stereocilia, the actin-rich, hair-like projections on the apical surface of sensory hair cells in the inner ear and vestibular epithelia. These structures are essential for mechanosensation, mediating both hearing and balance.

Stereocilia are arranged in bundles that form a staircase pattern, with the tallest taller than the shorter ones. Each stereocilium contains a core bundle of parallel actin filaments crosslinked by actin-associated proteins, giving rigidity and length. They are anchored to the apical cuticular plate and connected by extracellular links, including tip links that connect the tip of one stereocilium to the side of its taller neighbor. The tip links are composed of cadherin family proteins, notably protocadherin-15 and cadherin-23, and they cooperate with other lateral links to maintain bundle integrity.

Functionally, deflection of the stereociliary bundle toward the tallest row increases tension in the tip links, which opens mechanosensitive ion channels and allows cations, primarily potassium and calcium, to enter the hair cell. This generates a receptor potential and triggers neurotransmitter release to afferent neurons. Adaptation mechanisms, including motor activity of myosin proteins, modulate tension to adjust sensitivity to ongoing stimulation. Outer hair cells can also undergo electromotility via prestin, amplifying basilar-membrane movement, while inner hair cells serve as the primary sensory transducers.

Stereociliary structure and function are critical for normal audition and balance. Damage or genetic mutations affecting stereocilia or their links can lead to sensorineural hearing loss and vestibular dysfunction. Factors such as excessive noise exposure, ototoxic drugs, aging, and certain hereditary conditions can disrupt stereociliary integrity.