The hair cells of the cochlear organ of Corti transduce sound-induced basilar membrane vibration into electrical signals. Mechanotransduction depends on tip links connecting adjacent stereocilia. What ion predominantly enters through the tip-link-gated channels during stereocilia deflection toward the tallest row?
- A Na⁺, entering down its electrochemical gradient from endolymph
- B K⁺, entering down its electrochemical gradient from endolymph (high K⁺, +80 mV endocochlear potential) ✓
- C Ca²⁺, entering exclusively as a secondary messenger to trigger vesicle fusion
- D Cl⁻, entering through CFTR channels to depolarize the hair cell
Explanation
Unlike most cells where K⁺ exits, in cochlear hair cells K⁺ enters through mechanosensitive channels (MET channels, largely TMC1/TMC2) because endolymph has a unique ionic composition (high K⁺, ~150 mM) and a +80 mV endocochlear potential (maintained by stria vascularis), making the electrical driving force strongly inward for K⁺ into hair cells (resting potential ~-60 mV). Deflection toward the tallest stereocilia stretches tip links (cadherin-23/protocadherin-15), pulling MET channels open → K⁺ (and some Ca²⁺) influx → depolarization → basolateral voltage-gated Ca²⁺ entry → glutamate release onto auditory nerve. K⁺ then exits via basolateral K⁺ channels into the perilymph.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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