Each stereocilium is a rigid rod pivoting at its actin rootlet. A fine filament — the tip link — runs from the tip of a shorter stereocilium to a mechanotransduction (MET) channel on its taller neighbour. Deflecting the whole bundle toward the tallest row shears the rows past one another and stretches every tip link at once (the "gating spring" model, Corey & Hudspeth 1983; Howard & Hudspeth 1988).
P_open(x) = 1 / (1 + exp(-(x - x50) / x_s))
x_s = kBT / (k_gs · d) (Boltzmann steepness, nm)
T = k_gs · (x - x50) (tip-link tension, pN, clamped ≥ 0)
d(x50)/dt = (x - x50) / τ (adaptation motor tracks x)
I_MET = N · P_open · g · ΔV (transduction current, pA)
- x — bundle deflection in nanometres; positive = toward the tallest row (excitatory), negative = toward the shortest row.
- kgs — gating-spring stiffness; a stiffer spring converts less displacement into the same tension change, so Popen(x) becomes steeper and more switch-like.
- τ (adaptation) — myosin-1c motors at the tip-link's upper insertion slip down the actin core under sustained tension, dragging the operating point x50 toward the new x and letting the channels partially reclose even while the stimulus is held (fast + slow adaptation, simplified here to one time constant).
- Auto step stimulus — alternates x between 0 and +90 nm so you can watch Popen spike on the step and relax back down as x50 catches up, the signature adaptation transient recorded in real hair cells.
d ≈ 4 nm is the single-channel gating swing and kBT ≈ 4.1 pN·nm at body temperature; both are fixed physiological constants. This is the same mechanism that converts sound-driven basilar-membrane motion into the receptor current every cochlear hair cell uses to signal pitch and loudness.