Hair Bundle Stiffness Decomposition
Total hair-bundle stiffness is the sum of two parallel mechanical contributions:
- K_GS — stiffness from the gating springs that gate the MET channels. Sits in series with the tip links.
- K_SP — stiffness from stereociliary pivots (bending of the actin rootlets) plus horizontal lateral links between stereocilia.
Because the gating springs are in series with the tip links, cutting the tip links cleanly removes K_GS while leaving K_SP intact. This is the key experimental handle.
How to measure K_GS in practice
Tobin et al. (2019) used local iontophoresis of EDTA to chelate Ca²⁺ at the tip-link bond, disengaging the cadherin-related (CDH23 / PCDH15) adhesion. Measured quantities:
- Whole-bundle stiffness before EDTA → K_HB
- Bundle stiffness after tip-link disruption → K_SP
- K_GS = K_HB − K_SP
- Number of intact tip links N_TL inferred from MET current → per-tip-link gating-spring stiffness k_GS = K_GS / (γ² · N_TL), where γ ≈ 0.14 is the geometric gain factor converting bundle deflection to gating-spring extension.
Why the decomposition matters
Without separating K_GS from K_SP, any bundle-stiffness measurement conflates the transduction apparatus (what actually gates ion channels) with passive scaffolding (what holds the bundle upright). They have completely different developmental regulation, different disease phenotypes, and different therapeutic implications.
For STRC Hearing Loss: STRC maintains horizontal top connectors, which contribute to K_SP (the scaffolding). Losing STRC drops K_SP; the intact gating springs then operate over a softer scaffold and the OHC amplifier loop destabilises — even though the MET machinery itself is fine. Any rescue-stiffness claim (pharmacochaperone, hydrogel, synthetic HTC) should report both K_GS and K_SP separately, not just K_HB.