Distal separation of chick cochlear hair cell stereocilia: analysis of contact-constraint models

One model often used in the study of hair bundle micromechanics assumes simple geometric relationships between hair displacements, constrained by contact between neighboring hairs at their distal tips. Recent observations of hair bundle motion provided the opportunity to evaluate the contact-constraint model against measured displacements for the tallest and shortest sensory hairs. A contact-constraint model was developed based on the geometry of a single column of stereocilia. The model used morphological data from chick hair bundles for which displacement data in the excitatory and inhibitory directions were also available. For each hair bundle, a unique sensory hair radius was determined so that the calculated resting bundle morphology matched the measured values. The model was then evaluated against the displacement data for each hair bundle. In each case, the model underestimated the excitatory displacement of the shortest hairs. Failure of the model to accurately predict bundle motion raises the possibility of a distal separation between the hairs at rest. It is suggested that tip links pull the hairs through this separation during excitatory deflections. Perhaps at damaging levels of displacement, the hairs suddenly come into contact, tip-link tension dramatically increases, and the tip-links are fractured.