The tip link's role in asymmetric stereocilia motion of chick cochlear hair cells

The symmetry of chick cochlear hair bundle motion was examined in this stud y. Isolated segments from the basilar papilla were incubated in vitro in ei ther normal or low calcium medium, which is known to disrupt tip links. Ste reociliary bundles, stimulated with an oscillating water microjet, were ori ented in profile and viewed in slow motion at high magnification with strob oscopic illumination. The displacement of the tallest hair in the bundle wa s fixed to 20 degrees peak-to-peak (P-P) motion. The angular deflections of the shortest and tallest hairs were then measured in both the positive (to wards the tallest hair) and negative (towards the shortest) directions with respect to the non-stimulated position of the hair. The tallest hairs exhi bited nearly symmetric motion in medium containing normal and low calcium. The shortest hairs, in normal calcium, displayed considerable asymmetry wit h angular deflections in the positive direction significantly larger than i n the negative direction. This asymmetric motion disappeared after incubati on in low calcium. The shortest hair angular displacement in the negative d irection, however, was the same in both normal and low calcium conditions. These results indicated that the tallest and shortest hairs moved with equa l angular deflection in the negative direction, while in the positive direc tion the shortest hair moved through a significantly greater angular deflec tion than the tallest hair. The implication of this finding is that the tip links contributed significantly to hair bundle motion in the positive dire ction only. (C) 1999 Published by Elsevier Science B.V. All rights reserved .