Self Tuning of Hair Cell Bundle after Prolonged Deflection

Abstract

Active hair bundle motility is one of the signatures of the amplification mechanism in non-mammalian hair cells. Here we impose high-amplitude deflections on the hair bundle for 0.1-50 seconds. Due to incompleteness of the adaptation mechanism, this is expected to change the opening probability of the transduction channels, thus mechanically bringing the bundle out of its equilibrium state. Recovery of hair bundle motion was characterized, and the effects of calcium on the feedback mechanism were investigated. Traces of hair bundle motility recorded immediately post stimulation show a slow drift towards the equilibrium position, without oscillatory motion. Bundles recover their innate oscillation between 0.1 - 1 second after cessation of the stimuli, and the quiescent time (Tq) shows a strong dependence on stimulus duration. Relaxation of hair bundles is fitted by the sum of two exponentials. The shorter of the time constants is found to be in the range ∼20-50 ms, consistent with myosin-motor dynamics. The longer time constant is significantly slower, ∼0.5-1 s. Decreased calcium concentration leads to a slower spontaneous oscillation, consistent with prior results in the field. The quiescent interval induced by deflection of hair bundles in a low-calcium environment is shorter, recovering more rapidly post stimulus cessation. Blockage of the extrusion pumps that regulate its concentration inside the stereocilia was likewise seen to prolong the induced quiescence, indicating that calcium accumulation may underlie the suppression of spontaneous motility.