Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, California, United States of America.
PLoS One. 2012;7(3):e33862. doi: 10.1371/journal.pone.0033862. Epub 2012 Mar 27.
Active hair bundle motility has been proposed to underlie the amplification mechanism in the auditory endorgans of non-mammals and in the vestibular systems of all vertebrates, and to constitute a crucial component of cochlear amplification in mammals. We used semi-intact in vitro preparations of the bullfrog sacculus to study the effects of elastic mechanical loading on both natively coupled and freely oscillating hair bundles. For the latter, we attached glass fibers of different stiffness to the stereocilia and observed the induced changes in the spontaneous bundle movement. When driven with sinusoidal deflections, hair bundles displayed phase-locked response indicative of an Arnold Tongue, with the frequency selectivity highest at low amplitudes and decreasing under stronger stimulation. A striking broadening of the mode-locked response was seen with increasing stiffness of the load, until approximate impedance matching, where the phase-locked response remained flat over the physiological range of frequencies. When the otolithic membrane was left intact atop the preparation, the natural loading of the bundles likewise decreased their frequency selectivity with respect to that observed in freely oscillating bundles. To probe for signatures of the active process under natural loading and coupling conditions, we applied transient mechanical stimuli to the otolithic membrane. Following the pulses, the underlying bundles displayed active movement in the opposite direction, analogous to the twitches observed in individual cells. Tracking features in the otolithic membrane indicated that it moved in phase with the bundles. Hence, synchronous active motility evoked in the system of coupled hair bundles by external input is sufficient to displace large overlying structures.
主动毛束运动被认为是在非哺乳动物的听觉器官和所有脊椎动物的前庭系统中的放大机制的基础，并构成哺乳动物耳蜗放大的关键组成部分。我们使用牛蛙囊的半完整的体外培养物来研究弹性机械加载对天然偶联和自由振荡毛束的影响。对于后者，我们将不同硬度的玻璃纤维附着在纤毛上，并观察自发束运动引起的变化。当用正弦偏折驱动时，毛束显示出锁定相位的响应，表明存在 Arnold Tongue，其频率选择性在低幅度下最高，在更强的刺激下降低。随着负载的刚度增加，会看到模式锁定响应明显变宽，直到近似阻抗匹配，其中锁定相位响应在生理频率范围内保持平坦。当耳石膜留在制备物的顶部时，毛束的自然负载同样降低了它们相对于自由振荡束观察到的频率选择性。为了在自然加载和偶联条件下探测主动过程的特征，我们对耳石膜施加瞬态机械刺激。脉冲后，下面的束显示出相反方向的主动运动，类似于单个细胞中观察到的抽搐。跟踪耳石膜中的特征表明它与束同步运动。因此，由外部输入在偶联毛束系统中引发的同步主动运动足以使大的覆盖结构移位。
