Colclasure J Chris, Holt Jeffrey R
Departments of Neuroscience and Otolaryngology, University of Virginia, Charlottesville, VA, USA.
Gravit Space Biol Bull. 2003 Jun;16(2):61-70.
The human vestibular apparatus detects head movements and gravitational stimuli which impinge upon the mechanosensory hair cells of the inner ear. The hair cells, in turn, transduce these stimuli into electrical signals which are transmitted to the brain. These sensory cells are exquisitely responsive, signaling deflections of their mechanosensitive organelles as small as 1-2 nanometers. Remarkably, they are able to preserve this level of sensitivity even when confronted with large tonic stimuli, such as gravity. To accomplish this feat hair cells have devised a novel adaptation process that repositions the mechanotransduction apparatus on a millisecond time scale to allow high sensitivity over a broad operating range. Mechanotransduction in hair cells occurs via a direct gating mechanism in which hair bundle deflection focuses tension onto membrane-bound, cation-selective ion channels located near the tips of the hair bundle. Increased tension favors an open conformation of the channel and allows calcium to enter the cell. Elevated intracellular calcium promotes adaptation which has been hypothesized to result from the activity of a cluster of molecular motors that continually adjust the tension in the transduction apparatus. Although the transduction channel itself remains elusive, myosin Ic has recently been identified as a molecular component of the "adaptation" motor.
人类前庭器官可检测作用于内耳机械感觉毛细胞的头部运动和重力刺激。毛细胞继而将这些刺激转化为电信号并传输至大脑。这些感觉细胞极为敏感，能将其机械敏感细胞器小至1 - 2纳米的偏转转化为信号。值得注意的是，即便面对如重力这样的强持续刺激，它们仍能保持这种敏感程度。为实现这一壮举，毛细胞设计了一种新颖的适应过程，该过程能在毫秒时间尺度上重新定位机械转导装置，从而在广泛的操作范围内实现高灵敏度。毛细胞中的机械转导通过直接门控机制发生，其中毛束偏转将张力集中到位于毛束尖端附近的膜结合阳离子选择性离子通道上。张力增加有利于通道的开放构象并允许钙进入细胞。细胞内钙水平升高会促进适应，据推测这是由一组分子马达的活动导致的，这些分子马达不断调节转导装置中的张力。尽管转导通道本身仍未明确，但肌球蛋白Ic最近已被确定为“适应”马达的一个分子组成部分。
