前庭毛细胞的反相激活模式

Model of Phase-pposition Activation of Vestibular Hair Cells

分布在内耳前庭各个终器的感觉毛细胞可分为两种类型,它们分别是I型毛细胞和II型毛细胞。这两种毛细胞无论是在静纤毛的长度,还是毛细胞细胞体的形状,或者神经末梢的连接方式以及与之相联系的前庭神经元都不一样。前庭毛细胞感受重力或体位改变刺激的机制是由于其插入到覆盖在其上方耳石膜或终帽内的纤毛随着这些辅助结构的惯性位移而发生弯曲,纤毛的弯曲引起毛细胞膜电位发生去极化或超极化改变,从而促使毛细胞的神经冲动信号发放增强或减弱。静纤毛朝着动纤毛的方向弯曲形成对毛细胞的兴奋性刺激,而静纤毛朝着背离动纤毛的方向弯曲则构成对毛细胞的抑制。球囊斑和椭圆囊斑都被从其中心穿越的一条狭窄的细胞带划分为两个区域,这条狭窄的细胞带被称之为微纹区,在椭圆囊斑微纹区两侧周边区毛细胞的动纤毛都排列在朝向微纹区的位置,而球囊斑微纹区两侧周边区毛细胞的动纤毛都排列在背离微纹区的位置,由此可见,每一个囊斑微纹区两侧毛细胞的极性正好完全相反。当机体沿着一个囊斑的平面从垂直于微纹区的方向产生一个加速运动时,该囊斑一侧周边区的毛细胞会因为静纤毛朝着动纤毛的方向弯曲使该侧毛细胞发生去极化而处于兴奋状态,而另一侧周边区的毛细胞却因静纤毛朝着离开动纤毛的方向弯曲使该侧毛细胞发生超极化而处于抑制状态。三个壶腹嵴上毛细胞的动纤毛都是朝着或背离椭圆囊的统一方向,其中外半规管壶腹嵴上每个毛细胞的动纤毛都是统一朝着椭圆囊,而上半规管和后半规管壶腹嵴上的每个毛细胞的动纤毛都是统一朝着半规管的方向。壶腹嵴是一个位于壶腹腔内的横位的马鞍形隆起,当其上方覆盖的胶状质终帽随着内淋巴液的流动发生相对位置移动时,壶腹嵴一侧的毛细胞纤毛必然会随着终帽顶部的偏移而发生方向一致的弯曲,从而使这半侧壶腹嵴毛细胞产生同步的去极化或超极化反应,然而壶腹嵴另外一侧毛细胞的纤毛却会因为内淋巴液在壶腹嵴上方推动终帽时在被壶腹嵴遮挡一侧的底部形成一个反方向的漩涡式回流使壶腹嵴另外一侧毛细胞的纤毛发生方向相反的弯曲,从而使壶腹嵴两侧毛细胞处于一侧兴奋而另一侧抑制的截然相反的不平衡状态。这意味着同样的加速运动方向对与之相对应的前庭终器两侧感觉上皮产生的是截然不同的刺激信号,从而造成该前庭终器两侧感觉上皮输入信号的不平衡。这种不平衡刺激模式经前庭周边神经元传入到前庭中枢神经系统,使整个从周边到中枢的前庭系统都是以这种不平衡刺激方式传递和感知机体的平衡状态。

The vestibular sense organs contain two different types of sensory hair cells, type I and type II, that differ in the length of the stereocilia, shape of the cell body and their afferent and efferent synapses and nerve endings.The kinocilium and stereocilia of vestibular sensory hair cells are embedded into the otolithic membrane on the macula of saccule and utricle and the hair cells in the semicircular canal are inserted into the cupula overlying the cristae. When the otolithic membrane or cupula are displaced due to gravity or motion-induced inertial fluid flow, bending in the direction of the tallest cilia causes hair cell depolarization while bending toward the shortest cilia causes hair cell hyperpolarization, changes that lead to excitation or inhibition of the afferent vestibular neurons respectively. The hair cells in the macula of the saccule and utricle are divided into two areas by the striola, which curves through the middle of macula.Hair cells in the utricle are oriented with their kinocilium facing the striola, whereas hair cells in the saccule are oriented with their kinocilium facing away from the striola. Therefore, the polarity of the hair cells on opposite sides of the striola of the macula of the utricle and saccule are activated in phase opposition. When the body accelerates along a plane perpendicular to the striola of the macula, the hair cells on one side of striola will be excited(depolarized) whereas those on the other side will be inhibited(hyperpolarized). The polarity of the hair cells in the three ampulla have stereocilia oriented in the same direction. The tallest stereocilia on hair cells in the lateral semicircular canal are oriented toward the utricle, whereas those in the superior and posterior canal are oriented away from utricle. The crista of ampulla is saddle-shaped and the cupula covering the crista extends to the opposite wall of the ampulla. When the overlying cupula is displaced by the flow of endolymph, the stereocilia of the hair cells on one side of the saddle-shaped crista will be deflected toward in the direction of the crest. However, we speculate that the stereocilia on hair cells on the opposite of the crest of the crista will be deflected in the opposite direction due to fluid turbulence and backflow towards the crest of the crista. In this model, the hair cells on opposite sides of the cupula in the semicircular canals also respond in phase opposition, similar to that seen in the macula.