Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts 02115, USA.
J Neurosci. 2013 Feb 20;33(8):3706-24. doi: 10.1523/JNEUROSCI.4067-12.2013.
Afferent nerve fibers in the central zones of vestibular epithelia form calyceal endings around type I hair cells and have phasic response properties that emphasize fast head motions. We investigated how stages from hair-cell transduction to calyceal spiking contribute tuning and timing to central (striolar)-zone afferents of the rat saccular epithelium. In an excised preparation, we deflected individual hair bundles with rigid probes driven with steps and sinusoids (0.5-500 Hz) and recorded whole-cell responses from hair cells and calyces at room temperature and body temperature. In immature hair cells and calyces (postnatal days (P)1-P4), tuning sharpened at each stage. Transducer adaptation and membrane-charging time produced bandpass filtering of the receptor potential with best frequencies of 10-30 Hz and phase leads below 10 Hz. For small stimuli, electrical resonances sharply tuned the hair-cell membrane in the frequency range of 5-40 Hz. The synaptic delay of quantal transmission added a phase lag at frequencies above 10 Hz. The influence of spike thresholds at the calyceal spike initiation stage sharpened tuning and advanced response phase. Two additional mechanisms strongly advanced response phase above 10 Hz when present: (1) maturing (P7-P9) type I hair cells acquired low-voltage-activated channels that shortened the rise time of the receptor potential and (2) some calyces had nonquantal transmission with little synaptic delay. By reducing response time, the identified inner-ear mechanisms (transducer adaptation, low-voltage-activated channels, nonquantal transmission, and spike triggering) may compensate for transmission delays in vestibular reflex pathways and help stabilize posture and gaze during rapid head motions.
前庭上皮的中枢区传入神经纤维在 I 型毛细胞周围形成杯状末梢，具有相位反应特性，强调快速头部运动。我们研究了从毛细胞转导到杯状末梢放电的各个阶段如何为大鼠囊状上皮的中枢（条纹区）传入神经纤维提供调谐和定时。在离体标本中，我们用刚性探针驱动阶跃和正弦波（0.5-500 Hz）来偏置单个毛束，并在室温下和体温下从毛细胞和杯状末梢记录全细胞反应。在未成熟的毛细胞和杯状末梢（出生后第 1-4 天）中，每个阶段的调谐都变得更加尖锐。换能器适应和膜充电时间对感受器电位进行带通滤波，最佳频率为 10-30 Hz，相位超前低于 10 Hz。对于小刺激，电共振在 5-40 Hz 的频率范围内使毛细胞膜精确调谐。量子传递的突触延迟在高于 10 Hz 的频率下增加了相位滞后。在杯状末梢放电起始阶段的尖峰阈值的影响使调谐变尖锐并使响应相位提前。当存在两个额外的机制时，在高于 10 Hz 的频率上强烈地提前了响应相位：（1）成熟（P7-P9）的 I 型毛细胞获得了低电压激活通道，缩短了感受器电位的上升时间；（2）一些杯状末梢具有低突触延迟的非量子传递。通过减少响应时间，所识别的内耳机制（换能器适应、低电压激活通道、非量子传递和尖峰触发）可能补偿前庭反射通路中的传输延迟，并有助于在快速头部运动期间稳定姿势和注视。
