Kotler Oron, Miyazaki Kenichi, Khrapunsky Yana, Ross William N, Fleidervish Ilya A
Department of Physiology and Cell Biology, Faculty of Health Sciences and Zelman Center for Brain Science Research, Ben-Gurion University of the Negev, Beer Sheva, Israel.
Department of Physiology, New York Medical College, Valhalla, NY, United States.
Front Cell Neurosci. 2025 Sep 3;19:1662730. doi: 10.3389/fncel.2025.1662730. eCollection 2025.
Functional neuronal connectivity relies on long-range propagation of action potentials by myelinated axons. This process critically depends on the distribution and biophysical properties of ion channels clustered at specialized, regularly spaced domains, the nodes of Ranvier, where the signals are actively regenerated. Morphological and functional evidence indicates that voltage-gated Na channels, which directly support action potential conduction, are exclusively localized at nodes. While these domains also contain voltage-gated Ca channels that contribute to key intracellular signaling cascades, evidence regarding the presence of functional Ca channels in the internodal regions remains conflicting. Using high-speed fluorescence imaging, we characterized action potential-evoked Na and Ca dynamics at the nodes of Ranvier in myelinated axons of layer 5 pyramidal neurons in cortical brain slices. Spatially, both Na and Ca elevations were largely restricted to the nodal regions. The time-to-peak of the nodal Na transients was significantly shorter (3.7 ± 0.3 ms) than that of the Ca transients (10.3 ± 0.6 ms with OGB-1, 4.2 ± 0.5 ms with OGB-5 N), consistent with electrophysiological evidence indicating that Na influx occurs primarily during the action potential upstroke, whereas Ca influx predominantly takes place during and after the repolarization phase. The decay of Na transients, reflecting lateral diffusion into the internodes, was exceptionally fast in short nodes and became progressively slower in longer ones, consistent with computational models assuming diffusion-based clearance alone. In contrast, Ca transients decayed more slowly and showed no dependence on nodal length, consistent with clearance dominated by active transport. Finally, the post-spike recovery of nodal Na fluxes was rapid and temperature-dependent, consistent with the reactivation kinetics of voltage-gated Na channels. In contrast, the similarly rapid but temperature-independent recovery of Ca flux suggests that a single action potential does not induce Ca channel inactivation and therefore has minimal impact on their availability during subsequent spikes.
功能性神经元连接依赖于有髓轴突对动作电位的长距离传播。这一过程关键取决于聚集在特殊的、规则间隔区域（郎飞结）的离子通道的分布和生物物理特性，信号在郎飞结处被主动再生。形态学和功能学证据表明，直接支持动作电位传导的电压门控钠通道仅定位于郎飞结。虽然这些区域也含有对关键细胞内信号级联反应有贡献的电压门控钙通道，但关于节间区域功能性钙通道存在的证据仍存在矛盾。我们使用高速荧光成像技术，对皮层脑片第5层锥体神经元有髓轴突郎飞结处动作电位诱发的钠和钙动力学进行了表征。在空间上，钠和钙的升高主要局限于郎飞结区域。郎飞结钠瞬变的峰值时间（3.7±0.3毫秒）明显短于钙瞬变的峰值时间（用OGB-1时为10.3±0.6毫秒，用OGB-5N时为4.2±0.5毫秒），这与电生理证据一致，表明钠内流主要发生在动作电位的上升支，而钙内流主要发生在复极化阶段及之后。反映侧向扩散到节间的钠瞬变衰减在短节段中异常迅速，在长节段中逐渐变慢，这与仅假设基于扩散清除的计算模型一致。相比之下，钙瞬变衰减较慢，且与节段长度无关，这与以主动转运为主的清除方式一致。最后，郎飞结钠通量的峰后恢复迅速且依赖温度，这与电压门控钠通道的再激活动力学一致。相比之下，钙通量同样迅速但不依赖温度的恢复表明，单个动作电位不会诱导钙通道失活，因此对其在后续动作电位期间的可用性影响最小。
