López-Aguado L, Ibarz J M, Varona P, Herreras O
Departamento de Investigación, Hospital Ramón y Cajal, Madrid 28034, Spain.
J Neurophysiol. 2002 Nov;88(5):2809-20. doi: 10.1152/jn.00183.2002.
Action potentials (APs) in CA1 pyramidal cells propagate in different directions along the somatodendritic axis depending on the activation mode (synaptic or axonal). We studied how the geometrical inhomogeneities along the apical shaft, soma, and initial axon modulate the transmembrane current (I(m)) flow underlying APs, using model and experimental techniques. The computations obtained at the subcellular level during forward- and backpropagation were extrapolated to macroscopic level (field potentials) and compared with the basic in vivo features of the ortho- and antidromic population spike (PS) that reflects the sum total of all elementary currents from synchronously firing cells. The matching of theoretical and experimental results supports the following conclusions. Because the charge carried by axonal APs is almost entirely drained into dendrites, the soma invasion is preceded by little capacitive currents (I(cap)), the ionic currents (I(ion)) dominating I(m) and the depolarizing phase. The subsequent invasion of the tapering apical shaft is preceded, however, by significant I(cap), while I(ion) decayed gradually. A similar pattern occurred during backpropagation of spikes synaptically initiated in the axon. On the contrary, when the AP was apically initiated, the dendritic I(ion) was boosted by the apical flare, it was preceded by weak I(cap) and spread forwardly at a slower velocity. Soma invasion is reliable once the AP reached the main apical shaft but less so distal to the primary bifurcation, where it may be upheld by concurrent synaptic activity. The decreasing internal resistance of the apical shaft guided most axial current into the soma, causing its fast charging. There, I(ion) began later in the depolarizing phase of the AP and the reduced driving force made it smaller. This, in addition to a poor temporal overlapping of somatodendritic inward currents within individual cells, built a smaller extracellular sink, i.e., a smaller PS. In both experiment and model, the antidromic (axon-initiated) PS in the soma layer is approximately 30% larger than an orthodromic (apical shaft-initiated) PS contributed by the same number of firing cells. We conclude that the dominance of capacitive or ionic current components on I(m) is a distinguishing feature of forward and backward APs that is predictable from the geometric inhomogeneities between conducting subregions. Correspondingly, experimental and model APs have a faster rising slope during ortho than antidromic activation. The moderate flare of the apical shaft makes forward AP conduction quite safe. This alternative trigger zone enables two different processing modes for apical inputs.
CA1锥体细胞中的动作电位（APs）根据激活模式（突触或轴突）沿体树突轴在不同方向上传播。我们使用模型和实验技术研究了沿顶轴、胞体和初始轴突的几何不均匀性如何调节动作电位背后的跨膜电流（I(m)）流动。在正向和反向传播过程中在亚细胞水平获得的计算结果被外推到宏观水平（场电位），并与反映同步放电细胞所有基本电流总和的正向和逆向群体峰电位（PS）的基本体内特征进行比较。理论和实验结果的匹配支持以下结论。由于轴突动作电位携带的电荷几乎完全流入树突，因此在胞体侵入之前几乎没有电容电流（I(cap)），离子电流（I(ion)）主导I(m)和去极化阶段。然而，在逐渐变细的顶轴随后的侵入之前，存在显著的I(cap)，而I(ion)逐渐衰减。在轴突中突触引发的峰电位的反向传播过程中也出现了类似的模式。相反，当动作电位从顶端引发时，树突I(ion)因顶端扩张而增强，其之前是微弱的I(cap)，并以较慢的速度向前传播。一旦动作电位到达主要顶轴，胞体侵入是可靠的，但在初级分支远端则不太可靠，在那里它可能由同时发生的突触活动维持。顶轴内阻的降低将大部分轴向电流引导到胞体中，导致其快速充电。在那里，I(ion)在动作电位的去极化阶段开始得较晚，并且驱动力的降低使其变小。这与单个细胞内体树突内向电流的时间重叠不佳一起，形成了较小的细胞外汇，即较小的PS。在实验和模型中，胞体层中的逆向（轴突引发）PS比由相同数量的放电细胞贡献的正向（顶轴引发）PS大约大30%。我们得出结论，电容性或离子性电流成分在I(m)上的主导地位是正向和反向动作电位的一个显著特征，这可以从传导亚区域之间的几何不均匀性预测。相应地，实验和模型动作电位在正向激活期间的上升斜率比逆向激活时更快。顶轴的适度扩张使正向动作电位传导相当安全。这个替代的触发区为顶端输入提供了两种不同的处理模式。
