Myelinated axons conduct action potentials, or spikes, in a saltatory manner. Inward current caused by a spike occurring at one node of Ranvier spreads axially to the next node, which regenerates the spike when depolarized enough for voltage-gated sodium channels to activate, and so on. The rate at which this process progresses dictates the velocity at which the spike is conducted and depends on several factors including axial resistivity and axon diameter that directly affect axial current. Here we show through computational simulations in modified double-cable axon models that conduction velocity also depends on extracellular factors whose effects can be explained by their indirect influence on axial current. Specifically, we show that a conventional double-cable model, with its outside layer connected to ground, transmits less axial current than a model whose outside layer is less absorptive. A more resistive barrier exists when an axon is packed tightly between other myelinated fibers, for example. We show that realistically resistive boundary conditions can significantly increase the velocity and energy efficiency of spike propagation, while also protecting against propagation failure. Certain factors like myelin thickness may be less important than typically thought if extracellular conditions are more resistive than normally considered. We also show how realistically resistive boundary conditions affect ephaptic interactions. Overall, these results highlight the unappreciated importance of extracellular conditions for axon function.

有髓轴突以跳跃方式传导动作电位或尖峰。由朗飞的一个节点处发生的尖峰引起的内向电流轴向传播到下一个节点，当去极化足以使电压门控钠通道激活时，下一个节点会重新生成尖峰，等等。该过程进行的速率决定了尖峰传导的速度，并且取决于几个因素，包括直接影响轴向电流的轴向电阻率和轴突直径。在这里，我们通过修改的双电缆轴突模型的计算模拟表明，传导速度还取决于细胞外因素，其影响可以通过它们对轴电流的间接影响来解释。具体来说，我们表明，外层接地的传统双电缆模型比外层吸收性较低的模型传输的轴向电流更少。例如，当轴突紧密地包裹在其他有髓纤维之间时，就会存在阻力更大的屏障。我们证明，现实的电阻边界条件可以显着提高尖峰传播的速度和能量效率，同时还可以防止传播失败。如果细胞外条件比通常认为的更具抵抗力，那么某些因素（例如髓磷脂厚度）可能没有通常想象的那么重要。我们还展示了现实的电阻边界条件如何影响触觉相互作用。总的来说，这些结果凸显了细胞外条件对轴突功能的重要性未被充分认识。