查看Ch12 Physiology of Neurons的源代码

=== 动作电位通常在初始节段启动 ===
<b style=color:#0ae>Action potentials are usually initiated at the initial segment</b>

The soma, axon hillock, and initial segment of the axon together serve as a kind of focal point in most neurons. The many graded synaptic potentials carried by numerous dendrites converge at the soma and generate one electrical signal. During the 1950s, Sir John Eccles N12-2 and colleagues used glass microelectrodes to probe the details of this process in spinal motor neurons. Because it appeared that the threshold for action potentials in the initial segment was only ~10 mV above the resting potential, whereas the threshold in the soma was closer to 30 mV above resting potential, they concluded that the neuron’s action potentials would be first triggered at the initial segment. Direct recordings from various parts of neurons now indicate that spikes begin at the initial segment, at least for many types of vertebrate neurons. EPSPs evoked in the dendrites propagate down to and through the soma and trigger an action potential in a myelin-free zone of axon ~15 to 50 μm from the soma (Fig. 12-5A). The action potential then propagates in two directions: forward—orthrodromic conduction—into the axon, with no loss of amplitude, and backward—antidromic conduction—into the soma and dendrites, with strong attenuation, as we saw above in Figure 12-3. Orthodromic propagation carries the signal to the next set of neurons. The function of antidromic propagation is not completely understood. It is very likely that backwardly propagating spikes trigger biochemical changes in the neuron’s dendrites and synapses and they may have a role in plasticity of synapses and intrinsic membrane properties.

体细胞、轴突小丘和轴突的初始段共同作为大多数神经元的一种焦点。由众多树突携带的许多分级突触电位在胞体汇聚并产生一个电信号。在 1950 年代，John Eccles N12-2 爵士及其同事使用玻璃微电极来探测脊髓运动神经元中这一过程的细节。因为看起来初始段中的动作电位阈值仅比静息电位高 ~10 mV，而胞体中的阈值接近高于静息电位 30 mV，所以他们得出结论，神经元的动作电位将首先在初始段触发。来自神经元各个部分的直接记录现在表明，尖峰始于初始节段，至少对于许多类型的脊椎动物神经元来说是这样。树突中诱发的 EPSP 向下传播并通过胞体传播，并在距离胞体 ~15 至 50 μm 的轴突无髓鞘区触发动作电位（图 12-5A）。然后动作电位沿两个方向传播：向前 - 正常传导 - 进入轴突，没有幅度损失，以及向后 - 逆向传导 - 进入胞体和树突，具有很强的衰减，如上图 12-3 所示。顺向传播将信号带到下一组神经元。逆向传播的功能尚不完全清楚。向后传播的尖峰很可能会触发神经元树突和突触的生化变化，它们可能在突触的可塑性和内在膜特性中发挥作用。


The axon achieves a uniquely low threshold in its initial segment (see Fig. 12-5B) by two main mechanisms. First, the initial segment has a remarkably high density of voltage-gated Na+ channels (see Fig. 12-5C). The Na+ channel density in the axon initial segment is estimated to be 3-fold to 40-fold higher than in the membrane of the soma and dendrites, depending on neuron type and experimental methodology. The scaffolding protein ankyrin-G is a molecular marker for the initial segment and seems to be critical for organizing its unique distribution of ion channels. Second, initial segments often include Na+ channel types (see Table 7-1) such as Nav1.6 that activate at relatively negative voltages compared to channels such as Nav1.2 that are common in the soma and dendrites. The combination of large numbers of Na+ channels and their opening at relatively negative voltage allows the initial segment to reach action potential threshold before other sites on the dendrites and soma.

轴突通过其两个主要机制在其初始段（见图 12-5B）中达到独特的低阈值。首先，初始段具有非常高密度的电压门控 Na+ 通道（见图 12-5C）。轴突初始段中的 Na + 通道密度估计比胞体和树突膜中的 Na + 通道密度高 3 到 40 倍，具体取决于神经元类型和实验方法。支架蛋白锚蛋白-G 是初始片段的分子标记物，似乎对于组织其独特的离子通道分布至关重要。其次，初始段通常包括 Na + 通道类型（见表 7-1），例如 Nav1.6，与胞体和树突中常见的通道（如 Nav1.2）相比，它们在相对负的电压下激活。大量 Na + 通道的组合及其在相对负电压下的打开允许初始段在树突和体细胞上的其他位点之前达到动作电位阈值。

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