查看Ch12 Physiology of Neurons的源代码

=== 树突状膜具有电压门控离子通道 ===
<b style=color:#0ae>Dendritic membranes have voltage-gated ion channels</b>

All mammalian dendrites have voltage-gated ion channels that influence their signaling properties. Dendritic characteristics vary from cell to cell, and the principles of dendritic signaling are being studied intensively. Most dendrites have a relatively low density of voltage-gated channels (see pp. 182–199) that may amplify, or boost, synaptic signals by adding additional inward current as the signals propagate from distal dendrites toward the soma. We have already introduced the principle of an active cable in curve d of Figure 12-2B. If the membrane has voltage-gated channels that are able to carry more inward current (often Na+ or Ca2+) under depolarized conditions, a sufficiently strong EPSP would drive Vm into the activation range of the voltagegated channels. These voltage-gated channels would open, and their additional inward current would add to that generated initially by the synaptic channels. Thus, the synaptic signal would fall off much less steeply with distance than in passive dendrite. Voltage-gated channels can be distributed all along the dendrite and thus amplify the signal along the entire dendritic length, or they can be clustered at particular sites. In either case, voltage-gated channels can boost the synaptic signal considerably, even if the densities of channels are far too low for the generation of action potentials.

所有哺乳动物树突都具有影响其信号传导特性的电压门控离子通道。树突状特征因细胞而异，并且正在深入研究树突状信号转导的原理。大多数树突具有相对较低的电压门控通道密度（参见第 182-199 页），当信号从远端树突传播到体细胞时，可以通过增加额外的向内电流来放大或增强突触信号。我们已经在图 12-2B 的曲线 d 中介绍了有源电缆的原理。如果膜具有电压门控通道，能够在去极化条件下携带更多的向内电流（通常是 Na + 或 Ca2 + ），则足够强的 EPSP 会将 Vm 驱动到电压门控通道的激活范围内。这些电压门控通道将打开，它们额外的向内电流将添加到突触通道最初产生的电流中。因此，突触信号随距离的下降幅度比被动树突要小得多。电压门控通道可以沿树突分布，从而沿整个树突长度放大信号，或者它们可以聚集在特定位置。在任何一种情况下，电压门控通道都可以显着增强突触信号，即使通道的密度太低而无法产生动作电位。


An even more dramatic solution, used by a few types of dendrites, is to have such a high density of voltage-gated ion channels that they can produce action potentials, just as axons can. One of the best-documented examples is the Purkinje cell, which is the large output neuron of the cerebellum. As Rodolfo Llinás and colleagues have shown, when the dendrites of Purkinje cells are stimulated strongly, they can generate large, relatively broad action potentials that are mediated by voltage-gated Ca2+ channels (Fig. 12-3). Such Ca2+ spikes can sometimes propagate toward—or even into—the soma, but these Ca2+ action potentials do not continue down the axon. Instead, they may trigger fast Na+-dependent action potentials that are generated by voltage-gated Na+ channels in the initial segment. The Na+ spikes carry the signal along the axon in the conventional way. Both types of spikes occur in the soma, where the Na+ spikes are considerably quicker and larger than the dendritic Ca2+ spikes. The faster Na+ spikes propagate only a short distance backward into the dendritic tree because the rapid time course of the Na+ spike is strongly attenuated by the inherent filtering properties of the dendrites (i.e., the λAC is smaller for the rapid frequencies of the Na+ action potentials than for the slower Ca2+ action potentials). The dendrites of certain other neurons of the CNS, including some pyramidal cells of the cerebral cortex, can also generate spikes that are dependent on Ca2+, Na+, or both.

几种类型的树突使用的一种更引人注目的解决方案是具有如此高密度的电压门控离子通道，以至于它们可以产生动作电位，就像轴突一样。记录最充分的例子之一是浦肯野细胞，它是小脑的大输出神经元。正如 Rodolfo Llinás 及其同事所表明的那样，当浦肯野细胞的树突受到强烈刺激时，它们可以产生由电压门控 Ca2+ 通道介导的大的、相对广泛的动作电位（图 12-3）。这种 Ca2+ 尖峰有时可以向体细胞传播，甚至传播到体细胞中，但这些 Ca2+ 动作电位不会继续沿轴突向下移动。相反，它们可能会触发由初始段中的电压门控 Na+ 通道产生的快速 Na+ 依赖性动作电位。Na+ 尖峰以传统方式沿轴突传递信号。这两种类型的尖峰都发生在体细胞中，其中 Na+ 尖峰比树突状 Ca2+ 尖峰更快、更大。较快的 Na + 尖峰仅向后传播一小段距离到树突树中，因为 Na + 尖峰的快速时间过程被树突固有的过滤特性强烈衰减（即，对于 Na + 动作电位的快速频率，λAC 比对于较慢的 Ca2 + 动作电位）。CNS 的某些其他神经元的树突，包括大脑皮层的一些锥体细胞，也可以产生依赖于 Ca2+、Na+ 或两者兼而有之的尖峰。


Dendritic action potentials, when they exist at all, tend to be slower and weaker, and with higher thresholds, than those in axons. The reason is probably that one of the functions of dendrites is to collect and to integrate information from a large number of synapses (often thousands). If each synapse were capable of triggering an action potential, there would be little opportunity for most of the synaptic inputs to have a meaningful influence on a neuron’s output. The cell’s dynamic range would be truncated; that is, a very small number of active synapses would bring the neuron to its maximum firing rate. However, if dendrites are only weakly excitable, the problem of signal attenuation along dendritic cables can be solved while the cell still can generate an output (i.e., the axonal firing rate) that is indicative of the proportion of its synapses that are active.

树突动作电位（如果存在）往往比轴突中的动作电位更慢、更弱，并且阈值更高。原因可能是树突的功能之一是收集和整合来自大量突触（通常是数千个）的信息。如果每个突触都能够触发动作电位，那么大多数突触输入几乎没有机会对神经元的输出产生有意义的影响。单元格的动态范围将被截断;也就是说，极少量的活跃突触会使神经元达到其最大放电速率。然而，如果树突只是弱可激发的，则可以解决沿树突电缆的信号衰减问题，而细胞仍然可以产生指示其活跃突触比例的输出（即轴突放电率）。


Another advantage of voltage-gated channels in dendrites may be the selective boosting of high-frequency synaptic input. Recall that passive dendrites attenuate signals of high frequency more than those of low frequency. However, if dendrites possess the appropriate voltage-gated channels, with fast gating kinetics, they will be better able to communicate high-frequency synaptic input.

树突中电压门控通道的另一个优势可能是高频突触输入的选择性增强。回想一下，无源枝晶比低频信号更能衰减高频信号。然而，如果树突具有适当的电压门控通道，具有快速门控动力学，它们将能够更好地传达高频突触输入。

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