查看Ch12 Physiology of Neurons的源代码

=== 轴突专门用于快速、可靠和高效的电信号传输 ===
<b style=color:#0ae>Axons are specialized for rapid, reliable, and efficient transmission of electrical signals</b>

At first glance, the job of the axon seems mundane compared with the complex computational functions of synapses, dendrites, and somata. After all, the axon has the relatively simple job of carrying the computed signal—a sequence of action potentials—from one place in the brain to another without changing it significantly. Some axons are thin, unmyelinated, and slow; these properties are sufficient to achieve their functions. However, the axon can be exquisitely optimized, with myelin and nodes of Ranvier, for fast and reliable saltatory conduction of action potentials over very long distances (see p. 200). Consider the sensory endings in the skin of your foot, which must send their signals to your lumbar spinal cord 1 m away (see Fig. 10-11B). The axon of such a sensory cell transmits its message in just a few tens of milliseconds! As we see in our discussion of spinal reflexes on pages 392–395, axons of similar length carry signals in the opposite direction, from your spinal cord to the muscles within your feet, and they do it even faster than most of the sensory axons. Axons within the CNS can also be very long; examples include the corticospinal axons that originate in the cerebral cortex and terminate in the lumbar spinal cord. Alternatively, many central axons are quite short, only tens of micrometers in length, and they transmit their messages locally between neurons. The spinal interneuron between a sensory neuron and a motor neuron (see Fig. 10-11B) is an example. Some axons target their signal precisely, from one soma to only a few other cells, whereas others may branch profusely to target thousands of postsynaptic cells.

乍一看，与突触、树突和胞体的复杂计算功能相比，轴突的工作似乎很平凡。毕竟，轴突的工作相对简单，就是将计算出的信号（一系列动作电位）从大脑的一个地方传送到另一个地方，而不会发生显著的变化。一些轴突很薄，无髓且缓慢;这些特性足以实现它们的功能。然而，轴突可以通过髓鞘和 Ranvier 节点进行精细优化，以便在非常长的距离上快速可靠地进行动作电位的盐化传导（见第 200 页）。考虑足部皮肤的感觉末梢，它们必须将其信号发送到 1 m 外的腰椎脊髓（见图 10-11B）。这种感觉细胞的轴突在短短几十毫秒内就传递了它的信息！正如我们在第 392-395 页对脊髓反射的讨论中所看到的，相似长度的轴突将信号从脊髓传递到脚内的肌肉，而且它们比大多数感觉轴突都更快。CNS 内的轴突也可以很长;例子包括起源于大脑皮层并终止于腰脊髓的皮质脊髓轴突。或者，许多中央轴突很短，只有几十微米长，它们在神经元之间局部传递信息。感觉神经元和运动神经元之间的脊髓中间神经元（见图 10-11B）就是一个例子。一些轴突精确地靶向它们的信号，从一个胞体到几个其他细胞，而其他轴突可能会大量分支以靶向数千个突触后细胞。


Different parts of the brain have different signaling needs, and their axons are adapted to the local requirements. Nevertheless, the primary function of all axons is to carry electrical signals, in the form of action potentials, from one place to other places and to do so rapidly, efficiently, and reliably. Without myelinated axons, the large, complex brains necessary to control warm, fast mammalian bodies could not exist. For unmyelinated axons to conduct action potentials sufficiently fast for many purposes, their diameters would have to be so large that the axons alone would take up far too much space and use impossibly large amounts of energy.

大脑的不同部分有不同的信号需求，它们的轴突适应局部需求。然而，所有轴突的主要功能是将电信号以动作电位的形式从一个地方传输到另一个地方，并快速、高效、可靠地完成。没有有髓轴突，控制温暖、快速的哺乳动物身体所需的大型复杂大脑就不可能存在。为了使无髓轴突能够足够快地传导动作电位以达到多种目的，它们的直径必须非常大，以至于仅轴突就会占用太多空间并消耗难以置信的大量能量。

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