查看Ch12 Physiology of Neurons的源代码

=== 有髓轴突的传导速度随直径线性增加 ===
<b style=color:#0ae>Conduction velocity of a myelinated axon increases linearly with diameter</b>

The larger the diameter of an axon, the faster its conduction velocity, other things remaining equal. However, conduction velocity is usually much faster in myelinated axons than it is in unmyelinated axons (see p. 200). Thus, a myelinated axon 10 μm in diameter conducts impulses at about the same velocity as an unmyelinated axon ~500 μm in diameter. Myelination confers not only substantial speed advantages but also advantages in efficiency. Almost 2500 of the 10-μm myelinated axons can pack into the volume occupied by one 500-μm axon!

轴突的直径越大，其传导速度越快，其他条件保持不变。然而，有髓轴突的传导速度通常比无髓轴突快得多（见第 200 页）。因此，直径为 10 μm 的有髓轴突以与直径 ~500 μm 的无髓轴突大致相同的速度传导脉冲。髓鞘形成不仅具有显着的速度优势，而且具有效率优势。近 2500 个 10 μm 有髓轴突可以堆积到一个 500 μm 轴突所占据的体积中！


Unmyelinated axons still have a role in vertebrates. At diameters below ~1 μm, unmyelinated axons in the peripheral nervous system (PNS) conduct more rapidly than myelinated ones do. In a testament to evolutionary frugality, the thinnest axons of the peripheral sensory nerves, called C fibers, are ~1 μm wide or less, and all are unmyelinated. Axons larger than ~1 μm in diameter are all myelinated (Table 12-1). Every axon has its biological price: the largest axons obviously take up the most room and are the most expensive to synthesize and to maintain metabolically. The largest, swiftest axons are therefore used sparingly. They are used only to carry sensory information about the most rapidly changing stimuli over the longest distances (e.g., stretch receptors in muscle, mechanoreceptors in tendons and skin), or they are used to control finely coordinated contractions of muscles. The thinnest, slowest C fibers in the periphery are mainly sensory axons related to chronic pain and temperature sensation, for which the speed of the message is not as critical.

无髓轴突在脊椎动物中仍然发挥作用。在直径低于 ~1 μm 时，周围神经系统 （PNS） 中无髓轴突的传导速度比有髓轴突更快。为了证明进化节俭，周围感觉神经最细的轴突（称为 C 纤维）宽 ~1 μm 或更小，并且都是无髓的。直径大于 ~1 μm 的轴突都是有髓的（表 12-1）。每个轴突都有其生物学价格：最大的轴突显然占据了最多的空间，并且合成和维持代谢的成本最高。因此，最大、最快的轴突被谨慎使用。它们仅用于在最长距离上传递有关最快速变化的刺激的感觉信息（例如，肌肉中的拉伸感受器、肌腱和皮肤中的机械感受器），或者用于控制肌肉的精细协调收缩。外围最细、最慢的 C 纤维主要是与慢性疼痛和温度感觉相关的感觉轴突，对于它们来说，信息的速度并不那么重要。


The relationship between form and function for axons in the CNS is less obvious than it is for those in the PNS, in part because it is more difficult to identify each axon’s function. Interestingly, in the brain and spinal cord, the critical diameter for the myelination transition may be smaller than in the periphery. Many central myelinated axons are as thin as 0.2 μm. At the other extreme, very few myelinated central axons are >4 μm in diameter.

CNS 中轴突的形式和功能之间的关系不如 PNS 中的轴突明显，部分原因是更难识别每个轴突的功能。有趣的是，在大脑和脊髓中，髓鞘形成转变的临界直径可能小于外周。许多中央有髓轴突薄至 0.2 μm。在另一个极端，很少有有髓中央轴突的直径为 >4 μm。


The myelinated axon membrane has a variety of ion channels that may contribute to its normal and pathological function. Of primary importance is the voltage-gated Na+ channel, which provides the rapidly activating and inactivating inward current that yields the action potential. Nine isoforms of the α subunit of the voltage-gated Na+ channel exist (see Table 7-1). In normal central axons, it is specifically Nav1.6 channels that populate mature nodes of Ranvier at a density of 1000 to 2000 channels per square micrometer (see Fig. 12-5). The same axonal membrane in the internodal regions, under the myelin, has <25 channels per square micrometer (versus between 2 and 200 channels per square micrometer in unmyelinated axons). The dramatically different distribution of channels between nodal and internodal membrane has important implications for conduction along pathologically demyelinated axons (see below). K+ channels are relatively less important in myelinated axons than they are in most other excitable membranes. Very few of these channels are present in the nodal membrane, and fast K+ currents contribute little to repolarization of the action potential in mature myelinated axons. This diminished role for K+ channels may be a cost-cutting adaptation because the absence of K+ currents decreases the metabolic expense of a single action potential by ~40%. However, some K+ channels are located in the axonal membrane under the myelin, particularly in the paranodal region. The function of these K+ channels is unclear; they may set the resting Vm of the internodes and help stabilize the firing properties of the axon.

有髓轴突膜具有多种离子通道，可能有助于其正常和病理功能。最重要的是电压门控 Na+ 通道，它提供快速激活和失活的向内电流，从而产生动作电位。电压门控 Na + 通道的 α 亚基存在 9 种亚型（见表 7-1）。在正常的中央轴突中，特别是 Nav1.6 通道以每平方微米 1000 至 2000 个通道的密度填充 Ranvier 的成熟节点（见图 12-5）。在髓鞘下的结间区域，相同的轴突膜每 μm 有 <25 个通道（而无髓轴突每平方微米有 2 到 200 个通道）。淋巴结膜和淋巴结间膜之间通道的显着差异分布对沿病理脱髓鞘轴突的传导具有重要意义（见下文）。K+ 通道在有髓轴突中的重要性相对低于在大多数其他可兴奋膜中的重要性。这些通道很少存在于结膜中，快速的 K+ 电流对成熟有髓轴突动作电位的复极化几乎没有贡献。K+ 通道的这种作用减弱可能是一种降低成本的适应，因为 K+ 电流的缺失将单个动作电位的代谢费用降低了 ~40%。然而，一些 K+ 通道位于髓鞘下方的轴突膜中，特别是在结旁区域。这些 K+ 通道的功能尚不清楚;它们可以设置节间的静止 Vm 并帮助稳定轴突的放电特性。

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