Foster R E, Connors B W, Waxman S G
Brain Res. 1982 Mar;255(3):371-86. doi: 10.1016/0165-3806(82)90005-0.
Changes in conduction properties and in morphology were studied during rat optic nerve growth from birth (when no myelin is present and the glia have not differentiated) to adulthood (when the optic nerve is essentially 100% myelinated). Myelination begins around the sixth postnatal day and proceeds rapidly so that 85% of the fibers are myelinated at 28 days of age. Mean diameter of optic nerve axons remains about 0.2 micron for the first week and then increases rapidly if the fiber is being myelinated. Those axons not being myelinated remain about 0.2-0.3 micron in diameter. At birth the compound action potential has a single negative peak and a conduction velocity of about 0.2 m/s. The increase in conduction velocity prior to myelination is considerably greater than can be accounted for on the basis of increase in axonal diameter. There is no clear step increase in the velocity of the shortest latency peak correlated with the onset of myelination. During myelination the compound action potential develops multiple short latency components, which evolve into the adult-like 3 component compound action potential by 3-4 weeks of age. Durations of the relative refractory period and supernormal period decrease as age increases, but are not related to myelination in a simple manner. Sodium appears to be the only significant carrier of inward current at all ages. A measureable calcium conductance is not present at any age. Voltage-dependent potassium conductance contributes to the compound action potential at all ages, but the response to 4-aminopyridine in rapidly conducting fibers is apparently smaller than that in slowly conducting fibers. These results show that conduction can occur before myelination or the differentiation of glial cells. Moreover, changes in conduction velocity do not depend entirely on myelination or increases in axonal size. Finally, these results suggest a reorganization of axonal membrane properties during the development of rat optic nerve.
研究了大鼠视神经从出生（此时无髓鞘且神经胶质细胞未分化）到成年（此时视神经基本上100%有髓鞘）生长过程中传导特性和形态的变化。髓鞘形成在出生后第六天左右开始并迅速进行，以至于在28日龄时85%的纤维已被髓鞘化。视神经轴突的平均直径在第一周约为0.2微米，然后如果该纤维正在被髓鞘化则迅速增加。那些未被髓鞘化的轴突直径保持在约0.2 - 0.3微米。出生时复合动作电位有一个单一的负峰，传导速度约为0.2米/秒。髓鞘形成前传导速度的增加远大于仅基于轴突直径增加所能解释的程度。与髓鞘形成开始相关的最短潜伏期峰的速度没有明显的阶跃增加。在髓鞘形成过程中，复合动作电位发展出多个短潜伏期成分，到3 - 4周龄时演变成类似成年的三相复合动作电位。相对不应期和超常期的持续时间随年龄增加而减少，但与髓鞘形成没有简单的关系。在所有年龄段，钠似乎是内向电流的唯一重要载体。在任何年龄都不存在可测量的钙电导。电压依赖性钾电导在所有年龄段都对复合动作电位有贡献，但快速传导纤维对4 - 氨基吡啶的反应明显小于慢速传导纤维。这些结果表明，在髓鞘形成或神经胶质细胞分化之前就可以发生传导。此外，传导速度的变化并不完全取决于髓鞘形成或轴突大小的增加。最后，这些结果提示在大鼠视神经发育过程中轴突膜特性发生了重组。
