Tolhurst D J, Lewis P R
Physiological Laboratory, University of Cambridge, UK.
Ophthalmic Physiol Opt. 1992 Apr;12(2):241-3. doi: 10.1111/j.1475-1313.1992.tb00298.x.
It was proposed by Rushton in 1951, from theoretical considerations, that myelinated fibres less than 1 micron in diameter would conduct more slowly than unmyelinated fibres of the same size and that myelinated fibres below about 0.7 micron would not conduct at all. The experimental data on which he based his theory are all from the peripheral nervous system where small myelinated fibres are rare, and no experimental verification of Rushton's hypothesis has been attempted. In mammalian optic nerve, nearly all the fibres are myelinated; yet half have diameters below 1 micron, with many below 0.7 micron. The many studies of conduction velocities in the visual system enable a test of Rushton's hypothesis to be made. We have examined the correlations between conduction velocity and fibre diameter from a wide range of published studies of the mammalian visual system. The results of our analysis suggest that the small myelinated fibres of the optic nerve and optic tract conduct action potentials more rapidly than is predicted by Rushton's hypothesis, while the unmyelinated axons within the retina actually conduct more slowly than predicted. There is no reason to believe, in this case, that myelination of a small axon will reduce its conduction velocity.
1951年，拉什顿基于理论考量提出，直径小于1微米的有髓纤维传导速度会比同等大小的无髓纤维慢，且直径小于约0.7微米的有髓纤维根本无法传导。他提出该理论所依据的实验数据均来自外周神经系统，而外周神经系统中细小的有髓纤维很少见，且尚未有人尝试对拉什顿的假说进行实验验证。在哺乳动物的视神经中，几乎所有纤维都是有髓的；然而其中一半的直径小于1微米，许多还小于0.7微米。对视觉系统传导速度的众多研究使得对拉什顿假说的检验成为可能。我们从大量已发表的关于哺乳动物视觉系统的研究中，考察了传导速度与纤维直径之间的相关性。我们的分析结果表明，视神经和视束中的细小有髓纤维传导动作电位的速度比拉什顿假说所预测的要快，而视网膜内的无髓轴突实际传导速度比预测的要慢。在这种情况下，没有理由认为小轴突的髓鞘化会降低其传导速度。
