EFFECT OF MYELINATION ON THE CONDUCTION-VELOCITY OF OPTIC-NERVE FIBERS

It was proposed by Rushton in 1951, from theoretical considerations, that myelinated fibres less than 1-mu-m in diameter would conduct more slowly than unmyelinated fibres of the same size and that myelinated fibres below about 0.7-mu-m 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-mu-m, with many below 0.7-mu-m. 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.