Relative Conduction Velocities of Small Myelinated and Non-myelinated Fibres in the Central Nervous System

IN peripheral nerve, most axons with diameters of less than 1 µm do not have myelin sheaths, while most fibres more than 1 µm in diameter are myelinated^1,2. In the central nervous system, axons as small as 0.2 µm in diameter may be myelinated^2–5. In his paper on the effects of myelin on conduction velocity, Rushton⁶ concluded that 1 µm is the “critical diameter” above which “myelin increases conduction velocity” and below which “conduction is faster without myelination”. This conclusion is referred to widely (see, for example, refs. 7–9). In this communication we demonstrate that the analysis leading to this conclusion is based on morphological data¹⁰ which do not apply either to central or to peripheral fibres, so that myelinated fibres considerably smaller than 1 µm might be expected to conduct more rapidly than non-myelinated fibres of similar size.