Nodal axon diameter correlates linearly with internodal axon diameter in spinal roots of the cat

Nodal and internodal axon diameters of individual myelinated nerve fibres were measured electron microscopically in fibre samples from serially sectioned L7 ventral and dorsal spinal root of young adult cats. Axon cross-sectional area at the node of Ranvier in axons more than 4 micrometer in diameter was reduced to less than 20% of its internodal value. Internodal and nodal axon diameters showed a rectilinear distribution and linear regression analysis gave coefficients of correlation between 0.93 and 0.99. An individual nodal diameter value could be fitted to an internodal axon diameter value in a 95% prediction interval of +/- 1.06-2.41 micrometer.