Variations in Axonal Diameters of Myelinated Nerve Fibres: Apparent or Real?

Abstract

QUANTITATIVE histological methods are employed in the study of the myelinated fibre population of peripheral nerve and frequently a histogram is constructed to demonstrate the myelinated fibre diameter size frequency distribution. For this purpose fibres are grouped according to calibre, based upon measurements of diameter of the whole fibre, including the myelin sheath. Within any fibre group the relation of the axonal diameter to that of the total fibre is thought to be relatively constant. Thus, for fibres of 10–12μ, the proportion of axonal to external diameter is usually considered to be 0.7 : 1. This ratio was obtained by Donaldson and Hoke¹, and later workers^2,3, using stained transverse sections of fixed nerve, and by de Rényi⁴ using intact, unfixed fibres. The results of both these techniques need correction, the former for distortion inherent in the fixation and subsequent processing, and the latter for optical effects.