Axon-myelin relationships in rat cranial nerves III, IV, and VI: a morphometric study of large- and small-fibre classes.

The primary objectives of this study were to determine (1) if quantitative axon-myelin relationships are similar for large- and for small-fibre classes within individual nerves and (2) if the same axon-myelin relationships hold for equivalent fibre classes in closely similar nerves. The oculomotor, trochlear, and abducent nerves of the rat were examined since they each contain distinct large- and small-fibre classes and are similar in a wide range of anatomical and developmental respects. Accordingly, morphometric analyses of axon-myelin relationships were performed separately on large and small fibres of each of the three nerves. Within each nerve, the setting of the relationship between the two parameters was found to be different for the two fibre classes: Scatterplots relating sheath thickness to axon perimeter for large fibres were shifted upwards relative to those for small fibres. These differences were also reflected in the positions of the regression lines fitted to the plots and in the g-ratios. Significant differences were found between nerves in relation to their large fibres: Those of the abducent nerve had significantly thicker sheaths, those of the oculomotor nerve had significantly smaller axon perimeters, and the myelin sheath-axon perimeter relationship of the abducent nerve differed significantly from that of the other two. This study therefore shows that morphometric axon-myelin relationships may differ significantly between equivalent fibre classes of nerves that are closely similar in respect of morphological class, central origin, peripheral distribution, developmental environment, and function.