Diameter distribution spectral of myelinated axons in the median and ulnar nerves of the Soay sheep.

An ulnar and a median nerve were taken from each of three Soay sheep, two aged 13 months and one aged 6 years. The axon diameter distribution spectra in these six nerves were determined by measurement of transverse sections stained by the Weigert-Pal technique. The histograms of myelinated axon diameters, between 1-17 micron, were unimodal in all the ulnar nerves. The mode lay between 4-5 micron in the nerves from the young sheep and between 6-7 micron in the old animal. The histograms of the median nerve axon diameters, between 1-18 micron, were all tri-modal. The nerves from the young sheep had modes between 2-3, 4-5 and 9-11 micron, and in the nerve from the old animal the modes were between 3-4, 6-7 and 12-13 micron. These findings are correlated with the conduction velocities recorded from these nerves in an earlier series of experiments.