Characteristic variations of relative myelin sheath thickness in 11 nerves of the rat.

Computer-assisted measurements of relative myelin sheath thickness (the g ratio) were made in 11 peripheral nerves of the rat. The scatter diagrams showed nerve-specific variations in the distribution of relative myelin sheath thickness. Myelinated fibers of less than 3.5 microns axon diameter had relatively thin myelin sheaths, particularly in the splanchnic, vagus and glossopharyngeal nerves. The oculomotor nerve had two fiber populations clearly set apart in terms of relative myelin sheath thickness. Thickly myelinated fibers were found in facial and hypoglossal nerves. No single functional modality was evident for the thinly myelinated fibers.