Electron microscopic nerve fiber caliber analysis in the nerve branches to the M. mylohyoideus in mouse

Nerve fiber counts and caliber analysis of the nerve branches to m. mylohyoideus were performed with an electron microscope on 5 sides from 3 mice. The nerve branches contained an average of 93.6 (54.7%) myelinated fibers, 76.2 (44.6%) unmyelinated fibers and 1.7 (0.7%) undetermined fibers. 60.3% of the myelin sheaths of the myelinated fibers in the nerve branches were 3.5-6.0 microns in major diameter and 67.8% were 2.5-5.0 microns in minor diameter. 81.7% of the axons of the myelinated fibers were 1.5-4.5 microns in major diameter and 83.1% were 1.0-3.5 microns in minor diameter. The diameter frequency distributions of the myelin sheaths and axons of myelinated fibers in the nerve branches were similar in pattern to that of the facial trunk distal to the stylomastoid foramen. On the other hand, these frequency distributions of the nerve branches showed that the diameters of the myelinated fibers of the nerve branches were approximately 1-2 microns smaller than those of the r. digastricus, r. stylohyoideus and the nerve branch to venter rostralis of m. digastricus. Average ratios of the cross-sectional areas of myelin sheaths to those of axons of the myelinated fibers in the various axon area classes were 0.59-1.18, and those ratios tended to decrease in the larger axon classes.