Nerve fiber caliber analysis in the mouse facial trunk distal to the stylomastoid foramen with the electron microscope.

Nerve fiber counts and caliber analysis were performed with the electron microscope on 5 facial trunks distal to the stylomastoid foramen from 3 mice. On an average, about 89% of the total nerve fibers (3,205) were myelinated and about 11% unmyelinated. Majority (80.5%) of the myelin sheaths of the myelinated fibers in the nerves were 2.0-4.5 microns and majority (86.4%) of their axons were 1.0-3.0 micron in minor diameter. Their frequency distributions were similar in pattern to that of the large fiber zone of the motor root of the mouse facial nerve. Majority (86%) of the unmyelinated fibers in the facial trunk distal to the stylomastoid foramen were 0.1-0.5 micron in minor diameter. Its frequency distribution showed that the unmyelinated fibers of the facial trunk tend to be rather smaller than that of the other nerves examined in the previous studies. Ratios of the circumferential length of the myelinated fibers to that of the circles with the same areas ranged from 1.00 to 1.98, with 1.02-1.10 being most frequent (72%). Ratios of the axons of the myelinated fibers ranged from 1.00 to 2.54, with 1.04-1.14 being most frequent (51.8%). Ratios of the unmyelinated fibers ranged from 1.00-1.18 (84%), and those which over 1.20 were rare.