An analysis of axon caliber within the optic nerve of the cat: evidence of size groupings and regional organization.

Axon size in the optic nerves of three adult cats was measured using an accurate planimetric method. A sample of the axon complement was obtained from sets of more than 125 electron micrographs taken at each nerve. In contrast to previous reports, the distribution of axon diameter, as estimated from measurements of axonal cross-sectional area was clearly bimodal. The group of finest axons had a mode at approximately 1 micrometer, and the mode of the group of intermediate size axons was approximately 2 micrometers. Fibers with diameters larger than 3.5 micrometers formed the extensive tail of the distribution. The proportions of large, intermediate, and small axons were estimated to be 5%, 45%, and 50%. These groups probably correspond to the alpha, beta, and gamma classes of retinal ganglion cells. Regional distributions of axon caliber were also bimodal and, in a few instances, even trimodal. A peripheral crescent of each nerve contained a large number of fine fibers. Mean fiber diameter within discrete regions of the optic nerve varied inversely with the density of axon packing. The thickness of the myelin sheath was highly correlated with the inner fiber diameter, hence, myelin sheath width was also distributed in a bimodal manner. These results furnish the first clear evidence that the distribution of fiber caliber within the cat's optic nerve mirrors the principal classes of retinal ganglion cells.