Electron microscopic evidence of a ventronasal to dorsotemporal variation in fiber size in pigeon optic nerve.

Using a uniform sampling method, electron-microscopic examination of pigeon optic nerve revealed a dorsotemporal to ventronasal gradient in mean fiber size. In the dorsotemporal region small, myelinated axons (mean diameter = 0.87 micrometer, S.D. = 0.27) were predominant. Also unique to this region were scattered clusters of fine, unmyelinated fibers. A wider, bimodal spectrum in fiber size (mean diameter = 1.50 micrometer, S.D. = 0.66) was found in the ventronasal region of the nerve. Axons located in the area between these regions possessed a mean diameter of 1.15 micrometer, S.D. = 0.43. This gradient, constant throughout the length of the nerve, resulted from a nasal-to-temporal diminution in size and frequency of large fibers superimposed upon a relatively uniform small fiber base. The total number of fibers within the nerve, excluding those comprising the unmyelinated clusters, was calculated to be approximately 2.3 x 10(6). The overall distribution in fiber size appeared to be unimodal with the peak at 0.75 micrometer and mean diameter = 1.04 micrometer, S.D. = 0.44. Although previous investigators have reported that cross-sectional analysis of pigeon optic nerve revealed a homogeneous distribution in fiber size, the findings of the present study may be more compatable with anatomical and physiological features of pigeon retinotectal organization. These features include visual field acuity gradients, ganglion cell population pattern, and density fields of tectal afferent terminations.