DISTRIBUTION, SIZE AND NUMBER OF AXONS IN THE OPTIC PATHWAY OF GROUND-SQUIRRELS

The present study has examined the distribution of axons of differing sizes in the optic pathway of the ground squirrel. Axon diameters were measured from electron micrographs at various locations across sectio ns of the optic nerve and tract, and total distributions and numbers w ere estimated. In both the nerve and tract, roughly 1.2 million optic axons were present. The population of optic axons had a unimodal size distribution, peaking at 0.9 mu m in diameter and having an extended t ail toward larger diameters. Local axon diameter distributions in the optic tract indicated distinct (though partially overlapping) axon dia meter classes, including one of fine sizes peaking at 0.8-0.9 mu m, a second of medium sizes peaking around 1.7-1.8 mu m, and a third compos ed of the larger fibers with diameters up to 4.8 mu m. The fine calibe r axons were found at all locations in the tract, and were the only ax ons present immediately adjacent to the pia, while the medium-and coar se-caliber axons were found at deeper locations. Curiously, the larger axons were found primarily in the medial parts of the tract, where ax ons from the dorsal retina normally course. A similarly restricted dis tribution of the larger axons was observed in the dorsotemporal parts of the optic nerve, suggesting that this difference in the tract may r elate to an asymmetric distribution of ganglion cells on the retina gi ving rise to these axons. Measurements of axonal size taken within the optic fiber layer in dorsal and ventral parts of the retina confirmed this asymmetry, consistent with previous demonstrations of soma size differences in the dorsal versus ventral retina. The partial segregati on of axons by size in the optic tract of the ground squirrel then ref lects both the asymmetric distribution of retinal ganglion cell classe s and the chronotopic reordering of optic axons that occurs within the chiasmatic region.