Conduction velocity, size and distribution of optic nerve axons in the turtle, Pseudemys scripta elegans

Electrophysiological and morphological techniques have been used to characterize optic nerve axons in the red-eared turtle. Three distinct groups of axons are identified on the basis of conduction velocity and axon diameter. The first group (T1) is a small population of axons with large diameters (2.8-4.5 microns) and mean conduction velocities of 13 m/sec. The second group (T2) is a large population of axons with medium diameters (0.4-2.8 microns) and mean conduction velocities of 3 m/sec. The third group (T3) is a medium sized population of small diameter (0.2-0.6 micron), mostly unmyelinated axons with mean conduction velocities of 1 m/sec. There is a significant regional variation in the size, density and myelination of axons in the optic nerve. Large axons are found dorsally and ventrally, while smaller axons and the majority of unmyelinated fibers are found along a dorsotemporal to ventronasal axis through the nerve. Fink-Heimer techniques were used to trace the trajectories of axons of different sizes from the retina to the brain. Large diameter axons can be traced along the dorsal and ventral portions of the optic tract, with a dorsal group leaving the tract in the pretectum and a ventral group entering the basal optic tract. These observations suggest that the distribution of axons within the optic nerve reflects in part the distribution of ganglion cell somata in the retina. However, there is also some segregation of axons of different sizes according to their various central targets.