The arrangement of axons according to fiber diameter in the optic tract of the cat

Abstract

The topographic distribution of axons according to diameter classes in the optic tract of the cat has been studied by light and electron microscopic methods. The subdivision of the tract into a ventral and a dorsal sector, with the former containing the thickest fibers of the tract, has been confirmed. In addition, there is a ventrolateral marginal zone of the tract which contains many of the finest axons and there are gradients of fiber diameters definable for thick and intermediate axons. The gradients show the thinner fibers lying rostral to the thicker fibers. The major points demonstrable by fiber diameter measurements have been confirmed by counts of myelin lamellae. Comparison of fiber distributions in the optic tract with the distributions in the optic nerve on the one hand and with the geniculate terminations on the other shows that, in the region of the optic chiasm, retinofugal axons are sorted not only according to their crossed or uncrossed pathway but also according to their fiber diameter class and thus are presorted in terms of their final destinations.