Ultrastructure of visual callosal axons in the rabbit

Abstract

This study examines the morphology of visual callosal axons in the adult rabbit. Axons in the posterior 3 mm of the splenium of the corpus callosum were examined by electron microscopy. Nonmyelinated fibers comprise approximately 45% of the fiber population. These axons range from 0.08 to 0.6 μm in diameter, and usually occur in clusters of at least three to four axons. Myelinated fibers comprise 55% of the axons in the splenium. The diameters of myelinated fibers range from 0.3 to 1.85 μm. Values of the ratio g (axon diameter: total fiber diameter) range from 0.64 to 0.87. In the majority of myelinated axons, the inner mesaxon and outer tongue of glial cytoplasm are located in the same quadrant. The nonmyelinated gap at the nodes of Ranvier extends less than 2 μm, and an electron-dense undercoating is present subjacent to the axon membrane at the node. Branching of fibers was not observed. A physiological analysis of the conduction properties of these axons is presented in the following paper.