Bifurcating Axons Account for the Increase in Axonal Population in Posterior Human Optic Nerve

Abstract

Purpose: To demonstrate the presence of axonal bifurcation in the human optic nerve implicated by morphometric analysis. Methods: Six human optic nerves were visualized directly using light and electron microscopy. Results: Under the light microscope, features suggestive of bifurcating axons were found in about one in four 1-μ m-thick sections examined (or about one bifurcating axon per 0.024 mm3 of tissue). This amounted to about 18,750 bifurcations throughout the course of the human optic nerve. In most cases, the bifurcating axons branched off from the long axis of the main axon at an orthogonal angle before turning in 2–5 microns to continue coursing posteriorly. Ultrastructural analysis confirmed the axonal bifurcations observed with light microscopy. Conclusions: Axonal bifurcation in the human optic nerve is partly responsible for the increased axonal area and contributes to the increased optic nerve diameter as the nerve courses back from retrobulbar region to the pre-chiasmatic area. The present study adds bifurcating axons to the body of evidence supporting parallel processing in man since branching axons allow a single retinal ganglion cell to carry visual information to different nuclei in the central nervous system.