Walsh & Hoyt: Optic Nerve Axons

Nerve fibers represent roughly 90% of the tissue of the ONH. The topography of the fibers at the ONH is described above. The right-angle turn of the axons at the ONH is associated with an increased concentration of mitochondria in the prelaminar region. This accumulation of mitochondria, which had been thought to result from mechanical stricture of axons as they pass through the lamina cribrosa, may more likely reflect metabolic differences between unmyelinated and myelinated segments of the optic nerve. Unmyelinated nerves require more energy to conduct impulses than myelinated axons, which (mostly) only need to employ energy demanding voltage- gated channels at the nodes of Ranvier. Mitochondrial accumulation at the ONH can be induced by elevated intraocular pressure, which decreases the expression of microtubule- associated protein 1, which disrupts axonal ultrastructure and transport.