The optic nerve has several unique features. Unmyelinated ganglion cell axons travel from the retina through the highly organised, porous lamina cribosa becoming myelinated beyond for the rest of their journey. Previous researchers have found high numbers of mitochondria in these axons at the level of the lamina and have equated this with a physiological constriction of the nerve or some form of axoplasmic stasis. We have studied oxidative phosphorylation enzyme activity across the optic nerve head.
Results show a striking inverse relationship between myelination and cox activity.
In the human optic nerve, cox activity is high in pre lamina and lamina unmyelinated portions falling dramatically once the axons become myelinated. Rabbit optic nerve is myelinated throughout its course ending in the nerve fibre layer of the retina. A contrast in cox activity is seen at this point also. In the light of these findings we caution against the previous inference of a dynamic process of axoplasmic stasis at the lamina on the basis of observing increased numbers of mitochondria in the pre lamina human optic nerve.