[Morphofunctional characteristics of the optic nerve of the turtle Testudo graeca]

Electron microscopic investigation indicates that in T. graeca the optic nerve consists of about 216,000 fibers of which 11% are myelinated and 89%--unmyelinated ones. The diameter of myelinated fibers varies within 0.2-2.5 mu, the main bulk of fibers (42%) having the diameter 0.5-0.7 mu. The diameter of unmyelinated fibers varies between 0.1 and 0.35 mu. With respect to their diameter, 8% of the fibers of both groups overlap each other. During electrical stimulation of the optic nerve, action potential in the latter is recorded which consists of two high amplitude components. The first component (propagation velocity 1.5 m/sec) presumably corresponds to conduction of excitation along myelinated fibers, the second one (propagation velocity 0.6 m/sec)--along unmyelinated fibers.