The optic nerve (ON) of small whales (dolphins) is 2 to 3 times the diameter of the primate ON.  Recent LM (Dawson et al., J. Comp. Neurol., 1982) disclosed an unusual fiber spectrum where fiber diameters extended to greater than 15 µm in diameter.  Optic nerve fibers do not exceed 3 µm in primates.  EM was undertaken to examine the contribution of elements smaller than 1 µm and to determine the characteristics of the myelin investment.  EM photographs from central and peripheral portions of the nerve were digitally encoded by drawing the axis cylinder and sheath perimeters on a graphics transform pad.  Dimensions of 822 fibers were stored for off-line analysis.  The fiber spectrum, including the EM data, shows that dolphins have a larger representation of fibers greater than 1 µm where most terrestrial mammals emphasize smaller fiber dimensions.  Myelin sheath thickness was related linearly to axon diameter as in terrestrial species.  Rushton's g (sheath thickness/axon diameter = g) is a theoretical constant which describes the most efficient combination of electrical properties and is uniformly between 0.6 and 0.7 in terrestrial species. g in dolphins is 0.4 (±0.1 SE) for all major categories of the fiber spectrum.  This signifies unusually thick myelin sheaths in this large cranial nerve. Neuronal packing density in this optic nerve is lower than any known mammal.  The unusual aspects of packing density, fiber spectrum and g values in this species may be related to the requirements for fast conduction velocity in the presence of long periods of apnea.

Assisted by NSF grant BNS-7914129.