Morphological and electrophysiological characterization of the adult Siberian hamster optic nerve.

Electrophysiological recordings and transmission electron microscopy were used to characterize the compound action potential (CAP) and morphology, respectively, of the optic nerve in the Siberian hamster. The CAP was polyphasic in nature, comprising four separate but overlapping peaks, thereby implying that four sub-populations of axons defined by conduction velocity are present in the nerve. The histological analysis of nerves from four animals revealed a cross-sectional area of 128,171 μm(2) containing 78,109 axons. All of the axons were myelinated, and measurements of axon surface area revealed values ranging from 0.09 to 9.92 μm(2), although 68.3% were <1 μm(2). In the regions of the nerve sampled, the area occupied by axons varied from 10.2 to 80.1%, but in 72.5% of these regions the axons occupied between 50 and 70% of the total cross-sectional area. All regions of the nerve expressed small axons, but larger axons (>2.5 μm(2)) were selectively distributed throughout the nerve. We conclude that the CAP recorded from hamster optic nerve displays four distinct peaks; however, morphological analysis failed to reveal a similar distribution of axon sizes.