Spatial heterogeneity of the axolemma of non-myelinated fibers in the optic disc of the adult rat. Freeze-fracture observations.

This freeze-fracture study examined the structure of the axolemma of non-myelinated fibers in the optic disc region of the adult rat. Spatially heterogeneous patterns of intramembranous particles (IMPs) were observed on the E-fracture faces; particle distribution patterns ranged from clusters of 4-7 IMPs to linear arrays of particles 1-3 IMPs wide that apparently encircle the axon. These bands of particles displayed a periodicity of approximately 0.16 micrometers. The present findings demonstrate that, in specialized regions, the axolemma of non-myelinated fibers not in the region of synapses can exhibit distinct spatial heterogeneity.