Plasma membrane structure at the axon hillock, initial segment and cell body of frog dorsal root ganglion cells

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Abstract
Analysis of the plasmalemma of frog dorsal root ganglion cells by freeze-fracture demonstrates regional differences in the distribution of intramembranous particles. Although P-face particles are distributed rather uniformly, the E-face particle concentration at the cell body (∼300 μm−2) is much lower than that at the axon hillock (∼900 μm−2), proximal initial segment (∼1000 μm−2), or intermediate portion of the initial segment (∼800 μm−2). The particle concentrations in the latter regions approach that at the node of Ranvier and, moreover, particle size analysis reveals that the E-face particles, like those at the node, include a large number that are 10 nm or more in diameter. Thin sections reveal patches of a dense undercoating on the cytoplasmic surface of the axolemma in some regions of the initial segment but not the axon hillock. It is concluded from these results that the axon hillock and the initial segment of dorsal root ganglion cells have some of the structural characteristics of the node of Ranvier.