The Ranvier nodes in the neurogenic electric organ of the knifefish Sternarchus: A freeze-etching study on the distribution of membrane-associated particles

The two types of Ranvier nodes (type I with narrow gap, type II with giant gap) and internodes in nerve fibers composing the Sternarchus electric organ have been studied by means of freeze-etching electron microscopy. Numerical analysis of the distribution of membrane-associated particles revealed the following features: (1) the P-faces of both types of nodes and of the internodal axon bear a similarly high density of particles (1000-1200 particles/sq. μm on the average). (2) particle density is differential in E-faces: the histogram for type I nodes has a wider range of particle concentrations (114-1522 particles/sq. μm) than that for type II nodes (45-576 particles/sq. μm) whose density values are in the same range as those of the internodal axon. At least some type I nodes (narrow gaps) generate spikes and probably have a low resistivity; these nodes may be those with high particle density on E-faces. The low particle density on E-faces of type II nodes may be associated with high resistivity and absence of excitability. Similarly, the low particle density in internodes may reflect inexcitability. There is evidence that the transition from one nodal type to the next is gradual: as the gap width of type I nodes increases, there is an occurrence of surface elaborations and the density of E-face particles tends to drop towards the range of type II nodes.