The internodal axon membrane: electrical excitability and continuous conduction in segmental demyelination.

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Abstract
Longitudinal action currents were recorded from single, undissected myelinated nerve fibers in intact, perfused ventral roots of normal rats and ones treated with diphtheria toxin to produce demyelination. Closely spaced recording electrodes (120 .mu.m), signal averaging and the use of a calibrating current through the root permitted membrane currents to be determined over 240 .mu.m lengths of nerve. Contour plotting was used to plot membrane current density as a function of space and time. The previous result of Rasminsky and Sears (1972) of delayed saltation in demyelinated nerve fibers was confirmed. In addition a new phenomenon of continuous conduction was observed, along distances of up to 1 1/2 times the afferent internodal distance. The continuous spatial distribution of inward current in these cases showed that electrical excitability was distributed along the internodes. Internodal excitability was also revealed in demyelinated fibers by extra foci of inward current judged to be internodal on the basis of the spacing of the other (nodal) foci. Continuous conduction occurred at velocities in the range of 1.1-2.3 m/s or roughly 1/20th-1/40th of the velocities expected for normal stretches of the same fibers. The continuous conduction was attributed to conduction along lengths of demyelinated axon. This was supported by estimates of 0.86 and 1.5 .mu.F(frequency)/cm2 for membrane capacity from the foot of a continuously conducted action potential. The implications of internodal electrical excitability in demyelinated nerve fibers are discussed in relation to recent estimates of the density of Na channels in intact and homogenized normal nerves, and the pathophysiology of demyelinating disease.