Comparison of the after-effects of impulse conduction on threshold at nodes of Ranvier along single frog sciatic axons

Abstract

1. Single axons were teased from the distal end of whole frog sciatic nerve and impulses were recorded with a suction electrode. The whole nerve trunk was stimulated using a gross electrode that was slowly moved for several centimetres along the length of the nerve. The threshold for initiation of an action potential showed periodic minima which were interpreted as the location of nodes of Ranvier. 2. Internodal distances were uniform along individual fibres but differed among fibres having matching conduction velocities, suggesting that other individuating characteristics are also important in determining the spacing of nodes. 3. A standard protocol was used to measure the activity dependence of threshold. Nodes along any given fibre were found to be alike in the dependence of threshold on impulse activity. Both the superexcitable phase and the depressed phase of the after-effects of impulse activity were similar for successive nodes. This suggests that the activity dependence of an unbranched length of axon can be well characterized by looking at any one of its nodes. 4. Comparison of nodes from different axons showed large variations in activity dependence. Depressibility, denoting the relative tendency of an axon to show depression, was quantified either as the initial rate of rise in threshold (percentage increase/min) following the onset of repetitive stimulation or as the total rise in threshold (percentage increase) after 5 min of exposure to a standardized rate of repetitive stimulation. By either measure depressibility differed among axons more than it differed among nodes from a single axon. 5. Superexcitability following single impulses was measured in the absence of depression. Axons exhibiting a larger decrease in threshold during the superexcitable phase also tended to show larger depressions relative to other axons when stimulated at a given rate. 6. There was little correlation between conduction velocity and the magnitude of either the depressed phase or the superexcitable phase within the population of fibres studied. This suggests that axon diameter alone (as indicated by conduction velocity) cannot be responsible for the wide variations in the amplitude of the depressed phase or the superexcitable phase. 7. The results suggest that some process exists to constrain the nodes along a fibre to have a uniform activity dependence.