Role of potential wave spreading along myelinated nerve fiber in exictation and conduction.

T HE significance of the myelin sheath in electric excitation and in conduction of a nerve impulse has already been elucidated to a great extent by previous investigations (1-3). The results of these investigations are consistent, indicating that the myelin sheath has a distributed capacity and a very high leakage resistance. This capacity and resistance of the myelin sheath are considered to form, together with the axis-cylinder and the surrounding fluid medium, a cable-like electrical network. In a recent paper, one of us (4) has stressed the importance of the ‘potential wave’ spreading along this cable-like system in electrical excitation of the nerve fiber. It has been inferred that, when a stimulating voltage pulse is applied to a nerve fiber at a point some distance away from a node of Ranvier, the change in the potential difference across the myelin sheath spreads along the fiber at a finite rate and that this spread of potential wave is the process which precedes initiation of an impulse at the node. It has also been pointed out that the time required for this potential wave to spread from node to node could be the main factor determining the internodal conduction-time in saltatory conduction. In Lillie’s model of the myelinated nerve fiber (5, 6), the myelin sheath was compared to a thick layer of glass through which no capacitative current flow could take place. In this model, as well as in Frank’s more refined model (7), the ‘impulse’ jumps from a ‘node’ to the next at almost infinite speed, the over-all rate of conduction being determined solely by the rapidity of the process occuring at the nodes. Although the experiment of Huxley and Stampfli (3) demonstrated the existence of some delay within each internode, the large discontinuity in the time of appearance of the action current that they found at each node might be interpreted as supporting saltation zoiih f,espect lo time as opposed to the idea that the capacity of the myelin sheath could be the main factor determining the internodal conduction-time. The evidence brought forward by Tasaki and Takeuchi (I, 2) and others in support of the saltatory theory has dealt mainly with the discontinuity in the physiologically ‘active’ place of the fiber, namely with saltation with respect to space and not directly with saltation with respect to time. The present investigation was undertaken with a view to determining whether or not saltation with respect to time, besides that with respect to space, occurs in actual nervous conduction. In the light of a series of experiments showing the spread of a potential wave along the myelin sheath at a finite rate (4), we first examined whether or not the latent period of an action current (recorded from the site of stimulation) depends upon the distance from a node of Ranvier. By several different methods we could demonstrate a significant dependence of the latent period