The Significance of Internode Length for Saltatory Conduction: Looking Back at the Age of 90.

The development of peripheral nerve fibers involves interdependence between the timing of Schwann cell recruitment during myelination and elongation of the nerve. This adjusts the number and the length of internodes to the length of the fiber. Saltatory conduction in longer nerves involves longer saltations; this makes internode length the factor that determines conduction velocity, thereby adjusting impulse transmission in circuits of different lengths. Myelination increases conduction velocity by means of saltatory conduction but what determines the saltatory conduction is not so much the indispensable insulating adjunct of myelin as the length of the internodes that separate the excitable membrane segments. We have previously studied the development of the length and proportion of internodes in some detail. If the anatomical data are combined, the data fall in place for a revised understanding of conduction velocity and the system that adapts the conduction properties of peripheral nerves to fiber lengths and to body size.