Conduction properties of single nerve fibers in developing rat spinal nerve roots.

We have examined conduction properties and distribution of membrane currents in ventral spinal root axons of rats 12-47 days of age. Internodal length of the largest myelinated fibers increases at a steady 17-19 micron/day during this period as internodal conduction time decreases. Despite the rapid remodelling of fiber dimensions during the first few weeks of postnatal life, potassium channels are continually excluded from participation in action potential generation at the axon membrane of most nodes of Ranvier.