Maturation of axonal sprouts after nerve crush

We traced the gradual increase in conduction velocity of axons regenerating after nerve crush injury. Electrophysiologic recordings were made from populations of single myelinated lumbar dorsal root fibers in adult rats at various times after the sciatic nerve had been crushed. Conduction velocity was established for the parent fiber as well as for the maturing sprout at several points along its length. There was initially a sharp gradient in conduction velocity along the length of sprouts, but this returned to normal values by about the 80th day after injury. The conduction velocity of a regenerating sprout was always proportional to that of its parent fiber, even at very short survival times before the sprout had become connected to the periphery. The conduction velocity increased at a steadily decreasing rate until about the 150th day by which time it had achieved normal values. Sprouts of the fastest conducting parent fibers increased their conduction velocity at a greater rate than those of slowly conducting parent fibers. As a result, sprouts of all size-classes of myelinated parent fibers reached maturity at about the same time. Regenerating axons played a sensory role as soon as they arrived in the skin of the foot, long before they had reached full maturity.