Post reinnervation maturation of myelinated nerve fibers in the cat tibial nerve: chronic electrophysiological and morphometric studies.

The extent to which the long-term recovery of nerve fibers differs according to the cause of Wallerian degeneration is not clear, although outgrowth of axons is better after lesions with continuity of basal lamina of the Schwann cell tubes (nerve crush) compared with lesions with interruption of basal lamina (nerve section). Post-reinnervation maturation of myelinated nerve fibers of the cat tibial nerve was followed in chronic electrophysiologic studies after crushing, sectioning, and section+freeze lesions, and compared with morphometric analysis of the same nerves. The amplitudes of the compound nerve action potentials (CNAPs) recovered to a much lesser extent after sectioning than after crushing the nerve. This difference could be related to a smaller number of large fibers, a greater degree of sprouting after sectioning than after crushing, or less synchronization of conduction in regenerated fibers. In comparison, the compound muscle action potentials (CMAPs) recovered to a greater extent than the CNAP after sectioning and section+freeze, though not to the same degree or as fast as after crushing. The difference between the recovery of the CNAP and the CMAP could be due to better regeneration of motor fibers, to differences in the size of motor units or to a better summation of motor unit action potentials. The maximal conduction velocities (CV) in mixed nerve and in motor fibers increased faster after crushing than after sectioning and section+freeze to 60%-70% of control values. The diameters of the largest myelinated fibers increased in all lesions to about 80% of controls. The relation between fiber diameter and CV was influenced by remodeling of myelin during maturation. Hence, long-term functional recovery is influenced by the nature of the nerve lesion, and a smaller proportion of fibers recovered functionally after nerve section than after crush.