Reinnervation of normal and dystrophic skeletal muscle

A comparative study was conducted of resting membrane potential (RMP), extrajunctional acetylcholine (ACh) sensitivity, spontaneous and neurally evoked transmitter release, and directly and indirectly elicited action potentials in posterior latissimus dorsi (PLD) muscles of normal (line 412) and dystrophic (line 413) chickens during reinnervation after nerve crush. Control (nondenervated) dystrophic muscle fibers had a significantly greater RMP (-77.2 vs. -74.1 mV), extrajunctional ACh sensitivity (0.34 vs. 0.04 mV/nC), and incidence of repetitive firing of directly elicited action potentials than did normals. Miniature end-plate potential (MEPP) amplitude in dystrophic fibers was significantly reduced (0.26 vs. 0.38 mV). Six to 7 days after nerve crush, muscle fibers from both lines of chickens showed a significant reduction in RMP and an increase in extrajunctional ACh sensitivity. During this time spontaneous MEPPs were absent and the incidence of repetitive firing was decreased. No significant difference was noted between chicken lines in any of the properties studied. The return of normal properties associated with reinnervation occurred primarily between days 9 and 40. Repolarization of the RMP was clearly evident by day 9 in both lines, but dystrophic fibers showed a slightly earlier and greater degree of repolarization. Similarly, initial decreases in extrajunctional ACh sensitivity and the reappearance of MEPPs were observed on day 9 with dystrophic and day 12 with normal fibers. Neurally elicited action potentials were first recorded on day 11 for dystrophic and day 12 for normal fibers. Finally, multiple firing of directly elicited action potentials associated with reinnervation attained the same incidence (20 to 21% of fibers) on day 12 in dystrophic and day 14 in normal fibers. The results suggest that dystrophic chicken muscle has an enhanced capacity for reinnervation following nerve crush.