Effect of activity on the selective stabilization of the motor innervation of fast muscle posterior latissimus dorsi from chick embryo.

The role of neuromuscular activity in the maturation of the motor innervation was investigated in the fast focally innervated posterior latissimus dorsi (PLD) muscle of the chick embryo. The axonal supply in the PLD motor nerve, and the focal multiple innervation of the endplates were described on days 15 and 16 of embryonic life in normal and experimental embryos. In the first series of experiments, chick embryos were paralyzed by repeated injections between days 4 and 10 in ovo of the curare-like agent, flaxedil. Twice more axons in the PLD motor nerve and about twice more nerve terminal profiles at the endplates in the PLD muscles were found in paralyzed than in control embryos. In a second series of experiments, electrodes were implanted around the spinal cord of 7-day-old embryos and electric pulses delivered at 0.5 Hz frequency from day 10 to days 15-16 of incubation. At day 15.5, no change was observed in the axonal supply in the PLD motor nerve of stimulated embryos, while a two-fold decrease was observed in the number of motor nerve terminal profiles per endplate in the corresponding PLD muscle. The statistical distribution of the number of motor nerve terminal profiles per endplate was described from complete semi-serial sections in the PLD muscle from normal, paralyzed and stimulated chick embryos. In these three cases, the distribution of supernumerary nerve terminal profiles followed a Poisson law after one nerve ending had been subtracted from the number of nerve endings counted per endplate.