Failure to generate action potentials in newborn diaphragms following nerve stimulation

Action potentials were recorded intracellularly from single diaphragmatic fibers, in vitro, of newborn (3-10 d, n = 18) and older (> or = 21 d, n = 10) rats using flexible microelectrodes. At 20 and 50 Hz phrenic nerve stimulation (1 s duration), action potential transmission failure was significantly higher in the newborn than in the older fibers. During the failure periods, small and highly variable depolarizations were observed which were most likely EPPs. These results show that failure of action potential transmission across the neuromuscular junction is more prevalent in the newborn, and we speculate that this failure is due to inadequate release of neurotransmitter in newborn muscle fibers.