Muscle lacking nNOS is more excitable following denervation. ( A) Action potential traces from innervated and denervated wild-type and nNOS (NOS-1) null fibers. In innervated wild-type and nNOS null fibers, all or none action potentials are present. In the control 3 and 7 day denervated traces shown, no action potential is present and the only response is passive depolarization of the membrane potential in response to current injection. In the 3-day denervated nNOS null fiber shown a nearly normal action potential is present. In the 7-day denervated nNOS null fiber shown an action potential is present, but it is smaller and wider than action potentials from innervated muscle. ( B) Plot of the percent of excitable fibers in innervated and denervated wild-type and nNOS null fibers. In both wild-type and nNOS null innervated muscles 100% of fibers were excitable in all muscles studied ( n = 4 for wild-type, n = 3 for nNOS null). Three days following denervation only 13% of fibers were excitable in wild type muscles ( n = 4). In nNOS null muscles 73% of fibers were excitable 3 days following denervation ( P < 0.05 vs. wild-type, n = 4). Seven days following denervation 0% of wild-type fibers were excitable ( n = 4) while 43% of fibers from nNOS null muscles were excitable ( P < 0.05 vs. wild-type, n = 4).