Effects of 2 × 10⁻⁷ M TTX on the resting membrane potentials of soleus muscle fibers. Muscles were prepared from 12-wk-old WT and mutant mice, and membrane potentials were recorded with standard glass microelectrodes as described in the Materials and methods section. Each column represents the mean of membrane potentials recorded in the fibers of four soleus muscles from mutants and four from WT, with error bars indicating SEM. In soleus from WT, 94 recordings were made before and 80 after the addition of TTX. In soleus from mutants, 101 recordings were made before and 71 after the addition of TTX. The dashed line represents the resting membrane potential of WT controls.