Fig. 7. Amplitude of twitch contractions elicited by electrical stimulation during the 10 periods.  Upper graph , In control experiments, the saline vehicle was added between S3 and S10 (shaded bars).  Lower graph , tetrodotoxin (TTX; 0.1 μM), a selective inhibitor of neuronal fast Na⁺channels, was provided in periods S3-S10. Note the partial inhibitory action of tetrodotoxin. Values in the graphs are given as mean ± SD. *Statistical significance  versus saline control in the corresponding stimulation period (  P < 0.05). Six atria were tested in each group.