Science Topics – 179
Extensive studies over the years have firmly established the mechanisms of muscle contraction. Motor neurons excite the muscle membrane to generate the end-plate potential (EPP), which then triggers action potentials (APs) and ultimately results in muscle contraction (Fig.1A). Because amplitude of EPP is generally small and attenuated during the conduction, APs that amplify the potential are considered requisite for muscle contraction. In our study, we generated zebrafish lacking the voltage-gated sodium channels (NaV1.4s) essential for AP generation in muscles. Although it was expected that muscles would lose contractility, the swimming capability of the zebrafish was indistinguishable from that of wild-type fish, and the fish muscles contracted despite lacking APs. To reveal mechanisms by which the muscles contract without APs, we simulated the membrane potentials in the zebrafish muscle. The simulation showed that attenuation of the potential was not significant due to the small size of zebrafish muscle, and amplitude of EPP was large enough to induce the muscle contraction. Therefore, EPP can directly stimulate the contraction without APs in the zebrafish. These results raise a possibility that mechanisms of the muscle contraction are not universal among organisms. Studies on other animals will shed light on how the contraction mechanism was acquired during evolution. Given that unnecessary genes generally accumulate mutations to become pseudogenes in evolution, physiological roles of NaV1.4s other than muscle contraction may have preserved the genes intact in zebrafish.
Akiyama C, Sakata S, Ono F. Normal locomotion in zebrafish lacking the sodium channel NaV1.4 suggests that the need for muscle action potentials is not universal. PLoS Biology 23(4): e3003137. https://doi.org/10.1371/journal.pbio.3003137
＜Figure Legends＞
Fig. 1. Mechanism of muscle contraction (A) mechanisms of muscle contraction established. Excitation of motor neurons generates the end-plate potential (EPP), and it evokes action potentials (APs). APs induce muscle contraction. (B) mechanisms of contraction in zebrafish muscle. EPP can directly induce muscle contraction without APs.
Department of Physiology, Division of Life Sciences, Faculty of Medicine, Osaka Medical and Pharmaceutical University, Japan