{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nSCN4A encodes the pore‐forming α‐subunit of the skeletal muscle voltage‐gated sodium channel (NaV1.4), which is essential for initiating and propagating action potentials in muscle fibers. Recent structural analyses using cryo–electron microscopy have revealed the detailed architecture of the NaV1.4 channel complex—including the pore region and voltage–sensing domains—and have provided insight into the molecular basis of ion permeation and the allosteric mechanisms that govern fast inactivation. In addition, regulatory factors such as calmodulin binding and membrane sialylation have been shown to modulate channel gating and surface expression, thereby fine–tuning muscle excitability."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "5"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nMutations in SCN4A disturb the precise control of channel activation and inactivation, thereby leading to a wide spectrum of skeletal muscle channelopathies. Gain–of–function mutations can enhance aberrant sodium currents—such as resurgent or window currents—resulting in hyperexcitability that underlies non–dystrophic myotonias, paramyotonia congenita, and periodic paralysis, often with features such as cold sensitivity, episodic weakness, laryngospasm, or even essential tremor. Conversely, loss–of–function mutations may reduce channel availability to an extent that compromises action potential amplitude and manifests as congenital myopathy or myasthenic syndromes with early–onset weakness. Detailed biophysical and pharmacological studies in heterologous systems have demonstrated that these SCN4A mutations alter the balance between activation and inactivation kinetics, shift voltage–dependencies, and in some cases create abnormal gating pore currents. Moreover, modulation of channel function by pharmacological agents including ranolazine, mexiletine, flecainide, and cannabidiol has been explored as potential targeted treatments to restore normal excitability. Collectively, these findings, corroborated by genetic screening and in vivo experimental models, highlight the critical role of SCN4A in maintaining skeletal muscle function and emphasize that the phenotypic variability of SCN4A–related disorders reflects both the nature of specific mutations and their impact on sodium channel biophysics."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "6", "end_ref": "49"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Xiaojing Pan, Zhangqiang Li, Qiang Zhou, et al. 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