Pore-forming subunit of a voltage-gated sodium channel complex through which Na(+) ions pass in accordance with their electrochemical gradient. Alternates between resting, activated and inactivated states (PubMed:12766226, PubMed:29992740, PubMed:30190309, PubMed:15318338, PubMed:16890191, PubMed:18690054, PubMed:17898326, PubMed:19347921, PubMed:25707578, PubMed:26700687). Required for normal muscle fiber excitability, normal muscle contraction and relaxation cycles, and constant muscle strength in the presence of fluctuating K(+) levels (PubMed:12766226, PubMed:15318338, PubMed:16890191, PubMed:19347921, PubMed:25707578, PubMed:26700687, PubMed:26659129). Channel activity is regulated by the ancillary beta subunit SCN1B (PubMed:29992740). SCN1B strongly enhances the presence of the pore-forming alpha subunit at the cell surface (PubMed:29992740, PubMed:30190309). Interaction with SCN1B is required for rapid channel inactivation and rapid recovery after inactivation, and prevents decrease of channel activity in response to repetitive, high-frequency depolarizations (By similarity). The channel is inhibited by tetrodotoxin and saxitoxin (PubMed:30190309). Component of a voltage-sensitive sodium channel complex that consists of an ion-conducting pore-forming alpha subunit and one or more regulatory beta subunits (PubMed:29992740, PubMed:30190309). Interacts with SCN1B (PubMed:29992740, PubMed:30190309). Heterooligomer with SCN2B or SCN4B; disulfide-linked (By similarity). Interacts with the PDZ domain of the syntrophins SNTA1, SNTB1 and SNTB2 (By similarity). Interacts with the conotoxin GVIIJ (PubMed:24497506). The sequence contains 4 internal repeats, each with 5 hydrophobic segments (S1, S2, S3, S5, S6) and one positively charged segment (S4). Segments S4 are probably the voltage-sensors and are characterized by a series of positively charged amino acids at every third position. Phosphorylation at Ser-1328 by PKC in a highly conserved cytoplasmic loop slows inactivation of the sodium channel and reduces peak sodium currents. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. SCN4A mutations are the cause of an autosomal recessive neuromuscular disorder characterized by severe fetal hypokinesia, neonatal hypotonia and congenital myopathy of variable severity. The most severe clinical features include reduced or absent fetal movements, in-utero upper and lower limb contractures, talipes and hydrops, and intrauterine or early postnatal death. Mildly affected patients present with generalized hypotonia and weakness at birth or within the first few days of life, mild-to-moderate facial muscle weakness without ptosis, significant early respiratory and feeding difficulties, and skeletal abnormalities of the spine and palate. Symptoms improve over time in patients who survive infancy, resulting in gain of muscle strength and motor skills and concomitant resolution of early respiratory and feeding difficulties. In contrast to other SCN4A-related channelopathies, affected individuals manifest in-utero or neonatal onset of permanent muscle weakness, rather than later-onset episodic muscle weakness. Belongs to the sodium channel (TC 1.A.1.10) family. Nav1.4/SCN4A subfamily. SCN4A entry