Skeletal muscle voltage-gated sodium channels (Na_v1.4) are membrane proteins that dictate the action potential threshold and underlie the rapid depolarizing phase, or upstroke, of the action potential in this tissue. Alterations in sodium channel activity not only impact the threshold and the upstroke of the action potential, but can also affect its duration and the initiation of subsequent action potentials. The primary functional unit of the voltage-gated sodium channel is a 220 kDa polypeptide a-subunit and a single 36 kDa beta-subunit. Mutations in the SCN4A gene encoding the Na_V1.4 channel a-subunit cause five skeletal muscle disorders: potassium aggravated myotonia, paramyotonia congenita, hyperkalaemic periodic paralysis, hypo-kalaemic periodic paralysis, and a form of congenital myasthenic syndrome (Simkin & Bendahhou, 2011). Depending on the disease, the triggering factors could be rest following exercise, cold, alcohol, stress, potassium ingestion and carbohydrates.
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