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Molecular pathology of the skeletal muscle sodium channel.

Although the primary role of skeletal muscle is to do work, muscle contraction can only take place when signals arriving at the neuromuscular junction are successfully coupled to the release of Ca2+ from the sarcoplasmic reticulum. This coupling requires the spread of an action potential across the muscle fiber surface membrane and into the T tubule system. Should this signal fail, muscle paralysis will result in spite of normal neuromuscular junction and contractile protein function. The muscle action potential, like that in nerve, is initiated by the opening of membrane voltage-dependent sodium channels (1, 39). It has recently become evident that abnormalities in muscle membrane-sodium channel func­ tion can produce clinical disease (9, 40, 82); sodium channels that are too easily activated can produce repetitive membrane electrical activity or myo­ tonia, while channels that resist activation can result in periodic weakness or paralysis. The molecular characterization of muscle sodium channels and the genes that encode them has provided the tools necessary to explore the relationship between sodium channels and diseases of human skeletal muscle. The nature of this relationship has now been defined for a number of diseases, and several mutations in the sodium channel gene have been identified. These sodium channel disorders, although rare, are important not only for what they tell us