The electrical properties of muscle fiber membranes in dystrophia myotonica and myotonia congenita.

The disorder of skeletal muscle known as myotonia has repeatedly attracted the interest of clinical neurophysiologists. The condition is characterized by a prolonged contraction ofmuscle fibres following voluntary exertion; alternatively it may be induced by direct or indirect electrical stimulation of muscle or by mechanical percussion of the muscle belly. The condition may be an accompanying manifestation of several muscle disorders. In dystrophia myotonica the muscle fibres are dystrophic with evidence of necrosis and degeneration; in myotonia congenita the fibres show little sign of pathology though they are commonly hypertrophied; in paramyotonia the condition is induced by cold. Finally myotonia has been reported as a feature of adynamia episodica (Van der Meulen, Gilbert, and Kane, 1961) and polyneuritis (Worster-Drought and Sargent, 1952). The condition also occurs in an inherited form in goats (Kolb, 1938; Brown and Harvey, 1939) and is a frequent finding in hereditary mouse dystrophy (McIntyre, Bennett, and Brodkey, 1959; Lenman, 1963; McComas and Mossawy, 1965). It was the achievement of Lindsley and Curnen (1936) to resolve previous arguments by showing unequivocally that the prolonged myotonic contractions were accompanied by electrical activity in the affected muscles and they attributed this to continuing motor nerve activity. Evidence of a central nervous system component in myotonia was also found by Denny-Brown and Nevin (1941) and they showed that thiswas responsible for thephenomenon of afterspasm. On the other hand, the most striking electrophysiological feature of myotonic muscle, the rapid burst of action potentials which gradually lessens in frequency (dive bomber potentials), is a property of the muscle fibre membrane itself, since studies in man and in animals have shown that it resists not only motor nerve block but also curarization (Brown