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Myotonic discharges produce a distinctive sound on electromyography (EMG). This sound results from
(a) spontaneous firing of motor unit action potentials (MUAPs).
(b) intermittent blocking of muscle fibers during voluntary activity.
(c) firing of just a few muscle fibers per motor unit during voluntary activity.
(d) spontaneous firing of muscle fibers.
Answer: (d)
Commentary: The classic myotonic discharge of waxing and waning in frequency and amplitude is often described as either a dive-bomber or revving engine. The source generator of a myotonic potential is muscle fiber, and thus may take the form of either positive wave or a brief spike potential. Neuromyotonia and myokymia result in spontaneous firing of motor unit action potentials as opposed to muscle fiber action potentials. Intermittent blocking of some muscle fibers within a motor unit result in unstable MUAPs, which are characterized by changes in either amplitude or number of phases from potential to potential. Unstable MUAPs are seen in primary neuromuscular junction (NMJ) disorders or with new and immature NMJs, as can be seen in early reinnervation. Mypopathic MUAPs result from a decreased number of muscle fibers firing within a motor unit. These signals are seen as short-duration, small amplitude, and polyphasic potentials
2013 SAE
