Muscular dystrophy contrasted with denervation: different mechanisms underlying spontaneous fibrillations.

The data reviewed in this paper indicate that spontaneous fibrillations do not involve a cholinergic mechanism since non-depolarizing anticholinesterase drugs such as Mestinon fail to increase spontaneous fibrillations in denervated muscle. Fibrillation potentials are related to the changes in electrical properties of the membrane of denervated muscle fibres which lead to the appearance of spontaneous subthreshold depolarizations, sometimes triggering a propagated potential. Fibrillations seem to appear in cycles and this may depend on the depression of spontaneous depolarization by muscle activity itself. Fibrillations are also an important feature of Duchenne muscular dystrophy and polymyositis, but they have not been found in Landouzy-Dejerine muscular dystrophy. These "myopathic" fibrillations probably arise from subthreshold depolarizations in the membrane of muscle fibre segments which have been functionally or anatomically isolated from the end-plate by a pathological lesion (Fig. 4). Experimental demonstration of spontaneous fibrillations in baboon biceps muscles after extrajunctional myotomies indicates that such an isolated muscle fibre segment can indeed develop and sustain spontaneous fibrillation activities. Studies of motor unit potentials in myopathies by "coherent" electromyography disclose linked potentials after the main potential in Duchenne dystrophy, but not in Landouzy-Dejerine muscular dystrophy. The linked potentials are signs of collateral innervation by sprouts of the motor axons. The fact that linked potentials occur in Duchenne dystrophy, including in obviously dystrophic motor units (Fig. 5), shows that such motor axons are quite healthy and able to sprout efficiently. The muscle fibres thus innervated collaterally are probably the ones which fibrillated and were deprived of trophic motor control as a result of myopathic lesions of the type considered in Fig.4. This correlation receives support from the finding that both spontaneous fibrillations and linked potentials are lacking in Landouzy-Dejerine muscular dystrophy, which obviously presents a different type of muscle lesion.