**1. Introduction**

Rhythmic fibrillation potentials are the hallmark of denervated muscle fibres in needle EMG of a striated muscle (Conrad et al. 1972, Heckmann & Ludin 1982). They are readily activated by the insertion of an EMG needle electrode (Kugelberg & Petersén 1949). Irregular fibrillation potentials may also be present (Buchthal & Rosenfalck 1966, Purves & Sakmann 1974). There is, however, an obvious difficulty to discriminate between true irregular fibrillation potentials of a denervated muscle and end plate spikes, which occur in a normal muscle. This may lead to the conclusion that only rhythmic fibrillation potentials matter (Stöhr 1977). We have pointed out that fibrillation potentials, whether regular or irregular, have longer minimum interpotential intervals than end plate spikes (Partanen & Danner 1982). During long sequences, the mean interval between successive end plate spikes and rhythmic fibrillation potential tends to increase, whereas irregular fibrillations do not show this type of "self-inhibition" (Partanen & Danner 1982, Partanen & Nousiainen 1983). The aim of this chapter is to describe fibrillation potentials of different categories in either completely or partially denervated human limb muscles, or after a muscle injury. We also compare the characteristics of fibrillation potentials to neurally driven sequences, such as "myokymic" fibrillation potentials and end plate spikes (Brown & Varkey 1981, Partanen & Nousiainen 1983, Partanen 1999). "Myokymic" fibrillation potentials are a rare phenomenon of innervated muscle fibres. They have not been described earlier and are readily confused with end plate spikes. The term "myokymic" fibrillations is descriptive. The pathophysiology of "myokymic fibrillation" is different from true myokymia of whole motor units (see Willison 1982, Stålberg & Trontelj 1982).

© 2013 Partanen; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Partanen; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
