**4.4. Origin of regular and irregular fibrillation potentials**

Based on the pattern of discharges, two classes of spontaneously active fibres were found in experimentalstudyofratdiaphragm:rhythmicallydischargingfibres,andfibresinwhichaction potentialsoccuratirregularintervals(Purves&Sakmann1974).Themajorityofthesitesoforigin in both regular and irregular fibres were at the former end plate zone; however, there was no region along the length that could not be a site of origin. Regularly occurring action potentials wereassociatedwithoscillationsofthemembranepotential.Irregularlydischargingfibreswere brought to threshold by discrete non-propagated depolarizations called fibrillatory origin potentials (f.o.p.s.) (Purves & Sakmann 1974). F.o.p.s. are generated at the T-tubuli, since detubulation with glycerol abolishes the spontaneous activity (Smith & Thesleff 1976). Thus, the integrityofthe transverse tubular systemisaprerequisite forthepresenceofirregular spontane‐ ous activity. It was also observed, that these discrete depolarizations are caused by regenera‐ tive increase in the Na conductance of the membrane, similar to that associated with the normal action potential (Purves & Sakmann 1974, Smith & Thesleff 1976).

We may presume that in humans, fibrillations with regular rhythm also derive from the membrane potential oscillations of denervated muscle fibres (Thesleff 1982a) or the denervated part of a muscle fibre, as in muscular injury (Partanen & Danner 1982). Irregular fibrillations are accordingly caused by f.o.p.s reaching the firing threshold of an action potential. Imme‐ diately after a f.o.p. there is a period during which the probability of a second f.o.p. occurring is very low (Purves & Sakmann 1974). In denervated muscle fibres there are newly synthesized potassium channels, and they produce a longer duration of the hyperpolarization of the intracellular action potential compared to normal tissue. This hyperpolarization may last up to 100 ms and more (Thesleff 1982a, Dumitru 2000). Thus the refractory period after which a second action potential may occur is increased in denervated muscle fibres, compared to normal muscle fibres. Thus slightly irregular fibrillations with pauses may be fired by a muscle fibre eliciting a large number of f.o.p.s., which mainly reactivate the fibre immediately after the refractory period of a spontaneous potential. An occasional failure of a f.o.p. to occur may be seen as a pause in the fibrillation sequence. We have rarely observed a slightly irregular fibrillation sequence even without pauses, evidently representing a muscle fibre with a large number of f.o.p.s. On the other hand, random fibrillations may be associated with very infrequently occurring f.o.p.s. In any case, regular fibrillations are the first to be present also in experimental studies and irregular fibrillations arise later on (Purves & Sakmann 1974, Smith & Thesleff 1976).
