Fasciculations near the surface of a muscle may be visible at the skin those deep within the muscle and detected by an EMG needle are not. Fasciculations are isolated discharges that recur at irregular intervals, usually in the order of several seconds. They are larger and more complex than fibrillation potentials. Fasciculationįasciculations arise from the discharge of part or the whole of a single motor unit (fig 1C). They occur predominantly in neurogenic disease. The minimal jitter between potentials suggests that they arise by ephaptic transmission between adjacent fibres. Fibrillations are not found exclusively in neurogenic disease, however they also occur in inflammatory and dystrophic muscle disease.Ĭomplex repetitive discharges begin and end abruptly, may persist for several minutes (or until the electromyographer gets bored), have a constant frequency between 1–100 Hz, and consist of a stereotyped group of single fibre potentials. Any nerve lesion, complete or partial, from the spinal motor neurone to the intramuscular nerve branches can give rise to fibrillation. Fibrillation may persist for many months after a nerve lesion. They arise when the needle tip damages a fibre and spontaneous action potentials propagate up to the needle tip and then are extinguished. Positive sharp waves have the same origin as fibrillation and have the same significance. Fibrillation is not visible through the skin and is an electrical sign not a clinical sign. This is detected by the EMG needle as a single fibre discharge or fibrillation (fig 1A). The effect is to make the fibre supersensitive with the result that it discharges spontaneously. Acutely denervated muscle fibres have acetylcholine receptors over the whole of the muscle fibre membrane rather than these being limited to the neuromuscular junction. Muscle fibres themselves remain viable but after a period of 7–10 days become supersensitive and fibrillations will be detectable. SPONTANEOUS ACTIVITY Fibrillation, positive waves, and complex repetitive dischargesĪfter an acute nerve transection, nerve fibres degenerate from the site of the lesion distally.

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