S67 Special phenomena in the motor unit during voluntary and electrical activation

Abstract

Different information is obtained about the motor unit (MU) from various EMG methods. Single Fiber EMG (SFEMG) explores individual muscle fibers and endplates, conventional EMG a larger part of the MU and Macro EMG represents the entire MU. The presentation will first discuss consequences for routine EMG of the temporal and spatial variation in the activation of MUs that may lead to misinterpretation of some EMG features. It will also highlight some physiological parameters of conduction along a muscle fiber based on SFEMG. The muscle fiber velocity shows a short-term facilitation by previous activity, which gives rise to variation in arrival time to the recording electrode, seen both at voluntary and electrical stimulation. During long term activity on the other hand, the velocity decreases, seen and heard as change in frequency spectrum of the EMG signals. A special phenomenon is the decrease in action potential amplitude in some myotonic disorders. A surface recorded amplitude drop at repetitive stimulation seen in these cases not sign of neuromuscular junction disturbance, but reflects amplitude changes of individual fiber action potentials. Amplitude of single fiber action potentials also varies in relationship to just previous activity, with lower amplitude for short intervals, also seen in conventional EMG. Different types of reflexes involving just one or a few neurons can be studied with SFEMG. Different single cell phenomena can be studied in situ in man, and many of these details can be seen in routine EMG, once we are aware of them.