Modifica di Clinic Electromyography (sezione)

====Generation of the Motor Unit Potential (MUP)====
The MUP represents the summed activity of the individual action potentials generated by the muscle fibers belonging to the motor unit. The contribution of each muscle fiber depends on its spatial relationship to the recording electrode. A typical MUP is characterized by a triphasic shape, with an initial positive phase, the main negative peak, and a terminal phase.[[File:Figure 2 (Iani).jpg|thumb|'''Figure 2:''' Schematic representation of the MUP]]The single MUP recorded with a concentric needle represents the activity of 3-15 fibers adjacent to the electrode. The initial phase of the MUP is abrupt and coincides with the initial depolarization of the muscle fiber at the neuromuscular junction. The positive initial phase is generated by the approaching action potential signal, and the duration of the initial positive phase correlates with the distance of the recording electrode from the endplate region.<ref name="Stalberg2">Stalberg E., Bishoff C.: Outliers, a way to detect abnormality in quantitative EMG. Muscle and Nerve 1982; 5: 265-80.</ref> When the electrode is on this region, no positive initial phase forms, and the MUP starts directly in the negative direction from the baseline (isoelectric line). The main negative peak is generated by the summed action potential of the muscle fibers (≅ 15) closest to the electrode.  The terminal positive part of the MUP is analogous to the initial one and is generated by the action potentials moving away from the electrode. The small positive wave inserted in the terminal part is generated at the muscle-tendon junction, while the late negative wave that appears after the MUP’s terminal phase is due to an artifact caused by high-pass filters.  Satellite potentials are generally produced by a single muscle fiber and are temporally correlated with the main peak. These can be an expression of distal axons with slow conduction in the process of reinnervation, or ectopic endplate potentials, or small-diameter muscle fibers.<ref name="Stalberg3">Stalberg E., Bishoff C.: Reference values of motor unit potentials recorded with multi-MUP EMG. Muscle and Nerve 1994; 17: 842-851.</ref> They may also be present in healthy muscles but in numbers fewer than 1 in 20 MUPs.