MUAP

MUAP stands for motor unit action potential. It is the electrical signal generated by the muscle fibers within a single motor unit when its motor neuron fires. When the motor neuron discharges, action potentials propagate to all innervated fibers, and the summated extracellular activity of those fibers forms the MUAP. In electromyography, MUAPs are typically recorded with a needle electrode placed in the muscle or, less commonly, with surface electrodes using decomposition techniques to isolate individual motor unit potentials.

The appearance of a MUAP depends on motor unit size, fiber distribution, and recording technique. Normal MUAPs are usually of short to moderate duration and small to moderate amplitude with a relatively simple waveform. In neurogenic disorders, motor units may become enlarged through collateral reinnervation, causing MUAPs to have higher amplitude, longer duration, and often polyphasic morphology. In myopathic disorders, there is loss of muscle fibers within motor units, leading to reduced amplitude and shorter duration, with recruitment patterns that may be early or irregular.

MUAP analysis is used to distinguish neurogenic from myopathic processes, help localize lesions, and monitor disease progression or response to therapy. It complements measurements of the compound muscle action potential (CMAP), which reflects the summed response of many motor units. Interpretation depends on the recording technique, the muscle studied, and patient effort, and typically requires experienced clinical electrodiagnostic expertise.