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George E. Carvell, PhD, PT   University of Pittsburgh
 Motor Nerve Conduction Studies record responses from a distal muscle innervated by the nerve to be examined. Stimuli are delivered at several locations along the course of the nerve. Brief (0.1 msec pulses at 1/sec) but strong (supramaximal strength) stimuli are used. Responses are recorded with surface electrodes placed over the muscle belly (active electrode) and distal muscle tendon (reference electrode). A ground electrode is also placed over non-contractile tissue (not shown). The example shown is motor nerve conduction for the Median Nerve. Recording electrodes are placed over the Abductor Pollicis Brevis Muscle. Proximal stimulation is located at the elbow adjacent to the brachial artery. Distal stimulation is located at the wrist lateral to the palmaris longus tendon & proximal to the carpal tunnel. Stimuli are monophasic DC pulses; cathode (-) located distal to the anode (+). Stimuli build from submaximal to supramaximal strength. Recordings are simulated digital oscilloscope traces (note calibration).
Nerve conduction block can result from axonal degeneration, severe demyelination, or a transient physiologic conduction block due to nerve compression (neuropraxia). If all fibers are involved no evoked waveforms will result from nerve stimulation anywhere proximal to the lesion. If partial conduction block occurs, the waveforms have reduced amplitude but normal conduction time (surviving axons conducting at normal velocity). Partial conduction block may be found with a partial (1st, 2nd or 3rd degree) nerve injury or with certain peripheral neuropathies. Waveforms have short durations; waveform
Motor Nerve Conduction Block
areas are small.
GMOMM  2001