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George E. Carvell, PhD, PT   University of Pittsburgh
 
PERIPHERAL NERVE CONDUCTION: Motor Nerve Conduction Slowing
 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).
Slowing of nerve conduction may result from an acute ischemia, demyelination, or as a result of regeneration (conduction in immature axons with reduced diameter and immature myelination). Conduction time is longer than normal and the compound action potential waveforms tend to be temporally dispersed (less uniformity in conduction of  involved axons). Peripheral neuropathies often show slow nerve conduction in distal segments of upper and lower extremity peripheral nerves. Both sensory and motor axons are commonly involved. If the neuropathy is restricted to small fibers, NCV
Slow Motor Nerve Conduction
should be normal.
GMOMM  2001