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Premotor Potentials
1047-9651/98 $8.00
ELECTROMYOGRAPHY
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PREMOTOR POTENTIALS Jordan D. Mandel, MD
Premo tor potentials (PMPs) are small negative waveforms that can be observed preceding the compound muscle action potential (CMAP) during standard median and ulnar motor nerve conduction studies if the amplifier sensitivity is high. These potentials, initially deemed intramuscular nerve action potentials, were first observed by Simpson20 and Trojaborg21 in 1964 while they were studying motor nerve conduction. Since then, several additional observations and investigations have eventually clarified the origins of these potentials. INITIAL OBSERVATIONS
Both Simpson and Trojaborg20. 21 noted while measuring motor nerve conduction that a small negative potential might appear immediately before the main deflection of the muscle action potential. Simpson noted that this potential is not recorded from all parts of the muscle and postulated that it was a nerve action potential arising from the terminal motor fibers. He was concerned that this potential might lead to technical error by causing an examiner to measure the wrong response when calculating the latency. Trojaborg believed that the potential either represented a small portion of faster conducting fibers or a potential conducted from a more proximal muscle where the impulse arrives earlier. Simpson wrote, "The nature of this wave requires further investigation."20 A study by Bucthal and Rosenfalck evaluated this initial negative wave recorded before the main muscle action potential.3 They postulated that this waveform either originated from motor fibers or was an antidromically evoked sensory potential. Using concentric electrodes in the thenar and hypothenar eminences, this small initial negative wave was recorded in 12 out of 15 subjects. Bucthal and Rosenfalck felt that this potential most likely came from motor nerve fibers because they obtained it only when the recording electrode was in
From the Department of Physical Medicine and Rehabilitation, Zablocki VA Medical Center; and Capitol Rehabilitation Clinic, Milwaukee, Wisconsin
PHYSICAL MEDICINE AND REHABILITATION CLINICS OF NORTH AMERICA VOLUME 9 • NUMBER 4 • NOVEMBER 1998
765
ELECTROMYOGRAPHY
+
.00
PREMOTOR POTENTIALS Jordan D. Mandel, MD
Premo tor potentials (PMPs) are small negative waveforms that can be observed preceding the compound muscle action potential (CMAP) during standard median and ulnar motor nerve conduction studies if the amplifier sensitivity is high. These potentials, initially deemed intramuscular nerve action potentials, were first observed by Simpson20 and Trojaborg21 in 1964 while they were studying motor nerve conduction. Since then, several additional observations and investigations have eventually clarified the origins of these potentials. INITIAL OBSERVATIONS
Both Simpson and Trojaborg20. 21 noted while measuring motor nerve conduction that a small negative potential might appear immediately before the main deflection of the muscle action potential. Simpson noted that this potential is not recorded from all parts of the muscle and postulated that it was a nerve action potential arising from the terminal motor fibers. He was concerned that this potential might lead to technical error by causing an examiner to measure the wrong response when calculating the latency. Trojaborg believed that the potential either represented a small portion of faster conducting fibers or a potential conducted from a more proximal muscle where the impulse arrives earlier. Simpson wrote, "The nature of this wave requires further investigation."20 A study by Bucthal and Rosenfalck evaluated this initial negative wave recorded before the main muscle action potential.3 They postulated that this waveform either originated from motor fibers or was an antidromically evoked sensory potential. Using concentric electrodes in the thenar and hypothenar eminences, this small initial negative wave was recorded in 12 out of 15 subjects. Bucthal and Rosenfalck felt that this potential most likely came from motor nerve fibers because they obtained it only when the recording electrode was in
From the Department of Physical Medicine and Rehabilitation, Zablocki VA Medical Center; and Capitol Rehabilitation Clinic, Milwaukee, Wisconsin
PHYSICAL MEDICINE AND REHABILITATION CLINICS OF NORTH AMERICA VOLUME 9 • NUMBER 4 • NOVEMBER 1998
765