Recruitment of extensor-carpi-radialis motor units by transcranial magnetic stimulation and radial-nerve stimulation in human subjects

The responses of 34 extensor-carpi-radialis motor units to graded transcranial magnetic stimulation (TMS) and electrical stimulation of the radial nerve were investigated in six human subjects. Simultaneously with the recording of the single motor-unit discharges, motor-evoked potentials (MEPs) and...

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Bibliographic Details
Published in:Experimental brain research 1999-10, Vol.128 (4), p.557-562
Main Authors: MORITA, H, BAUMGARTEN, J, PETERSEN, N, CHRISTENSEN, L. O. D, NIELSEN, J
Format: Article
Language:English
Subjects:
Adult
Biological and medical sciences
Brain - physiology
Electric Stimulation
Evoked Potentials, Motor - physiology
Fundamental and applied biological sciences. Psychology
H-Reflex - physiology
Humans
Models, Neurological
Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration
Motor Neurons - physiology
Muscle, Skeletal - innervation
Muscle, Skeletal - physiology
Radial Nerve - physiology
Radial Nerve - radiation effects
Transcranial Magnetic Stimulation
Vertebrates: nervous system and sense organs
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Summary:The responses of 34 extensor-carpi-radialis motor units to graded transcranial magnetic stimulation (TMS) and electrical stimulation of the radial nerve were investigated in six human subjects. Simultaneously with the recording of the single motor-unit discharges, motor-evoked potentials (MEPs) and H-reflexes evoked by the two types of stimulation were recorded by surface electrodes and expressed as a percentage of the maximal motor response (Mmax). Ten motor units were activated in the H-reflex when it was less than 5% of Mmax, but not in the MEP even when it was 15% of Mmax. The opposite was observed for three motor units. Eleven motor units were recruited by both stimuli, but with significantly different recruitment thresholds. Only ten motor units had a threshold similar to TMS and radial nerve stimulation. From these observations, we suggest that caution should be taken when making conclusions regarding motor cortical excitability based on changes in the size of MEPs, even when it is ensured that there are no similar changes in background EMG-activity or H-reflexes.
ISSN:0014-4819
1432-1106
DOI:10.1007/s002210050881