Different patterns of excitation and inhibition of the small hand and forearm muscles from magnetic brain stimulation in humans.
OBJECTIVE
The objective of the present study is to quantify the effects of voluntary muscle contraction of the small hand (abductor pollicis brevis, first dorsal interosseus (FDI)) and forearm muscles (extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU)) on motor evoked potentials (MEPs).


METHODS
MEPs were recorded in 12 healthy subjects by a circular coil placed over the vertex at 1.2 times the resting motor threshold at different levels of the muscle contraction (0-100% of maximum voluntary contraction (MVC)). The effects of transcranial magnetic stimulation (TMS) on the onset latency, MEP area and silent period (SP) as a function of the %MVC were evaluated using a piecewise linear regression analysis.


RESULTS
The MEP areas for the small hand muscles were almost completely saturated at 20% of MVC. In contrast, the MEP areas for radial muscles (ECR, FCR) had a dual increase at 40% of MVC while the ulnar muscles (ECU, FCU) had a dual increase at 20% of MVC. A uniform latency shift (1.5-3 ms reduction) was observed in all muscles with a changing point at 10% of MVC. The SPs were the longest for FDI and were not significantly influenced by MVC for any muscles.


CONCLUSIONS
The excitatory and inhibitory effects of TMS on the MEPs differed for the small hand and forearm muscles and also between the ulnar and radial muscles. These results probably resulted from the different degrees of direct corticomotoneuronal inputs to each muscle and the inherent properties of the spinal motoneurons.
Department of Clinical Neurophysiology, Faculty of Medicine, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-Ku, Fukuoka, Japan.
Lin
L
Wu
Yoshinobu
Y
Goto
Takayuki
T
Taniwaki
Naoko
N
Kinukawa
Shozo
S
Tobimatsu
D000328:Adult; D001921:Brain / Q000502:physiology*; D004558:Electric Stimulation; D004574:Electromagnetic Fields; D019054:Evoked Potentials, Motor; D005260:Female; D005542:Forearm; D006225:Hand; D006801:Humans; D016014:Linear Models; D008280:Magnetics; D008297:Male; D009044:Motor Cortex / Q000502:physiology; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology