Cortical modulation of spinal excitability: an F-wave study.
F-waves are known to be highly sensitive to changes in the excitatory state of the spinal cord. This paper describes the effects of subthreshold transcranial magnetic stimulation on the F-waves evoked in hand and foot muscles. In the abductor pollicis brevis muscle, the F-wave was significantly enhanced when the cortical stimulus was given with a delay corresponding approximately to the expected time of collision, i.e., the difference between the mean latency of the F-wave and the mean latency of the motor evoked potential. A second, usually larger facilitatory phase follows the first phase after 2-3 msec, and later peaks of enhancement often occurred. After the enhancement periods, a significant inhibition of the F-wave was usually observed. In the extensor digitorum brevis muscle, the first facilitatory phase was observed some milliseconds earlier than expected in 4 of 5 subjects, and the inhibitory phase was less pronounced. We argue that the sequential arrival of I-waves at the spinal segment could be responsible for the changes observed in the F-wave recorded from the small hand muscle. In the foot muscle, anatomical and technical factors could contribute to the generation of a D-wave. The strong inhibition observed in the F-wave recorded from the hand muscle is likely to be due to the arrival on alpha-motoneurons of inhibitory postsynaptic potentials (IPSPs) generated by the cortical stimulus. Our data show that the F-wave ia a probe for changes in the spinal cord excitatory state.
Mercuri
B
B
Medical Neurology Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892-1428, USA.
Wassermann
E M
EM
Manganotti
P
P
Ikoma
K
K
Samii
A
A
Hallett
M
M
D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D006801:Humans; D008280:Magnetics; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*