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.