Intracortical inhibitory circuits and sensory input: a study with transcranial magnetic stimulation in humans

We compared intracortical inhibition (ICI) following paired transcranial magnetic stimulation (TMS) (interstimulus interval, ISI: 3 ms) and the inhibition of motor evoked potentials (MEPs) to TMS induced by stimulation of the median nerve (ISI: 200 ms) in six normal subjects. MEPs evoked by focal TMS were recorded in the relaxed opponens pollicis muscle and the size of the conditioned responses was expressed as a percentage of the size of the non-conditioned responses. Maximal ICI, ranging from 4 to 40%, and inhibition after median nerve stimulation, ranging from 11 to 68%, were significantly correlated. The results suggest that both phenomena are possibly mediated by the same gamma aminobutyric acid-ergic (GABAergic) inhibitory circuits and that afferent inputs to the cortex may contribute to their physiological activation.