Effects of Transcranial Magnetic Stimulation to the Reciprocal IA Inhibitory Interneurones in the Human Wrist

Abstract

In humans, which neural volleys strongly activate the reciprocal Ia inhibitory interneurones have not been clarified via the corticospinal tract or from the muscle spindles. We examined the inhibition from the corticospinal tract and antagonist group Ia fibres to alpha motoneurone pools using the combined method of transcranial magnetic stimulation (TMS) and the standard H-reflex technique. The test stimulus for the forearm H-reflex and the conditioning stimulus to antagonist muscle afferents were applied to the median and radial nerves, respectively. The transcranial magnetic stimulation was applied noninvasively over the left motor cortex. The radial nerve conditioning strongly suppressed the H-reflex rather than the transcranial magnetic stimulation. Transcranial magnetic stimulation-induced inhibition was disinhibited by the conditioning stimulus applied to the median nerve. To estimate the Subliminal inhibition produced by the transcranial magnetic stimulation, we used the following method: the radial nerve conditioning was altered among several different intensities, while transcranial magnetic stimulation intensity was fixed at that for which transcranial magnetic stimulation-induced inhibition was observable. A minor subliminal inhibition was observed. These results suggest that the corticospinal excitatory inputs to reciprocal Ia inhibitory interneurones in the human wrist are very weak relative to those of the originating group I muscle afferents.