Comparison of intracortical inhibition and facilitation in distal and proximal arm muscles in humans

1. Cortico-cortical inhibition and facilitation induced by paired transcran ial magnetic stimulation (TMS) of the human motor cortex were investigated in the distal muscle opponens pollicis (OP) and the proximal muscle biceps brachii (BB) of normal subjects. 2. The test response evoked by TMS (125% of motor threshold, MTh) in the re laxed OP and BE muscles was inhibited by a conditioning TMS (80% of MTh) at short interstimulus intervals (ISIs; 2-5 ms) and facilitated at longer ISI s (10-25 ms). The test response was significantly less inhibited at short I SIs and more facilitated at long ISIs in the BE than OF. 3. The MTh at rest was significantly lower for the OP than for the BE, indi cating a greater excitability of OP cortical area. However, the above patte rn of inhibition and facilitation was preserved both when the stimulus inte nsity was adjusted to evoke test responses of matched size in the two muscl es and within an ample range of conditioning stimulus intensities. 4. The use of a circular coil or a focal figure-of-eight coil produced no q ualitative differences in the pattern of inhibition and facilitation in eit her muscle. 5. The significant difference in MTh between muscles was lost during volunt ary activation. In both muscles, pre-innervation abolished the cortico-cort ical facilitation and reduced the cortico-cortical inhibition. However, the latter remained larger in the OP than BE muscle. 6. We suggest that the different potency of intracortical inhibitory and fa cilitatory circuits directed towards distal and proximal arm muscles is rel ated to their diverse prevalent functions.