Time-related changes of excitability of the human motor system contingent upon immobilisation of the ring and little fingers

Objectives: To examine possible changes of excitability of the human motor system contingent upon immobilisation of two hand fingers. Methods: Two series of 5 transcranial magnetic stimulation (TMS) sessions were carried out on different days (1, 2, 3, 4, and 7). In one series (fingers immobilised, FI), subjects wore for 4 days a device that kept immobilised the left fourth and fifth finger. In the other series (fingers free, FF), no constraining device was used. Focal TMS was applied over the right motor cortex and motor evoked potentials (MEPs) were recorded from left abductor digiti minimi (immobilised) and first dorsal interosseus (non-immobilised) muscles. Intensities of 10, 30, and 50% above the resting motor threshold (rMT), were used. Results: In FI series, rMT for both muscles showed significant increase on days 3, 4, and 7 with respect to day 1. At high stimulation intensity a clear decrease of MEPs amplitude was observed on days 3 and 4 for both muscles. Since no time-related changes of peripheral (M-wave) and spinal (F-wave) excitability were noted, MEPs and rMT changes are likely to have a cortical origin. In FF series, no changes of excitability were detected. Conclusions: Sensorimotor restriction of two fingers induces an early decrease of excitability, possibly at cortical level, which involves not only the immobilised muscle but also muscles with purportedly overlapping neural representations.