DIRECT AND CROSSED MODULATION OF HUMAN MOTOR CORTEX EXCITABILITY FOLLOWING EXERCISE

Rapid repetitive movements of the thumb (1 min duration) produce a rev ersible decrease in the activated primary motor cortex (MI) excitabili ty to transcranial magnetic stimulation (TMS) with recovery within 35- 40 min. In the present study we investigated (1) the role of periphera l sensory feedback in inducing such decrease and (2) possible effects of exercise on the non-activated MI. Stimulation of peripheral Ia affe rent fibres, induced by 1 min vibration of thenar muscles and 2 Hz ele ctrical stimulation of the median nerve at the wrist, have no effect o n motor evoked potential (MEP) amplitude to TMS suggesting no role for sensory feedback to produce MI excitability modulation. Exercise prod uces a significant (P < 0.01) decrease of MEPs for homologous non-exer cised muscles with concomitant contraction of corresponding motor cort ical output maps, suggesting that changes in MI excitability also occu r in the nonactivated hemisphere. This 'crossed' effect might relate t o an interhemispheric transfer of information, via homotopic connectio ns of the corpus callosum.