Modulation of Afferent Inhibition in Healthy Subjects

Object: To investigate factors that modulate afferent inhibition (AI) in healthy subjects. Methods: AI was evoked by electrical stimuli applied over digit V of the right hand given 15, 25, 35 and 50 ms prior to transcranial magnetic stimulation (TMS) over the left M1. Motor evoked potentials (MEP) of single and double pulse TMS were recorded from the right abductor digiti minimi (ADM) and right abductor pollicis brevis (APB). We first studied the influence of contralateral voluntary preinnervation of the left hand on AI recorded from the right hand. Secondly, we studied effects of electrical stimulation (three times sensory threshold, 12000 stimuli, frequency of 5Hz) over right digit V on paired pulse MEP amplitudes and AI. Results: We found that contralateral preinnervation decreases AI in ADM and APB. After 12000 electrical stimuli AI disappeared in the early interstimulus interval (ISI) of 25 ms whereas at 50 ms ISI AI was reduced. The electrical sensory stimulation had no effect on paired pulse and on single pulse MEP amplitudes. Conclusion: First, AI can be modulated by interhemispherical effects. Secondly, after 40 minutes of high frequency sensory-electrical stimulation we find afferent disinhibition. The absence of changes of paired pulse amplitudes and single pulse MEP amplitudes post-stimulation indicates that the sensory intervention did not modify excitability in the motor system. The change in AI suggests that the effect primarily occurs in the somatosensory cortex. An excitability change in S1 probably reduces the inhibitory effect of AI on M1.