Muscle pain differentially modulates short interval intracortical inhibition and intracortical facilitation in primary motor cortex

Excitability of the motor cortex can be suppressed during muscle pain. Yet the mechanisms are largely unknown. Short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were examined as possible candidate mechanisms to underpin this change. SICI and ICF were investigated in 11 healthy individuals before, during and after infusion of hypertonic saline into right first dorsal interosseous (FDI). Using paired-pulse transcranial magnetic stimulation (TMS), interstimulus intervals of 2, 3, and 13 ms were investigated. Pain intensity and quality were recorded using a 10-cm visual analogue scale and the McGill Pain Questionnaire. Resting motor threshold and motor-evoked potentials (MEPs) to single TMS stimuli were recorded before and after pain. Electromyographic recordings were made from right FDI and abductor digiti minimi. Participants reported an average pain intensity of 5.8 (1.6) cm. MEP amplitudes decreased in both muscles. Compared with the pre-pain condition, SICI was increased following pain, but not during. ICF was decreased both during and after pain when compared with the pre-pain condition. These findings suggest that muscle pain differentially modulates SICI and ICF. Although the functional relevance is unknown, we hypothesize decreased facilitation and increased inhibition may contribute to the restriction of movement of a painful body part.