To assess whether the motor system excitability can be modified by experimental tonic pain induced either in muscles or in subcutis.

Transcranial magnetic stimulation of the left primary motor cortex was used to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) muscle. Recordings were made before, during and after experimental pain induced by (1) injection of hypertonic (5%) saline into the right ADM, the right first dorsal interosseum (FDI) and the left ADM muscles, and (2) injection of hypertonic saline in the subcutaneous region of the right ADM. Both MEPs and H-reflex were recorded also from the right flexor carpi radialis (FCR) before, during and after muscle pain.

MEPs recorded from the ADM muscle were significantly reduced in amplitude during pain induced in the right ADM and right FDI muscles, but not during pain in the left ADM muscle or during subcutaneous pain. This inhibitory effect was observed during the peak-pain and persisted also after the disappearance of the pain sensation. In the FCR muscle, the MEP inhibition was observed during the peak-pain, while a significant reduction of the H-reflex's amplitude was observed starting 1 min after the peak-pain.

Tonic muscle pain can inhibit the motor system. The motor cortex inhibition observed at an early phase is followed by a reduction of the excitability of both cortical and spinal motoneurones.