Modification of Motor Evoked Potentials Caused by Electrical Peripheral Nerve Stimulation in Transcranial Magnetic Stimulation

Somatosensory cortex receives afferent inputs from skeletal muscles and joints while a voluntary movement is conducted. Although this sensory feedback may regulate the efferent motor control signals generated in the motor cortex, the relationship between the afferent sensory signals and the efferent motor signals is still unclear. In this study, we investigated the relationship between the afferent signals elicited by an electrical stimulus of a peripheral nerve and the efferent signals produced by a transcranial magnetic stimulation (TMS) of the motor cortex. The changes of motor evoked potentials (MEPs) elicited by TMS following an electrical stimulus of a median nerve were observed. The results showed that the MEPs were significantly attenuated when the inter-stimulus interval (ISI) between the electrical stimulus and the TMS was 20 ms, and that the MEPs were significantly enhanced when the ISI was longer than 35 ms. Furthermore, the brain condition which affected the MEPs was evaluated with the somatosensory evoked fields (SEFs) measured with magneto-encephalography (MEG). It was suggested that the activation and direction of the current dipole in the primary somatosensory cortex was related to the effect of the afferent signals on the motor function.