Can the Recording of Motor Potentials Evoked by Transcranial Magnetic Stimulation Be Optimized?

In TMS, a high-intensity brief magnetic pulse applied with a coil over the subject's scalp, induces an electric field across the cortical tissue that depolarizes a group of neuronal pools. [...]if a single pulse is applied over a particular spot of the primary motor cortex (M1), the generated action potentials travel down the corticospinal tract reaching a specific muscle or group of muscles, which in turn can be achieved by recording their myoelectric activities. [...]MUs action potentials (MUAPs) recorded over the skin surface may not be uniformly distributed in space as muscle force varies (Merletti and Parker, 2004; Rosa et al., 2008; Garcia and Vieira, 2011; Hodson-Tole et al., 2013). [...]sEMG amplitude arising from the algebraic temporal and spatial summation of the action potentials that emanate from the underlying recruited MUs may exhibit different spatial distributions throughout the muscle extent. Once the action potentials reach the motor end plates and propagate along the muscle fibers, they will distribute differently throughout the muscle, from both spatial and temporal points of view. [...]the electrode can record MEPs derived from the summation of MUAPs that emerge at this region before they propagate along the MUs to distinct muscle locations.