TMS-induced motor evoked potentials: Definitions and physiology

There has been a long history where neuroscientists have used magnetic fields to study the physiology of the human brain. This chapter reviews the use of transcranial magentic stimuluation (TMS) as an investigative tool to examine the excitability of the intracortical circuits of the motor cortex (M1) using single-pulse TMS. TMS is a non-invasive and painless technique that can be applied over the M1 to generate a motorevoked potential (MEP) in contralateral target muscles. Because TMS induces an electromagnetic field that passes relatively unimpeded through the scalp and skull, it also activates distinct intracortical circuits within the M1 providing a good mesaure of corticospinal excitability. This chapter will provide an overview of the technique of TMS, the underlying cortical physiology of MEPs and factors that can influence MEP amplitude.