Muscle responses to motor cortical stimulation: can we get more out of surface electromyography?

It is now close to 40 years since Anthony Barker and colleagues demonstrated that a pulsed magnetic field can be delivered through the scalp to the motor cortex of awake human volunteers and evoke brief movements in a contralateral hand muscle (Barker et al., 1985). Since then, the use of transcranial magnetic stimulation (TMS) to study the human motor cortex and corticospinal pathway has exploded, and TMS has greatly improved our understanding of the neural control of movement across healthy and clinical populations. Motor cortical TMS activates corticospinal cells directly, or indirectly through the activation of intracortical neurons that directly synapse onto these cells, thus generating descending volleys capable of recruiting spinal motoneurones. The near synchronous recruitment of motoneurones causes the activation of muscle fibres and generates a compound muscle action potential conventionally recorded with a single channel of surface electromyography (EMG), i.e. as a motor evoked potential (MEP). Importantly, the MEP provides an index of ‘excitability’ for the entire motor pathway and the size of the MEP is thus influenced by excitability changes at both the motor cortex and the spinal cord.