Motor Evoked Potentials (MEP)

Motor evoked potentials (MEP) are signals recorded from muscles following stimulation of motor cortex. The stimulation may be applied to exposed motor cortex, or transcranially through the skull. The stimulator may be magnetic or electrical.

Motor pathways

MEPs are used to monitor the functional integrity of the corticospinal motor pathway. The corticospinal pathway originates mainly in the primary motor cortex (Brodmann’s area 4), but fibers from other regions, such as the premotor, the supplemental motor, and somatosensory cortices, contribute as well. The corticospinal pathway can be subdivided into the lateral and anterior tracts, which mediate voluntary movement.

These motor pathways consist of upper and lower motor neurons. The axons of the upper motor neurons originate in the primary motor cortex, descend through the internal capsule, and project to the lower medulla. The upper motor neurons of the lateral tract cross the midline to the contralateral side and descend to the lateral column of the dorsal horn. After reaching the ventral horn of the spinal cord, the upper motor neuron axons form synaptic connections with lower motor neurons, the peripheral neurons that innervate the skeletal muscles. The axons of the anterior tract do not cross the midline in the medulla. These axons typically cross the midline as they approach their target area and synapse with lower motor neurons that innervate the trunk muscles.

Stimulation

1. Transcranial electrical stimulation

Transcranial electrical stimulation (tES). tES is a commonly used, non-invasive technique for generating MEPs. Stimulating electrodes for tES are placed on the scalp or subcutaneously above the primary motor cortex. Electrical current is then applied to the head to alter neuronal activity in the brain. However, due to the thickness and high resistance of the skull bone, only a small percentage of current reaches the brain tissue. Therefore, to record MEPs from muscle tissue, the stimulus strength must be set high enough to overcome that resistance and activate the underlying motor pathways. We call this type of stimulation supra-maximal, as the stimulus is higher than that required for the recruitment of all muscle fibers around the recording electrode. Train stimulation is required to elicit reliable MEPs under anesthesia, requiring the use of a multi-pulse stimulator. The stimulating electrodes for MEPs are placed at C1 (for right extremities) and C2 (for left extremities) using the 10-20 system. Unlike the cathodal stimulation used for SSEPs, clinicians use anodal (+) stimulation to elicit MEPs. Anodal tES is more effective at depolarizing the pyramidal neurons of the primary motor cortex due to the more vertical organization of these cells1.

1. Transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS). TMS is another non-invasive technique for generating MEPs. By applying a magnetic field over the scalp, TMS can induce electrical currents in brain tissue in awake patients. One disadvantage of TMS, however, is that TMS-induced MEPs are suppressed under anesthesia.

1. Direct cortical stimulation

Direct cortical stimulation. For some craniotomies, MEPs can also be induced by direct stimulation of the primary motor cortex. Without the resistance of the skull, direct cortical stimulation involves the use of much lower stimulation levels.

1. Spinal cord stimulation