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. Direct cortical stimulation
2. Transcranial electrical stimulation
3. Transcranial magnetic stimulation
4. Spinal cord stimulation