Editing Evoked potential (section)

== Motor evoked potentials ==
Motor evoked potentials (MEP) are recorded from muscles following direct stimulation of exposed motor cortex, or transcranial stimulation of motor cortex, either [[transcranial magnetic stimulation|magnetic]] or electrical.  Transcranial magnetic MEP (TCmMEP) potentially offer clinical diagnostic applications.  Transcranial electrical MEP (TCeMEP) has been in widespread use for several years for intraoperative monitoring of pyramidal tract functional integrity.

During the 1990s, there were attempts to monitor "motor evoked potentials", including "neurogenic motor evoked potentials" recorded from peripheral nerves, following direct electrical stimulation of the spinal cord.  It has become clear that these "motor" potentials were almost entirely elicited by antidromic stimulation of sensory tracts—even when the recording was from muscles (antidromic sensory tract stimulation triggers myogenic responses through synapses at the root entry level).{{Clarify | date = July 2019 | reason = Is the direct stimulation of the spinal chord carried 'antidromically' by the sensory fiber to the muscle, or is it carried normally and onward in the efferent tract through the synapse at the root?}}  TCMEP, whether electrical or magnetic, is the most practical way to ensure pure motor responses, since stimulation of sensory cortex cannot result in descending impulses beyond the first synapse (synapses cannot be backfired).

[[transcranial magnetic stimulation|TMS]]-induced MEPs have been used in many experiments in [[cognitive neuroscience]]. Because MEP amplitude is correlated with motor excitability, they offer a quantitative way to test the role of various types of intervention on the motor system (pharmacological, behavioral, lesion, etc.). TMS-induced MEPs may thus serve as an index of covert [[Premovement neuronal activity|motor preparation]] or facilitation, e.g., induced by the [[mirror neuron]] system when seeing someone's else actions.<ref>{{cite journal | author1 = Catmur C. | author2 = Walsh V. | author3 = Heyes C. | year = 2007 | title = Sensorimotor learning configures the human mirror system | journal = Curr. Biol. | volume = 17 | issue = 17 | pages = 1527–1531 | doi = 10.1016/j.cub.2007.08.006 | pmid = 17716898 | doi-access = free | bibcode = 2007CBio...17.1527C }}</ref> In addition, MEPs are used as a reference to adjust the intensity of stimulation that needs to be delivered by TMS when targeting cortical regions whose response might not be as easily measurable, e.g., in the context of TMS-based therapy.