What are motor evoked potentials (MEPs)?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Single- or repetitive-pulse stimulation of the brain causes the spinal cord and peripheral muscles to produce neuroelectrical signals known as motor evoked potentials (MEPs). [1] Clinical uses of MEPs include as a tool for the diagnosis and evaluation of multiple sclerosis and as a prognostic indicator for stroke motor recovery,

The most famous pioneers in MEP research were Penfield and Boldrey, who made direct observations by stimulating the human brain with weak electrical shocks in conscious patients who were undergoing surgery. [2] From 1950-1970, several other studies of electrical stimulation of the exposed motor cortex (ie, during neurosurgical procedures) were performed in animals and humans to study the pyramidal pathway and other corticospinal connections.

Noninvasive elicitation of MEPs was made possible by Merton and Morton in 1980. [3] They designed a high-voltage transcranial electrical stimulator that excited the motor cortex using cutaneous electrodes, which were placed over the scalp. Using their technique, a contraction of contralateral muscles is recorded in a conscious subject after transcranial electrical stimulation (TES). The clinical usefulness of this method has remained limited by the local discomfort of the electrical currents that are applied over the scalp. An exception to this limitation is its use for intraoperative monitoring.

The development of transcranial magnetic stimulation (TMS), in 1985, opened new possibilities for MEP studies. Barker et al created a new type of cortical magnetic stimulator, based on the principle of electromagnetic induction. [5] The device is composed of a main unit, which contains a bank of heavy-duty capacitors. The hand-held part is freely movable so that it can be placed over any part of the body. The investigator holds the stimulating coil tangentially over the motor cortex, and a technician holds a digitizing pen over the stimulating coil to record its 3-dimensional position; this allows stereotactic mapping of the motor cortex. MEPs are recorded with surface electrodes, which are placed over small hand muscles.

Although magnetic stimulation was first used to stimulate the peripheral nervous system (PNS) and muscles, cortical stimulation has become the focus of many studies.

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