What is the role of electrophysiology in the generation of motor evoked potentials (MEPs)?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Answer

Electrical stimulation of the exposed cortex has been studied in animal models for several decades. An initial D (ie, direct) wave is followed by several I (ie, indirect) waves, which come at periodic intervals (usually about 1 millisecond). D waves represent the direct excitation of corticospinal tract neurons, while I waves reflect indirect depolarization of the same axons via corticocortical connections. This propagation pattern of descending impulses has been confirmed and demonstrated in humans during surgical procedures, using epidural recordings made after cortical magnetic stimulation.

As these multiple volleys descend the corticospinal tracts, they summate at the anterior horn cells in the spinal cord. Although the first D wave may not bring the alpha motoneuron to fire, summation of subsequent I waves may reach the threshold and trigger neuronal firing. While this summation may result in a single discharge, the spinal motoneuron also may fire repeatedly after a sufficiently intense, single cortical stimulation. Consequently, the amplitude of an evoked potential after cortical stimulation may be larger than the response produced by supramaximal stimulation of the corresponding peripheral nerve.

Facilitation of motor evoked potentials

The excitability threshold for eliciting motor evoked potentials (MEPs) can be decreased by performing a voluntary contraction of the target muscle. In parallel to such threshold changes, a decrease in latency of the response to stimulation (2-6 milliseconds) can be observed with respect to the MEPs that are obtained with no contraction of the target muscle. The origin of this facilitation remains controversial. Some authors ascribe it, at least partially, to modifications of cortical excitability. Others support a segmental spinal mechanism, which seems to play a major role.

The conduction speed as measured in several studies (60-70 milliseconds) is compatible with propagation through fast corticospinal tract axons.


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