Neurophysiological mechanisms underlying motor evoked potentials in anesthetized humans. Part 2. Relationship between epidurally and muscle recorded MEPs in man.
OBJECTIVE
Direct (D) and transynaptic, (i.e. indirect) (I) corticospinal tract (CT) discharges were simultaneously recorded epidurally with muscle motor evoked potentials (MEPs) in patients under different levels of anesthesia. The effects of the one, two or more equal electrical stimuli, applied transcranially or directly to the motor cortex, were studied at different interstimulus intervals (ISIs) to determine the optimal conditions for eliciting I and MEP responses.


CONCLUSIONS
At anesthetic levels permiting large D and I responses to single stimuli, optimal D and I wave facilitation and MEPs occurred with two stimuli at ISIs greater than 4 ms (e.g. at 5.9 and 8 ms). When single electrical stimuli elicit only a D response, optimal MEP responses are determined by the number of stimuli and the recovery of CT fibers excitability (e.g. at an ISI of 4 ms).
Institute for Neurology and Neurosurgery, Beth Israel Medical Center - Singer Division, 170 East End Avenue, Rm. 311, New York, NY 10128, USA. vdeletis@bethisraelny.org
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D000758:Anesthesia; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D004824:Epidural Space; D019054:Evoked Potentials, Motor / Q000502:physiology*; D006225:Hand; D006801:Humans; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D011712:Pyramidal Tracts / Q000166:cytology / Q000502:physiology*