Neurophysiological mechanisms underlying motor evoked potentials in anesthetized humans. Part 2. Relationship between epidurally and muscle recorded MEPs in man

Objective and methods: Direct (D) and transynaptic, (i.e. indirect) (I) cor ticospinal tract (CT) discharges were simultaneously recorded epidurally wi th muscle motor evoked potentials (MEPs) in patients under different levels of anesthesia. The effects of the one, two or more equal electrical stimul i, applied transcranially or directly to the motor cortex, were studied at different interstimulus intervals (ISIs) to determine the optimal condition s for eliciting I and MEP responses. Results and conclusion: At anesthetic levels permiting large D and I respon ses 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 sin gle electrical stimuli elicit only a D response, optimal MEP responses are determined by the number of stimuli and the recovery of CT fibers excitabil ity (e.g. at an TSI of 4 ms). (C) 2001 Elsevier Science Ireland Ltd. All ri ghts reserved.