Intracortical inhibition of lower limb motor-evoked potentials after paired transcranial magnetic stimulation

• Published in: Experimental brain research Vol. 117; no. 3; pp. 437 - 443
• Main Authors: Stokic´, D. S., McKay, W. Barry, Scott, Lillian, Sherwood, Arthur M., Dimitrijevic´, Milan R.
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01.12.1997
• Subjects: Adult; Biological and medical sciences; Conditioning (Psychology) - physiology; Electric Stimulation; Electromagnetic Fields; Evoked Potentials, Motor - physiology; Fundamental and applied biological sciences. Psychology; Humans; Leg - physiology; Male; Middle Aged; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Cortex - physiology; Muscle, Skeletal - physiology; Neural Inhibition; Physical Stimulation; Vertebrates: nervous system and sense organs; Human; Motor pathway; Electrical stimulus; Central nervous system; Electrophysiology; Excitability; Lower limb; Anterior tibial muscle; Motor control; Motor cortex; Magnetic stimulus; Motor evoked potential; Inhibition; Brain (vertebrata)
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s002210050238

The aim of the present study was to determine the characteristics of intracortical inhibition in the motor cortex areas representing lower limb muscles using paired transcranial magnetic (TMS) and transcranial electrical stimulation (TES) in healthy subjects. In the first paradigm (n=8), paired magnetic stimuli were delivered through a double cone coil and motor evoked potentials (MEPs) were recorded from quadriceps (Q) and tibialis anterior (TA) muscles during relaxation. The conditioning stimulus strength was 5% of the maximum stimulator output below the threshold MEP evoked during weak voluntary contraction of TA (33+/-5%). The test stimulus (67+/-2%) was 10% of the stimulator output above the MEP threshold in the relaxed TA. Interstimulus intervals (ISIs) from 1-15 ms were examined. Conditioned TA MEPs were significantly suppressed (P<0.01) at ISIs of less than 5 ms (relative amplitude from 20-50% of the control). TA MEPs tended to be only slightly facilitated at 9-ms and 10-ms ISIs. The degree of MEP suppression was not different between right and left TA muscles despite the significant difference in size of the control responses (P<0.001). Also, conditioned MEPs were not significantly different between Q and TA. The time course of TA MEP suppression, using electrical test stimuli, was similar to that found using TMS. In the second paradigm (n=2), the suppression of TA MEPs at 2, 3, and 4 ms ISIs was examined at three conditioning intensities with the test stimulation kept constant. For the pooled 2- to 4-ms ISI data, relative amplitudes were 34+/-6%, 61+/-5%, and 98+/-9% for conditioning intensities of 0.95, 0.90, and 0.85x active threshold, respectively (P<0.01). In conclusion, the suppression of lower limb MEPs following paired TMS showed similar characteristics to the intracortical inhibition previously described for the hand motor area.