Proceedings of The Physiological Society

University of Manchester (2010) Proc Physiol Soc 19, C12

Oral Communications

The effects of contraction strength and stimulus intensity on long-interval intracortical inhibition in humans

S. Gandevia1,2, C. McNeil1,2, P. Martin1,2, J. Taylor1,2

1. Prince of Wales Medical Research Institute, Randwick,Sydney, New South Wales, Australia. 2. University of New South Wales, Sydney, New South Wales, Australia.

When two motor cortical stimuli are delivered at an interstimulus interval of 50-200ms, the response (motor evoked potential; MEP) to the second stimulus is typically suppressed. This phenomenon is termed long-interval intracortical inhibition. However, recent data demonstrate that the suppression can be mediated at a spinal rather than cortical level (McNeil et al., 2009) and that facilitation rather than inhibition is possible (unpublished observations). The aim of the present investigation was to explore the influences of voluntary contraction strength and test stimulus intensity on long-interval intracortical inhibition. In nine healthy subjects, MEPs were evoked in first dorsal interosseous by transcranial magnetic stimulation over the motor cortex. Single test and paired conditioning-test stimuli (100ms interstimulus interval) were delivered at rest or during brief contractions of 10, 25 or 100% maximal voluntary contraction (MVC) force. To quantify inhibition or facilitation, the conditioned MEP was expressed as a percentage of the unconditioned MEP recorded under the same conditions. Unconditioned MEP size increased markedly from rest to 10% MVC, was unchanged between 10 and 25% MVC, and decreased between 25 and 100% MVC. It increased with stimulus intensity at all levels of contraction but 100% MVC when all stimulus intensities produced an unconditioned MEP of the same size. Conditioned MEPs increased progressively with both contraction strength and stimulus intensity. At all stimulus intensities, inhibition increased from rest to 10% MVC, was maintained between 10 and 25% MVC, and decreased between 25 and 100% MVC. At all contraction levels, increasing the stimulus intensity decreased inhibition of the conditioned MEP. At rest and 100% MVC, the conditioned MEP was facilitated rather than inhibited with strong stimulus intensities. The data indicate that the effects of conditioning stimulation on an MEP vary greatly with voluntary contraction strength and test stimulus intensity. Unconditioned and conditioned responses to subcortical stimulation at the level of the mastoids collected in a subset of subjects (n=3) suggest that much of the suppression of the conditioned MEP occurs at a spinal rather than motor cortical level.

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