Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Abstract

Transcranial magnetic stimulation (TMS) was initially used to evaluate the integrity of the corticospinal tract in humans non-invasively. Since these early studies, the development of paired-pulse and repetitive TMS protocols allowed investigators to explore inhibitory and excitatory interactions of various motor and non-motor cortical regions within and across cerebral hemispheres. These applications have provided insight into the intracortical physiological processes underlying the functional role of different brain regions in various cognitive processes, motor control in health and disease and neuroplastic changes during recovery of function after brain lesions. Used in combination with neuroimaging tools, TMS provides valuable information on functional connectivity between different brain regions, and on the relationship between physiological processes and the anatomical configuration of specific brain areas and connected pathways. More recently, there has been increasing interest in the extent to which these physiological processes are modulated depending on the behavioural setting. The purpose of this paper is (a) to present an up-to-date review of the available electrophysiological data and the impact on our understanding of human motor behaviour and (b) to discuss some of the gaps in our present knowledge as well as future directions of research in a format accessible to new students and/or investigators. Finally, areas of uncertainty and limitations in the interpretation of TMS studies are discussed in some detail.

Utilizing Transcranial Magnetic Stimulation to Study the Human Neuromuscular System

“…Both interhemispheric facilitation (IHF) and interhemispheric inhibition (IHI) can be observed, but IHI is a stronger response. IHF does not have a well-defined protocol or mechanism, but it has been observed at interstimulus intervals of 4-8 ms 4 . IHI can be elicited at a wide range of interstimulus intervals (6-50ms), and it is partially mediated by GABA B .…”

“…IHI can be elicited at a wide range of interstimulus intervals (6-50ms), and it is partially mediated by GABA B . Pharmacological studies administering baclofen, a GABA B agonist, specifically suggest that longer interval IHIs were mediated by postsynaptic GABA B receptors 4 . In general, the paired pulse technique can be used to study a large variety of variables that provide insight into intracortical and interhemispheric properties.…”

“…Additionally, recent advances in magnetic stimulation now allows for partitioning of cortical versus spinal excitability 4,5 . For example, paired-pulse TMS can be used to assess intracortical facilitatory and inhibitory properties by combining a conditioning stimulus and a test stimulus at different interstimulus intervals 3,4,[6][7][8] . In this video article we will demonstrate the methodological and technical aspects of these techniques.…”

“…Single pulse TMS applied to the motor cortex excites pyramidal neurons transsynaptically 2 ( Figure 1) and results in a measurable electromyographic response that can be used to study and evaluate the integrity and excitability of the corticospinal tract in humans 3 . Additionally, recent advances in magnetic stimulation now allows for partitioning of cortical versus spinal excitability 4,5 . For example, paired-pulse TMS can be used to assess intracortical facilitatory and inhibitory properties by combining a conditioning stimulus and a test stimulus at different interstimulus intervals 3,4,[6][7][8] .…”

Postural challenge affects motor cortical activity in young and old adults

“…It is thought that inhibitory intracortical circuits modulate the excitability of the cortical neurons that project to the spinal motor neurons of the muscles involved in the task (Reis et al, 2008). The degree of reduction in intracortical inhibition is related to contraction intensity (Rantalainen et al, 2013;Ortu et al, 2008), contraction type (Howatson et al, 2011), and whether the movement starts or ends (Sidhu et al, 2013).…”