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Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex.
J Neurophysiol. 2003 Mar; 89(3):1256-64.JN

Abstract

Motor cortex stimulation has both excitatory and inhibitory effects on ipsilateral muscles. Excitatory effects can be assessed by ipsilateral motor-evoked potentials (iMEPs). Inhibitory effects include an interruption of ipsilateral voluntary muscle activity known as the silent period (iSP) and a reduction in corticospinal excitability evoked by conditioning stimulation of the contralateral motor cortex (interhemispheric inhibition, IHI). Both iSP and IHI may be mediated by transcallosal pathways. Their relationship to the contralateral corticospinal projection and whether iSP and IHI represent the same phenomenon remain unclear. The neuronal population activated by transcranial magnetic stimulation (TMS) is highly dependent on the direction of the induced current in the brain. We examined the relationship among iMEP, iSP, IHI, and the contralateral corticospinal system by examining the effects of different stimulus intensities and current directions. Surface electromyography (EMG) was recorded from both first dorsal interosseous (FDI) muscles. The iSP in the right FDI muscle was obtained by right motor cortex stimulation during voluntary muscle contraction. IHI was examined by conditioning stimulation of the right motor cortex followed by test stimulation of the left motor cortex at interstimulus intervals (ISIs) of 2-80 ms. The induced current directions tested in the right motor cortex were anterior medial (AM), posterior medial (PM), posterior lateral, and anterior lateral (AL). Contralateral MEPs (cMEPs) had the lowest threshold with the AM direction and the shortest latency with the PM direction. iMEPs were present in 8 of 10 subjects. Both iMEP and IHI did not show significant directional preference. iSP was observed in all subjects with the highest threshold for the AL direction and the longest duration for the AM direction. cMEP, iSP, and IHI all increased with stimulus intensity up to approximately 75% stimulator output. Target muscle activation decreased IHI at 8-ms ISI but had little effect on IHI at 40-ms ISI. iSP and IHI at 8-ms ISI did not correlate at any stimulus intensities and current directions tested, and factor analysis showed that they are explained by different factors. However, active IHI at 40-ms ISI was explained by the same factor as iSP. The different directional preference for cMEP compared with iMEP and IHI suggests that these ipsilateral effects are mediated by populations of cortical neurons that are different from those activating the corticospinal neurons. iSP and IHI do not represent the same phenomenon and should be considered complementary measures of ipsilateral inhibition.

Authors+Show Affiliations

Division of Neurology, Krembil Neuroscience Center and Toronto Western Research Institute, University Health Network, University of Toronto, Toronto, Ontario M5T 2S8, Canada. robert.chen@uhn.on.caNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

12611955

Citation

Chen, Robert, et al. "Organization of Ipsilateral Excitatory and Inhibitory Pathways in the Human Motor Cortex." Journal of Neurophysiology, vol. 89, no. 3, 2003, pp. 1256-64.
Chen R, Yung D, Li JY. Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex. J Neurophysiol. 2003;89(3):1256-64.
Chen, R., Yung, D., & Li, J. Y. (2003). Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex. Journal of Neurophysiology, 89(3), 1256-64.
Chen R, Yung D, Li JY. Organization of Ipsilateral Excitatory and Inhibitory Pathways in the Human Motor Cortex. J Neurophysiol. 2003;89(3):1256-64. PubMed PMID: 12611955.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Organization of ipsilateral excitatory and inhibitory pathways in the human motor cortex. AU - Chen,Robert, AU - Yung,Derek, AU - Li,Jie-Yuan, Y1 - 2002/10/30/ PY - 2003/3/4/pubmed PY - 2003/5/13/medline PY - 2003/3/4/entrez SP - 1256 EP - 64 JF - Journal of neurophysiology JO - J Neurophysiol VL - 89 IS - 3 N2 - Motor cortex stimulation has both excitatory and inhibitory effects on ipsilateral muscles. Excitatory effects can be assessed by ipsilateral motor-evoked potentials (iMEPs). Inhibitory effects include an interruption of ipsilateral voluntary muscle activity known as the silent period (iSP) and a reduction in corticospinal excitability evoked by conditioning stimulation of the contralateral motor cortex (interhemispheric inhibition, IHI). Both iSP and IHI may be mediated by transcallosal pathways. Their relationship to the contralateral corticospinal projection and whether iSP and IHI represent the same phenomenon remain unclear. The neuronal population activated by transcranial magnetic stimulation (TMS) is highly dependent on the direction of the induced current in the brain. We examined the relationship among iMEP, iSP, IHI, and the contralateral corticospinal system by examining the effects of different stimulus intensities and current directions. Surface electromyography (EMG) was recorded from both first dorsal interosseous (FDI) muscles. The iSP in the right FDI muscle was obtained by right motor cortex stimulation during voluntary muscle contraction. IHI was examined by conditioning stimulation of the right motor cortex followed by test stimulation of the left motor cortex at interstimulus intervals (ISIs) of 2-80 ms. The induced current directions tested in the right motor cortex were anterior medial (AM), posterior medial (PM), posterior lateral, and anterior lateral (AL). Contralateral MEPs (cMEPs) had the lowest threshold with the AM direction and the shortest latency with the PM direction. iMEPs were present in 8 of 10 subjects. Both iMEP and IHI did not show significant directional preference. iSP was observed in all subjects with the highest threshold for the AL direction and the longest duration for the AM direction. cMEP, iSP, and IHI all increased with stimulus intensity up to approximately 75% stimulator output. Target muscle activation decreased IHI at 8-ms ISI but had little effect on IHI at 40-ms ISI. iSP and IHI at 8-ms ISI did not correlate at any stimulus intensities and current directions tested, and factor analysis showed that they are explained by different factors. However, active IHI at 40-ms ISI was explained by the same factor as iSP. The different directional preference for cMEP compared with iMEP and IHI suggests that these ipsilateral effects are mediated by populations of cortical neurons that are different from those activating the corticospinal neurons. iSP and IHI do not represent the same phenomenon and should be considered complementary measures of ipsilateral inhibition. SN - 0022-3077 UR - https://www.unboundmedicine.com/medline/citation/12611955/Organization_of_ipsilateral_excitatory_and_inhibitory_pathways_in_the_human_motor_cortex_ L2 - https://journals.physiology.org/doi/10.1152/jn.00950.2002?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub=pubmed DB - PRIME DP - Unbound Medicine ER -