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Corticospinal control of antagonistic muscles in the cat

Authors

  • Christian Ethier,

    1. CRULRG Brain and Movement Laboratory, Department of Anatomy and Physiology, Faculty of Medicine, Laval University, F-6500, 2601 de la Canardière, Québec City (QC), Canada G1J 2G3
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  • Laurent Brizzi,

    1. CRULRG Brain and Movement Laboratory, Department of Anatomy and Physiology, Faculty of Medicine, Laval University, F-6500, 2601 de la Canardière, Québec City (QC), Canada G1J 2G3
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  • Dominic Giguère,

    1. CRULRG Brain and Movement Laboratory, Department of Anatomy and Physiology, Faculty of Medicine, Laval University, F-6500, 2601 de la Canardière, Québec City (QC), Canada G1J 2G3
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  • Charles Capaday

    1. CRULRG Brain and Movement Laboratory, Department of Anatomy and Physiology, Faculty of Medicine, Laval University, F-6500, 2601 de la Canardière, Québec City (QC), Canada G1J 2G3
    2. Dipartimento di Medicina Sperimentale, Sezione di Fisiologia Umana, Università di Pavia, Pavia, Italy
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Professor C. Capaday, 1CRULRG Brain and Movement Laboratory, as above.
E-mail: charles.capaday@anm.ulaval.ca

Abstract

We recently suggested that movement-related inter-joint muscle synergies are recruited by selected excitation and selected release from inhibition of cortical points. Here we asked whether a similar cortical mechanism operates in the functional linking of antagonistic muscles. To this end experiments were done on ketamine-anesthetized cats. Intracortical microstimulation (ICMS) and intramuscular electromyographic recordings were used to find and characterize wrist, elbow and shoulder antagonistic motor cortical points. Simultaneous ICMS applied at two cortical points, each evoking activity in one of a pair of antagonistic muscles, produced co-contraction of antagonistic muscle pairs. However, we found an obvious asymmetry in the strength of reciprocal inhibition; it was always significantly stronger on physiological extensors than flexors. Following intravenous injection of a single bolus of strychnine, a cortical point at which only a physiological flexor was previously activated also elicited simultaneous activation of its antagonist. This demonstrates that antagonistic corticospinal neurons are closely grouped, or intermingled. To test whether releasing a cortical point from inhibition allows it to be functionally linked with an antagonistic cortical point, one of three GABAA receptor antagonists, bicuculline, gabazine or picrotoxin, was injected iontophoretically at one cortical point while stimulation was applied to an antagonistic cortical point. This coupling always resulted in co-contraction of the represented antagonistic muscles. Thus, antagonistic motor cortical points are linked by excitatory intracortical connections held in check by local GABAergic inhibition, with reciprocal inhibition occurring at the spinal level. Importantly, the asymmetry of cortically mediated reciprocal inhibition would appear significantly to bias muscle maps obtained by ICMS in favor of physiological flexors.

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