title: Distinct interneuronal networks influence excitability of the surround during movement initiation.
abstract: Surround inhibition (SI) is a feature of motor control in which activation of task-related muscles is associated with inhibition of neighboring, nonprotagonist muscles, allowing selective motor control. The physiological basis for SI still remains unknown. In all previous studies, SI in the motor system was measured during movement initiation by using transcranial magnetic stimulation (TMS) to deliver a posteroanterior current at a single suprathreshold intensity. To expand our understanding of SI, we explored this phenomenon at a wide range of intensities and by stimulating motor cortex with currents along anteroposterior and lateromedial directions. Fifteen healthy volunteers performed a brief isometric index finger flexion on hearing a tone. Electromyography was recorded from the synergist and surround finger muscles. Single-pulse TMS was applied to stimulate the surround muscle at different intensities at rest or movement initiation. The motor evoked potential (MEP) amplitudes were then plotted against stimulation intensities to obtain the MEP recruitment curves for the rest and movement initiation conditions and for the three current directions for every subject. From the recruitment curves, we found that surround inhibition could be elicited only by the posteroanterior current. Hence, we postulate that surround inhibition is mediated by intracortical circuits in the motor cortex. Also, for the first time, we observed surround facilitation when the motor cortex was stimulated with anteroposterior current. Further studies are needed to investigate the mechanisms underlying both these phenomena individually in healthy subjects and patients with dystonia and other movement disorders.
authors.affiliation: Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; and.
authors.forename: Nivethida
authors.initials: N
authors.lastname: Thirugnanasambandam
authors.affiliation: Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; and.
authors.forename: Rohan
authors.initials: R
authors.lastname: Khera
authors.affiliation: Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; and Department of Neurology, Peking Union Medical College Hospital, Beijing, China.
authors.forename: Han
authors.initials: H
authors.lastname: Wang
authors.affiliation: Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; and.
authors.forename: Sahana N
authors.initials: SN
authors.lastname: Kukke
authors.affiliation: Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; and hallettm@ninds.nih.gov.
authors.forename: Mark
authors.initials: M
authors.lastname: Hallett
mesh_terms: D000328:Adult; D004576:Electromyography; D019054:Evoked Potentials, Motor; D005260:Female; D006225:Hand / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009434:Neural Pathways / Q000502:physiology; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: MEP recruitment curve; premotor-motor interaction; surround inhibition; transcranial magnetic stimulation