title: Cortico-muscular synchronization by proprioceptive afferents from the tongue muscles during isometric tongue protrusion.
abstract: Tongue movements contribute to oral functions including swallowing, vocalizing, and breathing. Fine tongue movements are regulated through efferent and afferent connections between the cortex and tongue. It has been demonstrated that cortico-muscular coherence (CMC) is reflected at two frequency bands during isometric tongue protrusions: the beta (β) band at 15-35Hz and the low-frequency band at 2-10Hz. The CMC at the β band (β-CMC) reflects motor commands from the primary motor cortex (M1) to the tongue muscles through hypoglossal motoneuron pools. However, the generator mechanism of the CMC at the low-frequency band (low-CMC) remains unknown. Here, we evaluated the mechanism of low-CMC during isometric tongue protrusion using magnetoencephalography (MEG). Somatosensory evoked fields (SEFs) were also recorded following electrical tongue stimulation. Significant low-CMC and β-CMC were observed over both hemispheres for each side of the tongue. Time-domain analysis showed that the MEG signal followed the electromyography signal for low-CMC, which was contrary to the finding that the MEG signal preceded the electromyography signal for β-CMC. The mean conduction time from the tongue to the cortex was not significantly different between the low-CMC (mean, 80.9ms) and SEFs (mean, 71.1ms). The cortical sources of low-CMC were located significantly posterior (mean, 10.1mm) to the sources of β-CMC in M1, but were in the same area as tongue SEFs in the primary somatosensory cortex (S1). These results reveal that the low-CMC may be driven by proprioceptive afferents from the tongue muscles to S1, and that the oscillatory interaction was derived from each side of the tongue to both hemispheres. Oscillatory proprioceptive feedback from the tongue muscles may aid in the coordination of sophisticated tongue movements in humans.
authors.affiliation: Department of Oral Physiology, Graduate School of Dental Medicine, Hokkaido University, Kita-ku, Sapporo 060-8586, Japan. Electronic address: maezawa@den.hokudai.ac.jp.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Maezawa
authors.affiliation: Human Brain Research Center, Graduate School of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-8507, Japan; Graduate School of Core Ethics and Frontier Sciences, Ritsumeikan University, Kita-ku, Kyoto 603-8577, Japan.
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Mima
authors.affiliation: Department of Systems Neuroscience, School of Medicine, Sapporo Medical University, Chuo-ku, Sapporo 060-8556, Japan.
authors.forename: Shogo
authors.initials: S
authors.lastname: Yazawa
authors.affiliation: Human Brain Research Center, Graduate School of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-8507, Japan.
authors.forename: Masao
authors.initials: M
authors.lastname: Matsuhashi
authors.affiliation: Department of Pediatrics, Graduate School of Medicine, Hokkaido University, Kita-ku, Sapporo 060-8638, Japan.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Shiraishi
authors.affiliation: Department of Oral Physiology, Graduate School of Dental Medicine, Hokkaido University, Kita-ku, Sapporo 060-8586, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Funahashi
mesh_terms: D000328:Adult; D004576:Electromyography; D005073:Evoked Potentials, Somatosensory / Q000502:physiology; D005260:Female; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009068:Movement / Q000502:physiology; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology*; D014059:Tongue / Q000294:innervation / Q000502:physiology*; D055815:Young Adult
keywords: Cortico-muscular coherence; Hypoglossal motor nucleus; Magnetoencephalography; Muscle spindle; Neural oscillation; Trigeminal nucleus