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Voluntary teeth clenching facilitates human motor system excitability
B Boroojerdi1, F Battaglia, W Muellbacher
1Human Cortical Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892-1428, USA.
Summary
Voluntary teeth clenching enhances motor system excitability in both upper and lower extremities. This effect involves both cortical and subcortical neural pathways, with varying contributions depending on the limb.
Area of Science:
- Neuroscience
- Motor Control
- Human Physiology
Background:
- Voluntary teeth clenching is a known maneuver to enhance peripheral monosynaptic reflexes.
- The precise neural sites responsible for this facilitatory effect remain incompletely understood.
Purpose of the Study:
- To investigate the neuroanatomical sites mediating the facilitatory effects of voluntary teeth clenching on motor reflexes.
- To differentiate between cortical and subcortical contributions to this phenomenon.
Main Methods:
- Utilized focal transcranial magnetic stimulation (TMS) to assess motor cortical excitability (recruitment curves, motor thresholds, intracortical inhibition/facilitation) in the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles.
- Investigated subcortical excitability using F waves, H reflexes, and brainstem magnetic stimulation during voluntary teeth clenching in seven healthy volunteers.
Main Results:
- Teeth clenching significantly facilitated motor evoked potentials (MEPs) and H reflexes, indicating enhanced subcortical excitability for both upper and lower extremities.
- Intracortical inhibition was reduced in the FDI (hand) but not TA (leg), suggesting a cortical contribution to hand muscle facilitation.
- No changes were observed in M wave amplitudes, ruling out peripheral nerve changes.
Conclusions:
- Voluntary teeth clenching exerts a similar facilitatory influence on motor pathways of both upper and lower extremities.
- Both cortical and subcortical neural structures contribute to the facilitatory effect in the hand, while primarily subcortical mechanisms appear involved in the lower extremity.