Abstract
The rabbit masseter muscle is composed of multiple anatomical partitions that produce different mechanical actions. The purpose of this study was to test the hypothesis that these compartments are differentially activated during the performance of different oral behaviors. Rhythmic activation of the masticatory muscles was elicited by stimulating the cortical masticatory area (CMA) while recording forces generated at the incisors in three dimensions with the mandible immobilized. Torques about the right temporomandibular joint (TMJ) were calculated using these forces recorded during isometric function. A set of 1–15 unique rhythmic behaviors was identified for each rabbit using torque phase plot patterns. Electromyographic recordings were made at nine different compartments in the right masseter, two compartments in the left masseter, two regions in the right digastric, and single locations in the left digastric and right and left medial pterygoid muscles. In activation cycles producing similar mechanical actions, activity patterns at the 16 recording sites were clustered into three to six groups using principal component analysis (PCA). To test for similarities in the activation of masseter compartments, pair-wise comparisons of the PCA assignment for the nine masseter compartments were conducted and frequencies of common assignment were compiled for each unique rhythmic behavior for each rabbit. Masseter muscle compartments were found to vary significantly in their PCA from the expected distribution of 100% common principal component (PC) assignment (i.e., similar activation pattern). This finding is consistent with the independent activation of masseter compartments.
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Abbreviations
- MSS1a:
-
Anterior superficial masseter, partition 1
- MSS1p:
-
Posterior superficial masseter, partition 1
- MSS2:
-
Superficial masseter, partition 2
- MSS3:
-
Posterior superficial masseter, partition 3
- MSS4:
-
Posterior superficial masseter, partition 4
- MSS5:
-
Anterior superficial masseter, partition 5
- MSM:
-
Intermediate masseter
- MPPo:
-
Posterior deep masseter
- MSPR:
-
Pars reflexa (inferior region of masseter)
- MedPT:
-
Medial pterygoid
- Dig:
-
Anterior digastric
- DigA:
-
Anterior digastric (anterior region)
- DigP:
-
Anterior digastric (posterior region)
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Acknowledgements
This work supported by USPHS/NIDCR grant DE-11536. Special thanks to J. Matos and R. Nair for their technical assistance during the data analyses and to Joyce Morris-Wiman and Michael Kutner for their valuable comments on earlier versions of the manuscript.
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Widmer, C.G., Carrasco, D.I. & English, A.W. Differential activation of neuromuscular compartments in the rabbit masseter muscle during different oral behaviors. Exp Brain Res 150, 297–307 (2003). https://doi.org/10.1007/s00221-003-1464-y
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DOI: https://doi.org/10.1007/s00221-003-1464-y