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Archives of oral biology (1)
Progress in brain research (1)
Morris-Wiman, J. (2)
English, A.W. (1)
Widmer, C. G. (1)
Widmer, C.G. (1)
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Limb, Respiratory and Masticatory Muscle Compartmentalization: Developmental and Hormonal Considerations
Widmer, C. G.
Progress in brain research
Neuromuscular compartments are subvolumes of muscle that have unique biomechanical actions and can be activated singly or in groups to perform the necessary task. Beside unique biomechanical actions, other evidence that supports the neuromuscular compartmentalization of muscles includes segmental reflexes that preferentially excite motoneurons from the same compartment, proportions of motor unit types that differ among compartments and a central partitioning of motoneurons that innervate each compartment. The current knowledge regarding neuromuscular compartments in representative muscles involved in locomotion, respiration and mastication is presented to compare and contrast these different motor systems. Developmental features of neuromuscular compartment formation in these three motor systems are reviewed to identify when these compartments are formed, their innervation patterns and the process of refinement to achieve the adult phenotype. Finally, the role of androgen modulation of neuromuscular compartment maturation in representative muscles of these motor systems is reviewed and the impact of testosterone on specific myosin heavy chain fiber types is discussed based on recent data. In summary, neuromuscular compartments are pre-patterned output elements in muscle that undergo refinement of compartment boundaries and muscle fiber phenotype during maturation. Further studies are needed to understand how these output elements are selectively controlled during locomotion, respiration and mastication.
Developmental and Functional Considerations of Masseter Muscle Partitioning
Archives of oral biology
The masseter muscle participates in a wide variety of activities including mastication, swallowing and speech. The functional demands for accurate mandibular positioning and generation of forces during incising or a power stroke require a diverse set of forces that are determined by the innate muscle form. The complex internal tendon architecture subdivides the masseter into multiple partitions that can be further subdivided into neuromuscular compartments representing small motor unit territories. Individual masseter compartments have unique biomechanical properties that, when activated individually or in groups, can generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint. The myosin heavy chain (MyHC) fiber-type distribution in the adult masseter is sexually dimorphic and is influenced by hormones such as testosterone. These testosterone-dependent changes cause a phenotype switch from slower to faster fiber-types in the male. The development of the complex organization of the masseter muscle, the MyHC fiber-type message and protein expression, and the formation of endplates appear to be pre-programmed and not under control of the muscle nerve. However, secondary myotube generation and endplate maturation are nerve dependent. The delayed development of the masseter muscle compared to the facial, tongue and jaw opening muscles may be related to the delayed functional requirements for chewing. In summary, masseter muscle form is pre-programmed prior to birth while muscle fiber contractile characteristics are refined postnatally in response to functional requirements. The motor control mechanisms that are required to coordinate the activation of discrete functional elements of this muscle remain to be determined.
masseter; masticatory muscle; tongue; compartments; development; myosin; endplate
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