Myosin heavy chain and fibre diameter of extrinsic tongue muscles in rhesus monkey.

Objective: The purpose of this investigation was to identify the myosin heavy chain (MHC) phenotype and fibre diameters of hypoglossal innervated extrinsic tongue muscles in rhesus monkey.

Methods: Genioglossus, styloglossus and hyoglossus muscle samples obtained from three female rhesus monkeys were analysed for MHC isoforms via gel electrophoresis and stained with MHC antibodies to measure least mean diameters.

Results: MHC phenotypes were consistent for all three muscles. Each muscle was predominantly composed of MHC type IIa and I. All three isoforms were significantly different from each other in fibre diameter for styloglossus and genioglossus (IIb>IIa and IIx>I; P<0.001). For hyoglossus, the MHC type II isoforms had larger diameters than the MHC type I isoform (P<0.001).

Conclusions: While the extrinsic tongue muscle MHC and/or muscle fibre type composition may be different between mammalian species, there are consistent similarities between the intrinsic and extrinsic tongue muscles. We suggest this is necessary for the highly coordinated activities performed by the tongue such as mastication, respiration and swallowing. The differences in fibre diameters among MHC isoforms suggest a large force gradation, which would be consistent with the coordination of these activities. The similarities among primates in MHC and/or muscle fibre composition as well as similar cortical inputs to the hypoglossal nucleus, suggest that we could expect to see similar MHC phenotype for extrinsic tongue muscles in human.