The muscle fibre composition of three human intrinsic tongue muscles, the longitudinalis, verticalis and transversus, was investigated in four anterior to posterior regions of the tongue using morphological and enzyme- and immunohistochemical techniques. All three muscles typically contained type I, IIA and IM/IIC fibres. Type I fibres expressed slow myosin heavy chain (MyHC), type II fibres fast MyHC, mainly fast A MyHC, whereas type IM/IIC coexpressed slow and fast MyHCs. Type II fibres were in the majority (60%), but regional differences in proportion and diameter of fibre types were obvious. The anterior region of the tongue contained a predominance of relatively small type II fibres (71%), in contrast to the posterior region which instead showed a majority of larger type I and type IM/IIC fibres (66%). In general, the fibre diameter was larger in the posterior region. This muscle fibre composition of the tongue differs from those of limb, orofacial and masticatory muscles, probably reflecting genotypic as well as phenotypic functional specialization in oral function. The predominance of type II fibres and the regional differences in fibre composition, together with intricate muscle structure, suggest generally fast and flexible actions in positioning and shaping the tongue, during vital tasks such as mastication, swallowing, respiration and speech.

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Immunohistochemistry was used to determine the myosin composition of defined fibre types of three embryologically different adult muscles, the oro-facial, masseter and limb muscles. In addition, the myosin composition in whole muscle specimens was analysed with biochemical methods. Both similarities and differences between muscles in the content of myosin heavy chains and myosin light chains were found. Nevertheless, each muscle had its own distinct identity. Our results indicated the presence of a previously undetected fast myosin heavy chain isoform in the oro-facial type II fibre population, tentatively termed 'fast F'. The masseter contained aberrant myosin isoforms, such as foetal myosin heavy chain and alpha-cardiac myosin heavy chain and unique combinations of myosin heavy chain isoforms which were not found in the limb or oro-facial muscles. The type IM and IIC fibres coexpressed slow and fast A myosin heavy chains in the oro-facial and limb muscles but slow and a fast B like myosin heavy chain in the masseter. While single oro-facial and limb muscle fibres contained one or two myosin heavy chain types, single masseter fibres coexpressed up to four different myosin heavy chain isoforms. Describing the fibres according to their expression of myosin heavy chain isozymes, up to five fibre types could be distinguished in the oro-facial and limb muscles and eight in the masseter. Oro-facial and limb muscles expressed five myosin light chains, MLC1S, MLC2S, MLC1F, MLC2F and MLC3F, and the masseter four, MLC1S, MLC2S, MLC1F, and, in addition, an embryonic myosin light chain, MLC1emb, which is usually not present in normal adult skeletal muscle. These results probably reflect the way the muscles have evolved to meet the specialized functional requirements imposed upon them and are in agreement with the previously proposed concept that jaw and limb muscles belong to two distinct allotypes.

It is well established that unilateral exercise can produce contralateral effects. However, it is unclear whether unilateral exercise that leads to muscle injury and inflammation also affects the homologous contralateral muscles. To test the hypothesis that unilateral muscle injury causes contralateral muscle changes, an experimental rabbit model with unilateral muscle overuse caused by a combination of electrical muscle stimulation and exercise (EMS/E) was used. The soleus and gastrocnemius muscles of both exercised and non-exercised legs were analyzed with enzyme- and immunohistochemical methods after 1, 3 and 6 weeks of repeated EMS/E. After 1 w of unilateral EMS/E there were structural muscle changes such as increased variability in fiber size, fiber splitting, internal myonuclei, necrotic fibers, expression of developmental MyHCs, fibrosis and inflammation in the exercised soleus muscle. Only limited changes were found in the exercised gastrocnemius muscle and in both non-exercised contralateral muscles. After 3 w of EMS/E, muscle fiber changes, presence of developmental MyHCs, inflammation, fibrosis and affections of nerve axons and AChE production were observed bilaterally in both the soleus and gastrocnemius muscles. At 6 w of EMS/E, the severity of these changes significantly increased in the soleus muscles and infiltration of fat was observed bilaterally in both the soleus and the gastrocnemius muscles. The affections of the muscles were in all three experimental groups restricted to focal regions of the muscle samples. We conclude that repetitive unilateral muscle overuse caused by EMS/E overtime leads to both degenerative and regenerative tissue changes and myositis not only in the exercised muscles, but also in the homologous non-exercised muscles of the contralateral leg. Although the mechanism behind the contralateral changes is unclear, we suggest that the nervous system is involved in the cross-transfer effects.

The present study provides novel data suggesting a role for pro-inflammatory cytokines and reduced ribosomal production in the development/maintenance of muscle contractures, possibly underlying stunted growth and perimysial ECM expansion. Muscle Nerve 58: 277-285, 2018.