Regional differences in fibre type composition in the human temporalis muscle

Abstract

Anatomical and electromyographic studies point to regional differences in function in the human temporalis muscle. During chewing and biting the anterior portions of the muscle are in general more intensively activated and they are capable of producing larger forces than the posterior portions. It was hypothetised that this heterogeneity in function is reflected in the fibre type composition of the muscle. The composition and surface area of different fibre types in various anteroposterior portions of the temporalis muscle were investigated in 7 cadavers employing immunohistochemistry with a panel of monoclonal antibodies against different isoforms of myosin heavy chain. Pure slow muscle fibres, type I, differed strongly in number across the muscle. In the most posterior portion of the muscle there were 24 % type I fibres, in the intermediate portion 57 %, and in the most anterior portion 46 %. The mean fibre cross-sectional area (m-fcsa) of type I fibres was 1849 µm#, which did not differ significantly across the muscle. The proportion of pure fast muscle fibres, type IIA and IIX, remained more or less constant throughout the muscle at 13 % and 11 % respectively ; their m-fcsa was 1309 µm# and 1206 µm#, respectively, which did not differ significantly throughout the muscle. Pure type IIB fibres were not found. The relative proportion of hybrid fibres was 31 % and did not differ significantly among the muscle portions. Fibre types IjIIA and cardiac αjIjIIA were the most abundant hybrid fibre types. In addition, 5 % of the type I fibres had an additional myosin isoform which has only recently been described by means of electrophoresis and was named Ia. In the present study they were denoted as hybrid type IjIa muscle fibres. It is concluded that intramuscular differences in type I fibre distribution are in accordance with regional differences in muscle function.

Morphology and Physiology of Masticatory Muscle Motor Units

“…It has been suggested that these hybrid fibers indicate fiber type transformation, as a result of, for example, an increase or decrease of activity (for review, see Pette and VrbovA, 1985). The following transition pathway of MHC isoforms has been suggested: I " I/IIA IIA " IIA/IID > IID/IIB " IIB (Gorza, 1990;DeNardi et al, 1993 Eijden, 1999. This parallels the finding that masticatory muscles contain a relatively large number of ATPaseintermediate (IM) fibers (Ringqvist, 1974;Eriksson and Thornell, 1983;Rowlerson et al, 1983).…”

Painful Unilateral Temporalis Muscle Enlargement: Reactive Masticatory Muscle Hypertrophy

“…Interestingly, the latter is similar to the median diameter of 57 lm in type 1 fibers, described by Lambert and colleagues in masticatory muscle myopathy (hypertrophic branchial myopathy) [14]. Masticatory muscles in humans have a somewhat different fiber type composition as compared to trunk or limb muscles [16,17]. In the latter, myofibrillar ATPase histochemical reaction yields two main fiber types of almost equal size: Type 1 (slow-twitch/oxidative) and type 2 (fast-twitch/ glycolytic).…”

“…Also, these may be confounded by regional heterogeneity of fiber type distribution in human temporalis muscle [16]. Hence, in the most anterior portion of the muscle, type 1 fibers constitute 46 % of all fiber types as compared to the intermediate and the posterior portions where they comprise 57 and 24 % respectively [16]. Collectively, fiber size and fiber type composition in the muscle biopsy of the present case represent a major departure from previously reported findings in normal control temporalis muscle.…”

“…Masticatory muscles have small type 2 fibers and a fiber type of intermediate staining intensity between type 1 and type 2 fibers (type 1 M fibers). Given that biopsies of temporalis muscle are exceptionally rare, normal reference values regarding fiber diameter and fiber type distribution in age matched controls are based mainly on postmortem data reported in a small number of publications [3,16,17]. Using myofibrillar ATPase histochemistry, Harriman reported preservation of the checkerboard pattern of fiber type composition, similar with that encountered in trunk and limb muscles, in a single temporalis muscle biopsy-derived normal control case from a 31-old-female [3].…”

“…However, reference values for normal fiber type composition and fiber size remain unclear as they are based on a very small number of postmortem control tissues from disparate ages [3]. Also, these may be confounded by regional heterogeneity of fiber type distribution in human temporalis muscle [16]. Hence, in the most anterior portion of the muscle, type 1 fibers constitute 46 % of all fiber types as compared to the intermediate and the posterior portions where they comprise 57 and 24 % respectively [16].…”