Heterogeneity of fiber and sarcomere length in the human masseter muscle

Abstract

This study deals with regional differences in the architectural design of the human masseter muscle. For a number of defined muscle regions the three-dimensional coordinates of origin and insertion points, and the lengths of the muscle fibers and the sarcomeres were determined in the closed jaw position. Measurements were made from cadavers and the data were used as input for a model predicting sarcomere length at other mandibular positions. At a closed jaw average muscle fiber length of the muscle regions ranged between 19.0-30.3 mm. The fibers appeared to be considerably longer (35%) anteriorly than posteriorly in the muscle, and deeply situated fibers were on average 5% shorter than superficially situated ones. Average sarcomere length of the regions ranged between 2.27-2.55 microns, indicating that at a closed jaw position sarcomeres are at suboptimum length and have different positions on the length-tension curve. In the deep layer of the muscle sarcomeres were significantly shorter (6%) than in the superficial layer. Within the superficial layer sarcomere lengths did not differ significantly, but in the deep layer sarcomeres were shorter (8%) posteriorly than anteriorly in the muscle. The model shows that jaw displacement had a different effect on sarcomere length in the muscle regions. When the jaw was rotated about a transverse axis (open/close rotation) sarcomere excursions were relatively small in the posterior muscle regions and large in the anterior regions. The reverse was true when the jaw was rotated contralaterally about a vertical axis. It is concluded that, due to heterogeneity in fiber and sarcomere lengths, the distribution of maximal isometric tension across the muscle at full effort is not uniform.

Architecture of the human jaw‐closing and jaw‐opening muscles

“…Both opener and closer should reach optimum length at a relatively small degree (5-10°) of jaw opening. However, there is evidence that muscles within the same functional group and portions of the same muscle do not use the same part of the length-tension curve, and this can be explained mainly by differences in architecture and different moment arm lengths (masseter: van Eijden and Raadsheer, 1992;pterygoids: van Eijden et al, 1995;temporalis: van Eijden et al, 1996). In the literature, a number of studies have been done in which SL has been measured in (some of) the jaw closers and jaw openers in animals (rat: Nordstrom et al, 1974;rabbit: Hertzberg et al, 1980;Weijs and van der Wielen-Drent, 1982;miniature pig: Herring et al, 1984).…”

“…Consequently, different muscles and different parts of the same muscle should operate on different parts of the length-tension relationship. This action should lead to a broadening of the relationship between jaw angle and the resultant jaw elevator or depressor force and to a broadening of the length-tension relationship of the separate muscles (masseter: van Eijden and Raadsheer, 1992;pterygoids: van Eijden et al, 1995;temporalis: van Eijden et al, 1996). The capability of each muscle of producing different mechanical actions is enhanced by a taskdependent regional contraction of muscle portions (masseter: Belser and Hannam, 1986;McMillan and Hannam, 1992;van Eijden et al, 1993;temporalis: van Eijden et al, 1990;McMillan, 1993;van Eijden, 1990, 1995) and by the heterogeneous distribution of fiber types (Mao et al, 1992).…”

“…Information about sarcomere length in the human is scarce. Recently, we reported about measured sarcomere lengths at the closed-jaw position in cadavers and estimated the effect of jaw position on sarcomere length and relative muscle tension by using a mathematical model (van Eijden and Raadsheer, 1992;van Eijden et al, 1995van Eijden et al, , 1996. The results pointed to differential stretching of sarcomeres and consequently to a heterogeneous distribution of tension when sarcomeres of the same muscle and of different muscles were compared.…”

Effects of strain on contractile force and number of sarcomeres in series of Xenopus laevis single muscle fibres during long-term culture

“…For instance, heterogeneity of sarcomere lengths within muscle has been demonstrated within in vivo muscle as well as in situ muscle (e.g. van Eijden and Raadsheer, 1992;Zuurbier and Huijing, 1993;Willems and Huijing, 1994;Pappas et al, 2002;Ahn et al, 2003). In addition, mechanical interaction between neighbouring muscles and extramuscular connective tissue, causes unequal forces to be exerted at proximal and distal tendons of muscles in the rat anterior tibial compartment (e.g.…”

Three-dimensional structure of the human temporalis muscle

“…Hence, during jaw movements, fiber and sarcomere excursions are probably not the same for various muscle portions, and as a consequence the maximum force and the excursion range of the muscle portions may differ. Such intramuscular differences have been demonstrated in other human jaw muscles (masseter: Van Eijden and Raadsheer, 1992;pterygoid: Van Eijden et al, 1995) but have thus far not been documented for the temporalis muscle. In addition, electromyographic studies in which fine wire electrodes have been inserted into different muscle portions of the temporalis demonstrate a differential activation of muscle portions, depending on the motor task that is executed Van Eijden, 1990, 1995;McMillan, 1993).…”