Muscle fibers comprise myofibrils consisting of repeating arrangements of sarcomeres (Greising et al., 2012). These myofibrils give the muscle its striated appearance as observed under transmission electron microscope (TEM) (Greising et al., 2012). Commonly, successive sarcomeres are delimited by a Z-disc. The adjacent segments of sarcomeres on both sides of the same Z-disc constitute the I-band. The A-band is intercalated between the two I-bands of a sarcomere. Transverse tubules (t-tubules) are plasmalemmal internalizations that extend transversely along the A–I junction of the sarcomere in mammals (Jayasinghe and Launikonis, 2013). At the level of A-I junctions of myofibrils, there are dyads and triads. In other words, on longitudinal/oblique cuts a sarcomere is flanked on both sides by such dyads/triads. In skeletal muscles, triads are the sites of excitation-contraction coupling. The triads consist of t-tubules connected to two adjacent terminal cisternae of the sarcoplasmic reticulum (SR) by junctional feet proteins (Han et al., 1992). Dyads are connections of a t-tubule with just one terminal cisterna of the SR, encountered in myocardium. The junctional feet consist of calcium channels of the SR physically connected to the calcium channels of the t-tubules (Rusu et al., 2019). According to (Greising et al., 2012), there is no evidence that at the level of t-tubules are there any fiber type differences in excitation-concetration coupling. As Porter and Palade wrote in 1957, “around and among the myofibrils [.] there is a complex structure consisting of three parallel, closely apposed tubules (a canalicular triad) [.]” which “is almost continuous transversely among the myofibrils [.]” (Porter and Palade, 1957).

Structurally, the fan-shaped temporal muscle in humans has a heterogeneous architectural design (van Eijden et al., 1996). This leads to intramuscular differences in the possible production of both force and excursion range (van Eijden et al., 1996). However, to the best of our knowledge, the ultrastructure of this muscle has been poorly specifically studied.

We aimed to uncover the specific fiber arrangement in the temporal muscle in order to search for a putative pattern that might point to a functional correlation with the movements the muscle is involved in. We have chosen rats as an animal model in order to focus on the ultrastructure of the temporalis muscle. In a future step, physiological aspects such as direction and distribution of forces should indicate the correlation with the fibrils pennation.