Three-dimensional structure of the human temporalis muscle

The maximal force a muscle is capable of producing is proportional to its physiological cross-sectional area and its excursion range to the length of the muscle fibers. The length of the sarcomeres is a major determinant for both force and excursion range. The human temporalis muscle is an architecturally complex muscle, and little is known regarding the possible heterogeneous distribution of these parameters throughout the muscle. The objective of this study was to determine this distribution for different muscle portions and to examine the functional consequences.

In eight cadavers, sarcomere lengths, fiber lengths, and physiological cross-sectional areas were measured for the closed mouth position in six different anteroposterior portions of the temporalis muscle. To determine the spatial position of the muscle portions, the three-dimensional coordinates of attachment sites of a number of fiber bundles were registered. These parameters were used as input for a mathematical model with which sarcomere length changes and the consequences for the production of active force at different open positions of the jaw were estimated.

At the closed-jaw position, average sarcomere length ranged between 2.26 and 2.34 microns and did not differ significantly among the muscle portions. Average fiber bundle length ranged between 21.7 and 28.9 mm and differed significantly among the muscle portions. The physiological cross-sectional area ranged between 1.82 and 2.93 cm2; the smallest values were found posteriorly, and the largest values anteriorly. The line of pull of the anteriormost muscle portion was slightly inclined anteriorly and medially, whereas the posteriormost portion was relatively strongly inclined backwardly and laterally. The model predicted that during jaw open-close movements a nonuniform change in length of the sarcomeres would occur; sarcomere excursions were smaller posteriorly than anteriorly. Different muscle portions seemed to function along different parts of the active length-force relationship.

The temporalis muscle is an architecturally heterogeneous muscle. Different muscle portions are capable of producing different maximum force and excursion range, and the portions have the capability of performing different mechanical actions.