Many mammalian muscles have a complex internal architecture. This type of structure could allow a single muscle to produce a variety of force vectors through selective regional contractions. This hypothesis was tested electromyographically in the multipinnate pig masseter by recording simultaneously from several intramuscular sites. It was found that the activity in different portions of the masseter varied systematically during the various phases of mastication. anatomical correlates of the differential activity included fasciculus orientation and length, sarcomere length in specific jaw positions, and histochemical fiber type. The usual assumptions made about muscles for biomechanical analysis, such as uniform contraction and constant line of action, are inappropriate for complex muscles such as the pig masseter.

Sarcomere lengths from 16 locations in the muscles of mastication were measured from pigs fixed in maximum excursion (wide jaw opening) and in postural position (near occlusion). These data, plus published data on sarcomere lengths in rabbit jaw muscles, were used to evaluate conflicting hypotheses about the factors which regulate serial sarcomere number in striated muscles. According to the most successful hypothesis, sarcomere number is adjusted so as to achieve an optimum sarcomere length when the muscle is experiencing a high level of tension. Most often, this occurs at jaw positions where the muscle is electrically active.

Sarcomere lengths were measured after pentobarbital anesthesia at five sites through the wall of the formaldehyde solution-fixed cadmium-arrested closed-chest rat left ventricle. Sections (250 micron) were cut from endocardium to epicardium with a freezing microtome. Selected sections were sonified, mixed with a gelatin-water solution, and placed on a glass slide. Sarcomere lengths were measured with an optical microscope at five sites through the wall. Sarcomere lengths progressively increased from section I (endocardium site) to section III (middle site) and IV. Sarcomere lengths were again shorter in section V (epicardium site). There was a progressive increase in sarcomere lengths with increasing intraventricular pressures. Sarcomere lengths did not significantly exceed optimum length.

Left ventricular dimensions were measured in Cd2+ arrested (presumably diastolic), open-chest rats. Aortic pressure was maintained at 137 cm H2O (100 mm Hg) and left-ventricular (luminal) pressures were established and maintained at their chosen values, each by means of reservoir systems. The selected left-ventricular pressures were chosen to be within or to even broaden the range of conceivable diastolic pressures (-3 to 48 cm H2O). After in situ fixation with 4% formaldehyde and gelatin embedding, the hearts were serially sectioned in the apex base direction to obtain information at 11 levels (10, 20, . . . 90, 100%). Tracings of selected sections were made along the edge of the left ventricular lumen and the pericardial surface. Volumes, surface areas, and mean external and internal radii of the left ventricle were derived. To quantify the circularity of sections a form factor (FF) was introduced (FF = 1 for a circular cross-section and less than one for other shapes). Ventricular lengths, radial dimensions, endocardial and epicardial surface areas, and total and luminal volumes increased with the increasing intraventricular pressures; as expected, the wall simultaneously thinned. Though its appearance was altered by the wall thinning, the curving muscle fascicular pattern was present over the entire pressure range examined. Endocardial surface areas increased more than did the epicardial surface areas. The endocardial FF value increased (more circular) at each section level as the pressure increased. The epicardial FF relationship was apparently constant (0.798 +/- 0.014) for all section levels from 10% through 90%, regardless of luminal pressure. These results, when taken in conjunction with the results of our previous published studies, prompted the following speculation. The wall of the diastolic ventricle is a fluid-filled chamber with intramyocardial pressures that may be higher than ventricular pressures.

Sarcomere lengths were measured with an optical microscope at five sites through the free wall of the open-chest rat left ventricle. After pentobarbital anesthesia the hearts were arrested with 5 mM cadmium chloride and 0.9% saline and fixed with formaldehyde solution. Serial sections (250 microns thick) were cut from endocardium to epicardium with a freezing microtome. Selected sections were sonified, mixed with a gelatin-water solution, and placed on a glass slide. There was a progressive increase in mean sarcomere lengths with increasing intraventricular pressures though the sarcomeres did not significantly exceed their optimum length. There was a distinct and statistically significant difference in the pattern of sarcomere lengths through the ventricular wall between our previous closed-chest study and the present open-chest study. In this open-chest animals, there was an almost linear pattern of increasing sarcomere lengths from endocardium to epicardium over the range of semiphysiologic diastolic intraventricular pressures (6, 12, and 24 cm H2O -- 0.59, 1.18, 2.35 kPa). These results appear to caution against extrapolations on ventricular function derived from open chest studies to the normal physiologic conditions.