Considering functional compartments in muscle

In-series fiber architecture in long human muscles

The fiber architecture of adult human sartorius and gracilis muscles was examined using a combination of fiber microdissections and histological methods. Intact fibers were dissected from fascicles of muscle strips that were digested in nitric acid. All of these fibers terminate intrafascicularly by tapering to a fine strand at one or both ends. They measure 4-20 cm after correction for shrinkage. Systematic dissections of 1 cm long blocks sampled at intervals along the muscle length suggest that tapered fiber endings occur at all locations along the muscle but are most common centrally; here they accounted for up to 14% of dissected fibers in each block. Transverse sections of muscle confirm that fiber profiles with small diameters occur at all levels of the muscle but are especially common in sections more than 5 cm from its origin or insertion. The architectural arrangement demonstrated here suggests that long human muscles, like muscles in other species, are composed of relatively short, in-series fibers. This has many implications for the neural activation and force-developing behavior of these muscles that must be considered when paralyzed muscles are reanimated using electrical stimulation. Further, it may predispose long muscles to certain types of neuromuscular damage and dysfunction.

Interfiber tension transmission in series-fibered muscles of the cat hindlimb

Several muscles of the cat hindlimb, including biceps femoris and tenuissimus, are composed of short, in-series muscle fibers with tapered intrafascicular terminations. Tension generation and transmission within such muscles requires that active fibers should be mechanically coupled in series via myomyous junctions, specialized connective tissue attachments, or the endomysium. This report establishes that the tapered fibers of the cat biceps femoris and tenuissimus muscles have insignificant numbers of either myomyous or specialized connective tissue junctions. Tension appears to be transmitted in a distributed manner across the plasmalemma of the tapered (and probably the non-tapered) portions of the fibers to the connective tissue of the endomysium, which is therefore an essential series elastic element in these muscles. Subplasmalemmal dense plaques were identified and may play a role in transmembrane force transmission. In addition to the endomysium, passive muscle fibers may also serve to transmit tension between active fibers, and therefore should also be considered to be series elastic elements.