Architectural design, fiber‐type composition, and innervation of the rat rectus abdominis muscle

Abstract

The rectus abdominis muscle is architecturally compartmentalized by tendinous intersections and is supplied by multiple thoracic nerves. In this study, the rectus abdominis of the rat has been qualitatively and quantitatively examined with regard to muscle dimensions, fiber organization, fiber-type composition, and innervation. The muscle exhibits architectural heterogeneity and different patterns of innervation among its thoracic, epigastric, and hypogastric parts. The epigastric part, adherent to the rectus sheath via tendinous intersections, represents relatively simple design. It is formed by serially arranged compartments with shorter fibers, compared with the other parts. These compartments are segmentally supplied by thoracic nerves. The hypogastric part is more complex, forms an interdigitation of muscular slips, and has segmental distribution of thoracic nerves in mediolateral direction. The thoracic part much differs from the other parts. It has smaller cross-sectional areas, compartments composed of abundant nonspanning fibers with intrafascicular termination, and non-segmental distribution of thoracic nerves. In addition to these craniocaudal specializations among the three parts, the muscle exhibits mediolateral differences in fiber-type composition. Slow-twitch oxidative fibers are more densely distributed in the medial half region than the lateral, whereas fast-twitch glycolytic fibers follow an inverse pattern. The mediolateral differences in fiber-type composition as well as the craniocaudal specializations in architectural design and innervation imply regionally differentiated recruitments of the muscle in various behaviors.

In vivo and in vitro Examination of the Tendinous Inscription of the Human Semitendinosus Muscle

“…Particularly, some neck and trunk muscles display multiple long regions with in-series fibers which display multiple attachments to vertebrae or ribs, each innervated by a different bundle of motor axons [Krogh and Towns, 1984;Richmond and Armstrong, 1988;Hijikata et al, 1992;Herring et al, 1993]. This arrangement may allow integration of forces exerted by multiple regions as the muscles contract.…”

“…Although there is not enough information on the mechanical role of the TI for human ST muscle function, evidence from other muscles of the human body underlies the importance of compartmentalization for muscle function [Krogh and Towns, 1984;Richmond and Armstrong, 1988;Hijikata et al, 1992;Herring et al, 1993;Kamibayahsi and Richmond, 1998;Brown and McGill, 2009]. Particularly, some neck and trunk muscles display multiple long regions with in-series fibers which display multiple attachments to vertebrae or ribs, each innervated by a different bundle of motor axons [Krogh and Towns, 1984;Richmond and Armstrong, 1988;Hijikata et al, 1992;Herring et al, 1993].…”

Functional combination of tapering profiles and overlapping arrangements in nonspanning skeletal muscle fibers terminating intrafascicularly

“…Diagrams on the right of the thoracic part, as our previous studies predicted (see Hijikata et al, 1992). The one-to-two correspondence in the fiber connection makes it impossible for a similar CSA to exist between overlapping and non-overlapping portions.…”

Morphometry and Histochemistry of the Rectus abdominis Muscle Fibers of Tufted Capuchin Monkeys (Cebus apella Linnaeus, 1758)

“…In terms of compartimentalization, Hijikata et al (1992) observed that in the rectus abdominis muscle of the rat, the percentage of SO fibers increases in a lateral to middle direction, whereas the opposite occurs for the frequency of FG fibers. In the rectus abdominis muscle of the tufted capuchin monkey, it was observed that both frequency and area of FG fibers increase in a proximal to distal direction, but not with a statistically significant difference.…”