MUSCLE-FIBER ARCHITECTURE OF THE DOG DIAPHRAGM

Previous measurements of muscle thickness and length ratio of costal d iaphragm insertions in the dog (A. M. Boriek and J. R. Rodarte. J. App l. Physiol. 77: 2065-2070, 1994) suggested, but did not prove, discont inuous muscle fiber architecture. We examined diaphragmatic muscle fib er architecture using morphological and histochemical methods. In 15 m ongrel dogs, transverse sections along the length of the muscle fibers were analyzed morphometrically at x20, by using the BioQuant System I V software. We measured fiber diameters, cross-sectional fiber shapes, and cross-sectional area distributions of fibers. We also determined numbers of muscle fibers per cross-sectional area and ratio of connect ive tissue to muscle fibers along a course of the muscle from near the chest wall (CW) to near the central tendon (CT) for midcostal left an d right hemidiaphragms, as well as ventral, middle, and dorsal regions of the left costal hemidiaphragm. In six other mongrel dogs, the macr oscopic distribution of neuromuscular junctions (NMJ) on thoracic and abdominal diaphragm surfaces was determined by staining the intact dia phragmatic muscle for acetylcholinesterase activity. The average major diameter of muscle fibers was significantly smaller, and the number o f fibers was significantly larger midspan between CT and CW than near the insertions. The ratio of connective tissues to muscle fibers was l argest at CW compared with other regions along the length of the muscl e. The diaphragm is transversely crossed by multiple scattered NMJ ban ds with fairly regular intervals offset in adjacent strips. Muscle fas cicles traverse two to five NMJ, consistent with fibers that do not sp an the entire fascicle from CT to CW. These results suggest that the d iaphragm has a discontinuous fiber architecture in which contractile f orces may be transmitted among the muscle fibers through the connectiv e tissue adjacent to the fibers.