The Relationship Between Myofilament Packing Density and Sarcomere Length in Frog Striated Muscle

Abstract

In cross-sections of single fibers from the frog semitendinosus muscle the number of thick myofilaments per unit area (packing density) is a direct function of the sarcomere length. Our data, derived from electron microscopic studies, fit well with other data derived from in vivo, low-angle X-ray diffraction studies of whole semitendinosus muscles. The data are consistent with the assumption that the sarcomere of a fibril maintains a constant volume during changes in sarcomere length. The myofilament lattice, therefore, expands as the sarcomere shortens. Since the distance between adjacent myofilaments is an inverse square root function of sarcomere length, the interaction of the thick and the thin myofilaments during sarcomere shortening may occur over distances which increase 70 A or more. The "expanding-sarcomere, sliding-filament" model of sarcomere shortening is discussed in terms of the current concepts of muscle architecture and contraction.

Disproportionate loss of thin filaments in human soleus muscle after 17-day bed rest

“…Brandt et al demonstrated in frog striated muscle fibers that density is directly proportional to sarcomere length, and spacing is inversely proportional to the square root of sarcomere length. 5,17 Similar relationships were observed for human muscle fibers after plotting density and spacing values against sarcomere length (Fig. 5).…”

“…It has been shown previously for frog and rabbit muscles that the packing density (number per unit area) of thick and thin filaments is directly altered by sarcomere length, i.e., higher density and closer spacing (interfilament distance) result as sarcomeres are lengthened, and packing density decreases in shortened sarcomeres. 5,17 To determine whether packing density in human muscle fibers is directly related to sarcomere length, the correlation of thick filament spacing and sarcomere length was assessed by linear regression.…”

Myofilament spacing and force generation in intact frog muscle fibres.

“…This implies that both the number of active cross-bridges and the force per cross-bridge are unaffected by the decrease in filament separation MS 8083 which occurs when an intact fibre is stretched (Huxley, 1953;Elliott, Lowy & Worthington, 1963;Elliott, Lowy & Millmann, 1965, 1967Brandt, Lopez, Reuben & Grundfest, 1967;Elliott, 1967;April, Brandt & Elliott, 1971). Indeed, several experimental results in the literature can be taken to support this view.…”

Historical perspective on heart function: the Frank–Starling Law

“…Alterations in interfilament lattice spacing, such as increases in the lattice spacing as a result of shortening, leads to sarcomere thickening and intracellular volume redistribution (Elliott et al 1963(Elliott et al , 1967Brandt et al 1967). In turn, the developed active tension decreases via decreased approximation of myosin and actin filaments, and thus less strong binding cross-bridges are formed (Rome 1972;Matsubara and Millman 1974b).…”