Muscle structure sarcomeres

Figure 49-1. The structure of voluntary muscle. The sarcomere is the region between the...

Clearly there is a still much to learn about the contractile mechanism, force production, the strain dependence of the crossbridge cycle and, indeed, how force and movement are actually generated. As shown in Squire et al., Granzier and Labeit, Brown and Cohen, and Geeves and Holmes (all in this volume), and in this article, a great deal is already known about muscle structure and function. However, there is also little doubt that the sarcomere still has some major surprises in store for us. 

The structure of the contractile unit is fundamental to the interpretation of the relationship between muscle length and isometric tension in skeletal muscle. Each sarcomere unit contains bipolar thick filaments with a central bare zone surrounded by pairs of actin filaments of opposite polarity. The actin filaments are anchored at Z-bands that are aligned in parallel. The length-tension behavior of skeletal muscle has been interpreted in terms of changes in the overlap between the thick eind thin filaments as... 

Muscle force generation is believed to arise from the formation of crossbridge bonds between thick and thin myofilaments within the basic building block of muscle, the sarcomere. These structures, in the nanometer to micrometer range, must be viewed by electron microscopy or x-ray diffraction, limiting study to fixed, dead material. Consequently, muscle contraction at the sarcomere level must be described by models that integrate metabolic and structural information. 

Smooth muscles, as the name implies, do not contain sarcomeres. In fact, it was initially difficult to demonstrate the presence of thick filaments in smooth muscle, although their presence is now well-established. On the other hand, it is very difficult to demonstrate thick filaments in highly motile cells, such as macrophages and neutrophils, and this may reflect the necessity to rapidly form and redistribute cytoskeletal elements during migration. Thick filaments in smooth muscles appear to be considerably longer than those in striated muscles. They run diagonally in smooth muscle cells and attach to the membrane at structures known as dense bodies. Thus, there is a cork-screw effect when smooth muscles contract (Warshaw etal., 1987). 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

Figure 3. Structure of a muscle sarcomere. In a polarizing microscope muscle appears to have dark (A) and light (I) bands. The l-band region only contains thin filaments. The A-band region contains both thick and thin filaments. One sarcomere is the distance between two Z-lines. In cross section, the hexagonal packing of the thick and thin filaments can be seen.

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Because there are no sarcomeres in smooth muscle, there are no Z lines. Instead, the actin filaments are attached to dense bodies. These structures, which contain the same protein as Z lines, are positioned throughout the cytoplasm of the smooth muscle cell as well as attached to the internal surface of the plasma membrane. Myosin filaments are associated with the actin filaments, forming contractile bundles oriented in a diagonal manner. This arrangement forms a diamond-shaped lattice of contractile elements throughout the cytoplasm. Consequently, the interaction of actin and myosin during contraction causes the cell to become shorter and wider. 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Fig. 4. Electron myograph of skeletal muscle. The band structure caused by the sarcomeres, two capillaries (1) and several small lipid droplets (2) are shown. The intramyocellular lipid droplets (size approximately 0.1-1 pm) make up the IMCL content mentioned in the context.

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