Muscle contractile units

Many factors influence the contractile activity of smooth muscle. The strength of contraction of multiunit smooth muscle may be enhanced by stimulation of a greater number of cells, or contractile units. This mechanism is directly comparable to motor-unit recruitment employed by skeletal muscle. As the number of contracting muscle cells increases, so does the strength of contraction. However, this mechanism is of no value in single-unit smooth muscle. Due to the presence of gap junctions, all of the muscle cells in the tissue are activated at once. 

Figure 1.12 Diagrammatic interpretation of contraction in a myo-fibril of skeletal muscle. The diagram shows a single sarcomere, the basic contractile unit, limited at each end by a Z-disc. Muscle fibres are packed with hundreds of parallel myofibrils, each of which consists of many, often thousands, of sarcomeres arranged end to end. Contraction is the conseguence of the thin actin filaments being pulled over the thick filaments to increase the region of overlap and telescope the sarcomere.

The striation of the muscle fibers is characteristic of skeletal muscle. It results from the regular arrangement of molecules of differing density. The repeating contractile units, the sarcomeres, are bounded by Z lines from which thin filaments of F-actin (see p. 204) extend on each side. In the A bands, there are also thick parallel filaments of myosin. The H bands in the middle of the A bands only contain myosin, while only actin is found on each size of the Z lines. 

D. Filament Organization in the Contractile Units of Different Muscle Types... 

The thick filaments and the thin filaments of a myofibril partly interdigitatc with six thin filaments surrounding each thick filament. A single contractile unit, the sarcomere. is depicted in Fig. 5-32 and is about 1.5 yum in diameter and 2.2 /nm long. A linear arrangement of sarcomeres forms the myofibril, which, along with other myofibrils packed in parallel, can run the length of the muscle cell (up to several centimeters). 

The skeletal muscle cell contains two types of filaments, thin and thick. The basic contractile unit of the muscle might be considered to be a thick filament surrounded by thin filaments, as shown in Figure 10.46. The sarcomere is the name of this unit, This basic unit is 23 pm long. The skeletal muscle cell may be about 500 nun long and 50 pm wide. EKiring contraction, the thin and thick filaments slide past each other at about 15 pm/sec. 

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... 

FIGURE 22.3 Contractile apparatus in cells, (a) A sarcomere is ahighly organized contractile unit in muscle cells that uses actin thin filaments and myosin thick filaments to shorten the length of a muscle, (b) A stress fiber is a loosely organized contractile structure in nonmuscle cells that is essential for migration and contraction. 

The hundreds of small contractile units found in each muscle fiber. Myofibrils are composed of the muscle protein filaments, myosin and actin. 

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction. 

Myoepithelial cells are contractile elements found in salivary, sweat, and mammary glands that show a combined smooth muscle and epithelial phenotype (Foschini et al. 2000). In the normal breast, the ductal and acinar units are lined by two cell layers the inner layer lining the lumen and an outer layer of myoepithelial cells. An intact myoepithelial cells layer is seen in both benign and in situ lesions, whereas loss of the myoepithelial cells layer is considered the rule for the diagnosis of invasive cancer (Kalof et al. 2004 and citations therein). 

The anatomical unit of muscle is an elongated cell called a fibre. Each individual fibre cell consists of myofibrils which are bundles of contractile protein filaments composed of actin and myosin (Figure 7.1). Differences in structure indicate that muscles have evolved to perform particular functions. Although the structure of fibres, myofibrils and filaments of actin and myosin, is similar in all muscle types, their arrangement, action and control allow identification of three tissue types ... 

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