Skeletal contractile apparatus

The structure of the contractile apparatus of smooth muscle at the next higher level is also characteristically different from other muscles. The concentrations of actin and myosin in smooth muscle are about three times higher for actin and four times lower for myosin than in skeletal muscle. Correspondingly, in smooth muscle the ratio of the numbers of moles of actin to moles of myosin, and the ratio of the number of actin filaments to those of myosin filaments, are about 12 times larger than for other muscles. Thus, the arrangements of the two sets of filaments are bound to be quite different just on the basis of numbers of actin and myosin... 

The compliance in series with the active force. Force exerted by the activated elements must be transmitted or borne by whatever structural elements are in series with them. In skeletal muscle there is clearly a tendon in series but not so with smooth muscle. In smooth muscle, the total length of contractile apparatus is broken up into individual cells with intercalating extracellular connective structures. In addition, the portions of the crossbridges in series with the pulling site must also be stretched before force can rise to isometric levels. Taken together, the... 

Smooth muscle cells are small and spindle shaped (thin and elongated see Table 12.1). Similar to skeletal muscle, the contractile apparatus in smooth... 

Skeletal muscle relaxation and paralysis can occur from interruption of function at several sites along the pathway from the central nervous system (CNS) to myelinated somatic nerves, unmyelinated motor nerve terminals, nicotinic acetylcholine receptors, the motor end plate, the muscle membrane, and the intracellular muscular contractile apparatus itself. 

Focal adhesions (in muscle often referred to as costameres) are regions that are associated with the sarcolemma of skeletal muscle fibres and comprise proteins of the dystrophin-glycoprotein complex and vinculin-talin-integrin system. Focal adhesions play both a mechanical and a signalling role, transmitting force from the contractile apparatus to the extracellular matrix in order to stabilise skeletal-muscle fibres during contraction and relaxation. Several focal adhesion constituent proteins have been shown to be defective in muscular dystrophies and cardiomyopathies. 

The role of a particular myosin in vivo is related to its tail domain. For example, the tail domains of myosins I, V, VI, and XI bind the plasma membrane or the membranes of intracellular organelles as a result, these molecules have membrane-related activities (Figure 19-16a). In contrast, the coiled-coil tail domains of myosin II dimers associate to form bipolar thick filaments in which the heads are located at both ends of the filament and are separated by a central bare zone devoid of heads (Figure 19-16b). The close packing of myosin molecules into thick filaments, which are a critical part of the contractile apparatus in skeletal muscle, allows many myosin head domains to interact simultaneously with actin filaments. 

Skeletal muscles contract when depolarized by neural signals transmitted at the neuromuscular junction. Each muscle fiber has one synaptic connection, but each motor neuron typically branches and controls several muscle fibers. The larger and more powerful muscles are controlled by neurons branched many times. The contractile apparatus composed of motor neuron and all the fibers it controls is called a motor unit. 

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