Muscle filament structure and muscle contraction.

Abstract

Research into the mechanism of muscle contraction has reached a very intriguing stage. It now seems likely that all types of muscle may operate in basically the same way. But despite the present detailed knowledge of the molecular arrangements in certain muscles, the precise way in which a muscle generates force is still a mystery. According to the now generally accepted sliding filament model (1, 2), muscular contraction occurs when a force is generated between two types of muscle filament: one containing the protein actin, the other containing the protein myosin. The force causes the two types of filament to slide past each other and this, in turn, results in an overall shortening of the muscle. Results from a large variety of muscle types are consistent with this sliding filament model of contraction (3-9). Because of their regularity, the muscles that have provided most of the present information on the contractile mechanism are the striated muscles, in particular vertebrate striated muscle. For this reason the present review concentrates primarily on the structure of the filaments in this type of muscle ; reference to other muscle types is made only when a comparative study demonstrates general principles involved in muscle design. Vertebrate skeletal muscle consists of many long parallel fibers (multi­ nucleate cells) some of which may extend the whole length of the muscle (Figure 1). At each end of the muscle the fibers are usually attached via a tendon to the skeleton on which the muscle acts. Each fiber comprises many long thin fibrils each about 1-2.um in diameter. In several muscles these run closely parallel to the muscle length and this makes them ideally suited to structural studies. Each fibril shows a pattern of cross-striations commonly about 2-3 .urn apart. The main features of the repeating unit (the sarcomere) contained between successive striations are shown in Figure Ie. The central region of each sarcomere contains an array of myosin filaments packed side by side in a hexagonal lattice (Figure Id). Each filament is about 1 .6j1m long and about 140 A in diameter (10, 11). Adjacent myosin filaments are joined together halfway along their length by a cross-linking structure called the M-line comprising M-protein (12). Interdigitating with the array of myosin filaments are two