CHAPTER 14 MUSCLE CONTRACTION t is impossible to overemphasize the same way. For example, skeletal muscles of importance of muscle in vertebrates. vertebrates all appear to initiate contractions IThe very life style of every one demands with sodium spikes, whereas striated movement, impossible without muscle. In muscles of some invertebrates initiate fact, in man about 40% of the body mass is contractions with calcium spikes. We will striated muscle, making it the most abundant confine our discussion primarily to tissue. Striated muscle is so named vertebrate skeletal muscle, pointing out the because of its characteristic cross-striped distinctive features of structure and function appearance. Most striated muscle is skeletal of cardiac and smooth muscle. muscle, involved in rotation of bones around joints and therefore responsible for most of the movements of which we are aware. Other striated muscles move the eyes and serve as valves to check the flow of blood or other fluids, e.g., the bulbospongiosus aids erection of the penis or clitoris by compressing the deep dorsal vein. Cardiac muscle is also striated in appearance, but it differs significantly from other striated muscle in both its structure and its behavior. Still other muscles, called smooth muscles, lack the characteristic cross-striations, but Figure 14-1. Two arrangements of muscle contain the same contractile proteins. The fibers within a muscle. A. Parallel smooth muscles are important as linings of arrangement: Tendons are lines radiating the gastrointestinal tract that churn and from rectangles (muscle fibers) at each end. B. Pennate arrangement: Tendons are propel food through the tract, as linings of vertical lines extending from the two sides blood vessels that control their diameters of the parallelogram. Double headed and thus flow through them, as valves that arrows (f) indicate direction of force exerted by individual muscle fibers; control the passage of gases and fluids in the single-headed arrows (F) indicate direction body, and as controllers at many other places of force exerted by whole muscle. (Zierler in the body. KL: Mechansims of muscle contraction and its energetics. In: Mountcastle VB [ed]: Of the three types of muscle, skeletal and Medical Physiology. 13th ed, Vol. 1. St. cardiac muscle have been studied most Louis, C.V. Mosby, 1974). thoroughly. It is presumed that the mechanism of contraction is the same for both types and only the details of initiating and controlling the contraction differ. Not Muscle structure. all striated muscle, however, behaves in the Skeletal muscles are composed of masses 14-1 of fibers, each an individual cell. There are several types of muscles, each with different arrangements of fibers, but these can be divided into two major classes: those with fibers arranged in parallel and those with a pennate arrangement. Figure 14-1 shows these two classes. In the parallel arrangement (A), each muscle fiber, or a small group of fibers, is attached to its own tendon, the tendons converging on a common point1. The muscle fibers are side- by-side, i.e., in parallel, but the name of the class comes from the fact that the muscle fibers shorten in a direction (double headed arrow, f) parallel to the direction of shortening of the muscle (single-headed arrow, F). The pennate muscle fibers (B) attach to a common tendon, so that the direction of shortening of the individual fibers (double- headed arrow, f) is different from the direction of shortening of the whole muscle (single-headed arrow, F). As a result, the pennate muscle cannot shorten as much as the parallel muscle. Pennate muscles are located in positions requiring small but powerful movements; parallel muscles are located in positions requiring longer movements with less power or faster movements. 1 In Fig. 14-1, muscle fibers are rectangles or parallelograms, tendons are lines radiating from the muscle fibers or vertical lines extending from the two sides of muscle fibers, arrows labeled f indicate direction of force exerted by fibers, and arrows labeled F indicate direction of force exerted by the whole muscle. 14-2 Figure 14-2 - Levels of organization within a skeletal muscle, including (counterclockwise from top left) whole muscle and fascicles, bundles of muscle fibers, myofibrils, thin and thick filaments, and myosin and actin molecules. (Warwick R, Williams PL [ed]: Grays' Anatomy. 35th British ed, Edinburgh, Churchill Livingston, 1973; modified from a drawing by Professor D Fawcett) Muscles, fibrils and filaments. To in register (the same stripes are lined up). understand how a muscle works it is The myofibrils are striated because the necessary to understand the fine-structure of myofilaments are not homogeneously muscle cells because it is the internal parts distributed within them, but rather occur in of the cells that do the work. The relevant regular, repeating arrays. internal structures are the myofibrils, the Myofibrils. Figure 14-2 shows, on the left, myofilaments and the sarcoplasmic the whole muscle, a bundle of muscle fibers, reticulum. Muscles are composed of muscle and their subunits, the myofibrils. Note the fibers; fibers are composed (in part) of striated appearance of all three. Each myofibrils; and myofibrils are composed of muscle fiber contains about 1000 myofibrils myofilaments. Skeletal muscles have a that are 1 :m in diameter and run the length characteristic striated appearance because of the fiber. Myofibrils have no membrane, the myofibrils are characteristically striated being simply surrounded with cytoplasm. and because the myofibrils are more or less The cross-striations of the myofibrils are 14-3 serially repeating units called sarcomeres. subunits projecting out at approximately A sarcomere can be from 1.5-3.5 :m in right angles with the filament. The structure length, depending upon the contractile state has been likened to two golf clubs with their of the muscle, and it is bounded on each end shafts twisted together. Drawings of a by a disc, called the Z disc or Z line. Each myosin molecule, and its position within the sarcomere contains an anisotropic (doubly thick filaments are shown in Figure 14-2. refractive, therefore dark in phase micro- The myosin molecules of thick filaments are scopy) band bounded by two isotropic arranged in a sheaf with heads oriented (singly refractive, therefore light) bands. toward each end and tails toward the center. The anisotropic band is called the A band; Each subsequent myosin molecule attaches the isotropic band is called the I band. 14 nm further toward the end of the Actually, each sarcomere contains two half-I filament, and its head is rotated 60° around bands (one at each end) because a single I the filament from its predecessor. Thus, the band straddles the Z line and therefore is thick filament is studded with projections part of two adjacent sarcomeres. In the except at its center, which contains only center of the A band, there is a lighter region myosin tails. Note that myosin molecules at known as the H zone or H band. During opposite ends of the thick filament are contraction the A band does not change oriented in opposite directions–sort of like a length2, though the sarcomere shortens, the bundle of unsorted golf clubs, some with distance between Z lines lessens, and the I heads at the right end, some with heads on and H bands narrow. Any theory of muscle the left. The thick filaments are coincident contraction must account for these with the A band of the sarcomere. observations. The myofibrils, as shown in Figure 14-2, are composed of proteinaceous structures called myofilaments. One filament is thick, about 11 nm in diameter and 1.5 :m long, whereas the other is thin, 5 nm in diameter and 1 :m long. These filaments are referred to as the thick filaments and thin filaments, respectively. Thick filaments are made up of several hundred myosin molecules, proteins of a molecular weight of about 500,000, and some other minor proteins whose function is unknown. The myosin molecule has a tail region that is rodlike, and head region, with two globular 2 Actually, it is generally accepted that in Figure 14-3. Organization of the sarcomeres. A. Pattern of Limulus, the horseshoe crab, the A bands do cross-striation in skeletal muscle with bands labeled. B. change length when contractions occur at Arrangement of thick and thin filaments that accounts for the pattern of cross-striations. C. Hexagonal arrays of thick and lengths less than the resting length. They thin filaments in cross sections through the sarcomere in the apparently do not in mammalian muscle. A band, H band and I band. 14-4 Each thin filament contains three protein 3, the thin filaments are organized into molecules: actin, troponin, and regular hexagonal arrays within the tropomyosin. A single thin filament is myofibrils, with a thick filament at the composed of 300 to 400 actin molecules and center of each array in the A band. Three 40 to 60 troponin and tropomyosin thick filaments are equidistant from each molecules. Actin is a small, nearly spherical thin filament, whereas six thin filaments are molecule that is arranged in the filament into equidistant from each thick filament as two helical strands, as shown in Figure 14-2, shown in the left panel. A cross section with about 13 actin molecules per complete through the I band shows only the thin turn of the helix. Troponin and tropomyosin filament array; a section through the H band are sometimes called regulator proteins shows only the thick filament array (plus the because of their central role in regulating slender, thread-like processes muscle contraction. Tropomyosin is a interconnection thin filaments). filamentous protein that is thought to form two strands that lie in the grooves formed between the actin strands.