Understanding Muscle Contraction: The Role of Sarcomeres and Motor Units, Study notes of Physiology

Download Understanding Muscle Contraction: The Role of Sarcomeres and Motor Units and more Study notes Physiology in PDF only on Docsity! 1 The Sarcomere The sarcomere is the functional unit of the contractile system in muscle, and the events that take place in one sarcomere are duplicated in the others. Various sarcomere build a myo- fi bril, myofi brils build the muscle fi ber, and muscle fi bers build a muscle. The sarcomere is composed of thin fi laments c h a p t e r 5 Impaired Muscle Performance LORI THEIN BRODY AND CARRIE M. HALL MORPHOLOGY AND PHYSIOLOGY OF MUSCLE PERFORMANCE Improving muscle performance often translates into improvements in functioning by the patient. A thorough understanding of muscle morphology and physiology is required to prescribe an appropriate exercise program that proceeds to the ultimate goal of a functional outcome for each patient. Gross Structure of Skeletal Muscle Each of more than 430 voluntary muscles in the body consists of various layers of connective tissue. Figure 5-1 illustrates a cross section of a muscle consisting of thousands of muscle cells called muscle fi bers (Fig. 5-1A and B). These multinu- cleated muscle fi bers lie parallel to one another and are sepa- rated by the innermost layer of connective tissue, called the endomysium. As many as 150 fi bers are arranged into bundles called fasciculi. Fasciculi are surrounded by perimysium, the next layer of connective tissue. The entire muscle is encased by the outermost layer of connective tissue, the epimysium. This connective tissue sheath tapers at the ends as it blends into and joins the intramuscular tissue sheaths forming the tendons. The tendons connect to the outermost covering of the bone, the periosteum. The force of muscle contraction is transmitted directly from the muscle’s connective tissue to the point of attachment on the bone. Beneath the endomysium and surrounding each muscle fiber is a thin, elastic membrane, called the sarcolemma, enclosing the fiber’s cellular contents. The aqueous pro- toplasm or sarcoplasm contains the contractile proteins, enzymes, fat and glycogen particles, the nuclei, and various specialized cellular organelles. Embedded in the sarcoplasm is an extensive network of interconnecting tubular channels known as the sarcoplasmic reticulum. This highly specialized system provides the cell with structural integrity and also serves important functions in muscular contractions. Ultrastructure of Skeletal Muscle The ultrastructure of skeletal muscle consists of different levels of subcellular organization (see Fig. 5-1). Each muscle fi ber consists of small fi bers called myofi brils (see Fig. 5-1C). Myofi brils are composed of even smaller threads called myo- fi laments (see Fig. 5-1D–F). The myofi laments are composed primarily of two proteins, actin and myosin. Six other proteins have been identifi ed that have a structural or physiologic pur- pose. The contractile unit of the entire myofi bril is known as the sarcomere. Muscle Myofibril A. B. C. D. E. F. Sarcomere Thick filament: myosin Cross section at level of A-band Myosin filament Actin filament Thin filament: actin Sarcomere A I myofibril z = zline A = A-band I = I-band A-band I-band Z Z Muscle fibers with capillaries FIGURE 5-1. Schematic drawing of the structural organization of skel- etal muscle. (A) A fibrous connective tissue, epimysium surrounds the muscle, which is composed of many fascicles. The fascicles are encased in a dense connective sheath, the perimysium. (B) The fascicles are com- posed of muscle fibers surrounded by capillaries and covered by a loose connective tissue called the endomysium. Each muscle fiber is composed of numerous myofibrils. (C) Myofibrils consist of smaller filaments that form a repeating banding pattern along the length of the myofibril. One unit of this serially repeating pattern is called a sarcomere. (D–F) The banding pattern of the sarcomere is formed by the organization of thick and thin filaments, actin and myosin. HALL_Chap05_Webpages.indd 1 6/22/2010 8:20:45 PM 2 Therapeutic Exercise: Moving Toward Function (approximately 5 nm in diameter) formulated from the pro- tein actin and the thick fi laments (approximately 15 nm in diameter) formulated from the protein myosin. Figure 5-1C illustrates the structural pattern of myofila- ments within a sarcomere. The lighter area is referred to as the I band and contains the portion of the thin filaments that do not overlap with the thick filaments. The darker zone is known as the A band and is the region where actin and myo- sin overlap. The Z line bisects the I band and adheres to the sarcolemma to give the entire structure stability. The repeat- ing unit between two Z lines represents the sarcomere. The actin and myosin filaments within the sarcomere are primarily involved in the mechanical process of muscular contraction and therefore in force development. Each myosin cross- bridge is an independent force generator. Actin-Myosin Orientation Figure 5-2 illustrates the actin-myosin orientation within a sarcomere at resting and contracted lengths. Actin, the chief component of the thin fi lament, has the shape of a double helix and appears as two strands of beads spiraling around each other. Two additional proteins, troponin and tropomyosin, are important constituents of the actin helix because they appear to regulate the making and breaking of contacts between the actin and myosin fi laments during contraction. Tropomyosin is a long polypeptide chain that lies in the grooves between the helices of actin. Troponin is a globular molecule attached at regular intervals to the tropomyosin (Fig. 5-3). Intracellular Tubule System The sarcoplasmic reticulum and transverse tubule (T-tubule) system within the muscle fi ber can be seen in Fig. 5-4. The sarcoplasmic reticulum lies parallel to the myofi brils, whereas the T-tubule system runs perpendicular to the myofi bril. The lateral end of the sarcoplasmic reticulum terminates in a saclike vesicle that stores calcium. The T-tubule system appears to function as a microtransportation network for spreading the action potential (i.e., wave of depolarization) from the fi ber’s outer membrane inward to the deep regions of the cell. Chemical and Mechanical Events During Contraction and Relaxation The most widely held theory of muscle contraction is the slid- ing fi lament theory. According to this theory, active shorten- ing of the sarcomere, and hence of muscle, results from the relative movement of the actin and myosin fi laments past one another while retaining its original length. Excitation-contraction is the physiologic mechanism whereby an electric discharge at the muscle initiates the chemical events that lead to contraction. When a muscle fiber is stimulated to contract, there is an immediate increase in the intracellular calcium concentration. Arrival of the action potential at the T-tubules causes calcium to be released from the lateral sacs of the sarcoplasmic reticulum. The inhibitory action of troponin (i.e., preventing actin-myosin interaction) ceases when calcium ions bind rapidly with troponin in the actin filaments. The globular head of the myosin cross-bridge provides the mechanical means for the actin and myosin filaments to slide past each other. During contraction, each cross-bridge undergoes many repeated but independent cycles of movement. Thus at any given moment only approxi- mately half of the cross-bridges actively generate force and displacement, and when these detach, others take up the task so that shortening is maintained. Display 5-1 summarizes the events during excitation, contraction, and relaxation of the muscle. Muscle Fiber Type Skeletal muscle is not a simple homogenous group of fi bers with similar metabolic and functional properties. Distinct fi ber types have been identifi ed and classifi ed by their Sarcomere relaxed Contraction Thick filament (myosin) Thin filament (actin) Sarcomere contracted FIGURE 5-2. Actin-myosin relationships in relaxed and contracted position. Troponin Tropomyosin Actin filament FIGURE 5-3. The relationship of actin, troponin, and tropomyosin. Myofibrils SarcolemmaT-tubules T-tubule Sarcoplasmic reticulum FIGURE 5-4. Relationships of the sarcoplasmic reticulum, T-tubule system, and myofibrils. HALL_Chap05_Webpages.indd 2 6/22/2010 8:20:46 PM