Chapter 8 Muscle, Bone and Skin.pdf

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University of Florida
Biology - Biological Sciences
BSC 3096

Chapter 8 Muscle, Bone and Skin Monday, July 15, 2013 10:18 PM 1. Skeletal muscle a. A voluntary muscle tissue that can be controlled b. Unique in that they contract only in response to a signal from a somaticmotorneuron i. They cannot initiate their own contraction, and their contractionis not influenced by hormones c. Makes up the bulk of muscle in the body d. They position and move the skeleton e. Skeletal muscles are usually attached to bones by tendons made of collagen f. Typically, muscle stretchesacross a joint i. The origin of a muscles is the end of the muscle that is attached closest to the trunk or to the more stationary bone (the point at which it attaches to a bone) ii. The insertion of the muscle is the more distal or more mobile attachment to the bone, usually on the smaller bone that moves relative to the larger bone g. Muscles work in groups i. The agonist (the muscle responsible for movement) contracts,while a second muscle, the antagonist, stretches (relaxes) 1) ii. When the antagonist contracts, the bone movesin the opposite direction, stretching the agonist iii. Synergistic muscles assist the agonist by stabilizing the origin bone or by positioning the insertion bone during the movement h. Contraction of skeletal muscle may squeeze blood and lymph vessels aiding circulation i. Contraction of skeletal muscle also produces large amounts of heat 2. Physiology of Skeletal Muscle contraction a. The cell membrane of a muscle fiber (cell) is called the sarcolemmaand the cytoplasm is called the sarcoplasm b. The main intracellular structures in striated muscles are myofibrils,highly organized bundles of contractile and elastic pro teins that carry out the work of the contraction c. Skeletal muscles also contain extensive sarcoplasmicreticulum, a form of modified ER that wraps around each myofibril like a piece of lace i. The SR consists of longitudinal tubules, which release calcium ions, into enlarged regions at the ends of the tubules called terminal cisternae d. The terminal cisternae are adjacent to and closely associated with a branching network of transverse tubules, aka t-tubules i. ii. One t-tubule and its two flanking terminal cisternae are called a triad iii. T-tubules rapidly move action potentials from the cell surface into the interior of the fiber iv. Without t-tubules, the action potential could reach the center of the fiber only by the diffusion of positive charge around the cytosol, a slower process e. In-between myofibrils are an abundance of glycogen granules and mitochondria f. One muscle fiber (cell)contains thousands or more myofibrilsthat occupy most of the intracellular volume i. g. Each myofibril is composedof several types of proteins; i. The contractile proteins myosin and actin, the regulatory proteins tropomyosin and troponin, and the giant accessory proteins titin and nebulin h. Myosin is a motorprotein with the ability to create movement Biology Page 1 h. Myosin is a motorprotein with the ability to create movement i. Each myosin molecule is composedof protein chains that intertwine to form a long tail and a pair of tadpole heads ii. iii. The rodlike tail is tuff, but the protruding myosinheads have an elastic hinge region where the heads join the rods i. In skeletal muscle, about 250 myosin molecules join to make a thick filament i. Each thick filament is arranged so that the myosin heads are clustered at each end of the filament, and the central region is just a bundle of myosin tails j. Actin is a protein that makes up the thin filaments of the muscle fiber i. One actin molecules is a globular protein, g-actin ii. Usually g-actin molecules polymerize to form long chains or filaments, called f-actin iii. F-actin polymers twist together like a double strand of beads, creating the thin filaments of myofibril iv. k. Most of the time, the parallel thick and thin filaments of the myofibril are connected by myosincross bridges (the heads) th at span the space between the filaments i. ii. Each G-actin molecule has a single myosin-binding site, and each myosinhead has one actin-binding site and one binding site for ATP iii. Cross bridges have two states: low-force (relaxed muscle) and high-force (contracting muscle) l. Under a light microscope,the arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternati ng light and dark bands m. One repeat of the pattern is called a sarcomere i. ii. Z disks are zigzag protein structures that serve as the attachment site for thin filaments iii. I bands are the lightest color bands of the sarcomereand represent a region occupied by only thin filaments 1) A Z disk runs through the middle of every I band, so each half of an I band belongs to a different sarcomere iv. A bands are the darkest of the sarcomeresbands and encompassthe entire length of thick filament 1) At the outer edge of the A band, the thick and thin filaments overlap 2) The center of the A band is occupied by thick filaments only v. H zone is the central region of the A band that is lighter than the outer edges of the A band because the H zone is occupied by thick filaments only vi. M line is the protein line for thick filament attachment Biology Page 2 vii. viii. The proper alignment of filaments within a sarcomere is ensure by titin and nebulin 1) Titin stabilizes the position of the contractile filaments and its elasticity returns stretched muscles to their resting length 2) Titin is helped by nebulin and helps align the actin filaments 3) 3. Muscle contractions a. Events at the neuromuscular junction convert an acetylcholinesignal from a somatic motorneuron into an electrical signal in the muscle fiber b. If you examine a myofibril at its resting length, you see that within each sarcomere,the ends of the thick and thin filament s overlap slightly c. As the muscle contracts, the thick and thin filaments slide past each other, moving the Z disks of the sarcomere closer toget her i. d. As contractionoccurs, the sarcomere shortens (HI contract) e. The two Z disks at each end move closer together, while the I band and H zone, almost disappear f. Despite shortening of the sarcomere, the A band length remains constant g. The rotation of the myosin cross bridges provides the force that pushes the actin filaments during contraction i. Myosin heads bind to actin molecules ii. A calcium signal initiates the power stroke, when myosin cross bridges swivel and push the actin filaments towards the center of the sarcomere iii. At the end of each power stroke, each myosin head releases actin, then swivels back and binds to a new actin molecule, ready to start another contraction h. Myosin converts the chemical bond energy of ATP into the mechanical energy of cross bridge motion i. A calcium signal is responsible for the contractionof muscle j. In resting muscle, tropomyosinwraps around actin filaments and partially coversactin's myosin binding sites Biology Page 3 i. k. This is tropomyosinblacking or off position l. Weak, low-force actin-myosinbinding can still take place, but myosin cannot complete its power stroke m. Before contraction can occur, tropomyosinmust be shifted to the on position n. The off/on positioning is regulated by troponin o. When contraction begins, one protein of the calcium binding complex called troponin binds reversibly to Ca2+ p. The calcium -troponin complex pulls tropomyosincompletely away from the actins myosin binding site q. r. A muscle contraction beings with an action potential i. A neuron attaches to a muscle cell forming a neuromuscular synapse ii. The action potential of the neuron releases Ach into the synaptic cleft iii. The action potential travels down the T-tubules to the SR, which suddenly becomespermeable to Ca2+ ions iv. 4. A motor unit a. The muscle fibers of a single muscle do not all contract at one b. Instead, from 2 -2000 fibers spread throughout the muscle are innervated by a single neuron c. The neuron and the corresponding muscle fibers are called a motorunit d. Motorunits are independent of each other e. The force of a contracting muscle depends upon the number and size of the active motorunits, and the frequency with which th ey are fired f. Muscles requiring fine movement have smaller motorunits g. Typically smaller motorunits are the first to be activated, then larger as needed i. This allows for a smooth increase in the force generated by the muscle h. 5. Skeletal Muscle Types a. Skeletal muscle fibers are classified by speed and resistance to fatigue b. Includes slow-twitchfibers (type I), fast-twitch oxidative-glycolyticfibers(type IIA), and fast-twitch glycolyticfibers(type IIB) c. Type II muscle fibers develop tension two to three times faster than type I muscle fibers d. The speed with which a muscle fiber contracts is determined by the isoform of myosin ATPase present in the fibers thick filam ents i. Fast-twitch fibers split ATP more rapidly and can therefore complete multiple contractile cycles more rapidly than slow-twitch ii. This speed translates into faster tension development in the fast-twitch fibers e. The duration of contraction also varies according to fiber type i. Twitch duration is determined largely by how fast the SR removesCa2+ from the cytosol ii. When Ca2+ is removed,the muscle begins to relax Biology Page 4 ii. When Ca2+ is removed,the muscle begins to relax f. Another difference between the muscle types is their ability to resist fatigue i. Type IIB rely primarily on anaerobic glycolysisto produce ATP ii. The accumulation of H+ from ATP hydrolysis, however,contributes to acidosis, a condition implicated in the development of fatigue iii. As a result, glycolytic fibers fatigue more easily than to oxidative fibers, which do not depend on anaerobic metabolism g. Type I fibers have large amounts of
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