BSCI440 CHAPTER 12 NOTES for Exam 1 Muscles I Skeletal Muscle Skeletal muscles are composed of muscle fibers muscle cells Each skeletal muscle fiber is a long cylindrical cell with up to several hundred nuclei largest cell in the body Muscle Fiber Anatomy Sarcolemma the cell membrane of a muscle fiber Sarcoplasm the cytoplasm Myofibrils main intracellular structures in striated muscle highly organized bundles of contractile and elastic proteins that carry out the work of contraction Sarcoplasmic reticulum SR modified EP that wraps around each myofibril consists of longitudinal tubules with enlarged end regions called terminal cisternae Concentrates and sequesters Ca2 with the help of Ca2 ATPase in the SR membrane Ca2 release plays a role in contraction of muscles Transverse tubules a branching network with membranes that are a continuation of the muscle fiber membrane Allow A P s to move rapidly from the cell surface into the interior of the fiber so they reach the terminal cisternae simultaneously Myofibrils Are Muscle Fiber Contractile Structures Each myofibril is composed of several types of proteins the contractile proteins myosin and actin the regulatory proteins tropomyosin and troponin and the giant accessory proteins titin and nebulin Myosin motor protein with the ability to create movement composed of protein chains that form a long tail and two heads tail and heads joined by a hinge region that allows head to swivel around attachment point Multiple myosins form a thick filament Actin protein that makes up the thin filament Contains myosin head binding site Parallel thick and thin filaments are connected by myosin crossbridges Sarcomere Z disks I bands A band H zone M Line Z disk is attachment site for thin filaments I bands represent region occupied by only thin filaments A band encompasses the entire length of a thick filament H zone is the central region of the A band that is occupied by thick filaments only M line represents proteins that form the attachment site for thick filaments Proper alignment of filaments within sarcomere is ensured by titin and nebulin Muscle Contraction Creates Force The force created by contracting muscle is called muscle tension Relaxation is the release of tension created by a contraction Events at the neuromuscular junction convert an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscle fiber 1 Excitation contraction coupling E C coupling process in which muscle A P initiates Ca2 signals that in turn activate a contraction relaxation cycle Contraction relaxation cycle sliding filament theory of contraction muscle twitch Actin and Myosin Sliding filament theory of contraction overlapping actin and myosin filaments of fixed length slide past one another in an energy requiring process resulting in muscle contraction When muscle contracts the thick and thin filaments slide past one another but do not change length I band and H zone disappear length of A band stays the same Tension generated in a muscle fiber is directly proportional to the number of high force crossbridges between the thick and thin filaments During contraction the myosin heads use the power stroke to push actin towards the center the myosin heads do not detach at the same time or the actin filament would go back to their original position Myosin ATPase provides the ATP energy for the power stroke energy released by ATP puts myosin in the cocked position 2 Signals Initiate Contraction Ca Troponin Ca2 binding complex that controls positioning of an elongated protein polymer tropomyosin At rest tropomyosin wraps around actin filaments and blocks actin s myosin binding sites When Ca2 binds to troponin C reversibly troponin undergoes a conformational change and pulls tropomyosin away from actin s myosin binding sites The on position allows myosin to bind to actin forming strong high force crossbridges so they can carry out their power strokes Actin moves toward the center and contractile cycles repeat as long as the binding sites are available For muscle relaxation to occer Ca2 must decrease in the cytosol then Ca2 unbinds from troponin and tropomyosin can block the actin s myosin binding sites Myosin Heads Step Along Actin Filaments Starting from rigor state 1 ATP binds and myosin detaches ATP binding decreases the actin binding affinity of myosin 2 ATP hydrolysis provides energy for the myosin head to rotate and reattach to actin 3 The power stroke release of Pi allows myosin head to swivel 4 Myosin releases ADP myosin is tightly bound in the rigor state cycle is ready to begin once more ATP binds to myosin 2 Acetylcholine Initiates Excitation Contraction Coupling 1 Ach is released from the somatic motor neuron 2 Ach initiates an A P in the muscle fiber T tubules a DHP receptor when the A P reaches the DHP receptor it changes b RyR ryanodine receptors conformation change opens the RyR Ca2 conformation release channels in the SR 3 The muscle A P triggers Ca2 release from the SR 4 Ca2 combines with troponin and initiates contraction 5 To end contraction Ca2 is removed from cytosol it is pumped back into the SR by Ca2 ATPase A single contraction relaxation cycle in a skeletal muscle fiber is called a twitch Skeletal Muscle Is Classified by Speed and Fatigue Resistance Slow twitch fibers slower used for maintaining posture standing or walking Fast twitch fibers oxidative and glycolytic develop tension faster complete contractile cycles more rapidly pumps Ca2 into SR faster used for fine quick movements Oxidative glycolytic fibers have more mitochondria more blood vessels more myoglobin smaller diameters oxidative phosphorylation for ATP synthesis Glycolytic fibers have lower myoglobin content larger in diameter fewer blood vessels rely on anaerobic glycolysis for ATP synthesis Force Produced v Length of Fiber Each sarcomere contracts with optimum force if it is at optimum length before contraction begins If sarcomere is too long not enough crossbridges are formed and less force is generated If sarcomere is too short too many crossbridges and not enough space for actin to move Force of Contraction increases with Summation If repeated action potentials are separated by long intervals of time the muscle fiber has time to relax completely between stimuli Summation if the interval of time between A P s is shortened the muscle fiber does not have time to relax completely between stimuli resulting in a forceful contraction If A P s continue to