1 5 1 Muscle Structure and Function BIO 361T Fall 2014 1 Each muscle is a group of muscle cells myocytes that are connected Smooth and cardiac muscles are held together via gap junctions while skeletal muscles are held together by only connective tissue Muscle cells have cytoskeletal networks of actin and myosin proteins that allow them to change shape comprising thin and thick filaments respectively In striated muscle these filaments are arranged into sarcomeres which are connected to form myofibrils that run the length of the cell 2 How does membrane depolarization lead to changes in myocyte shape Use the following bank of proteins to assemble the molecular mechanism of excitation contraction coupling Some proteins may be used more than once a Dihydropyridine receptor DHPR a voltage dependent Ca2 channel in the sarcolemma b Ryanodine receptor RyR a Ca2 dependent Ca2 channel in the SR membrane c Voltage gated Na channel d Actin e Myosin 1 2 3 4 5 An efferent neuron depolarizes releasing neurotransmitter onto the myocyte Neurotransmitter binds a receptor opening ion channels and depolarizing the sarcolemma This opens a allowing extracellular Ca2 into the myocyte Ca2 interacts with d binding proteins to allow actin myosin crossbridges to form As long as ATP and Ca2 are available the muscle will stay contracted ATP will be hydrolyzed by e heads as it binds pulls and re binds actin shortening the sarcomeres 6 c also open allowing the depolarization to spread down the sarcolemma contracting the entire cell 7 In cardiac muscle cells influx of Ca2 opens b further increasing cytosolic Ca2 Ca2 induced Ca2 release 8 In skeletal muscle fibers DHPR in the sarcolemma and RyR in the SR membrane are physically connected When the sarcolemma depolarizes a will open which will cause b to open further increasing cytosolic Ca2 depolarization induced Ca2 release One action potential in a muscle fiber causes one contraction event Explain why the duration of this contraction is limited by the availability of two things ATP and Ca2 Without ATP no energy to form new cross bridges Without Ca2 caldesmon or tropomyosin covers the myosin binding sites on actin preventing cross bridges from forming Smooth muscle cells stop at step 5 Why do you think this is sufficient Because they contract slowly over long periods and do not need intense bursts of Ca2 Why do you think this is insufficient for most striated muscles Because they have a higher contraction speed and are larger so this passive spread of Ca2 from the membrane is insufficient to permeate all of the myofibrils in the cell 2 3 Draw a figure that illustrates all of the steps of excitation contraction coupling in smooth cardiac and skeletal muscle cells Smooth 4 Cardiac Skeletal Once a myocyte has contracted how does it relax Use the following bank of proteins to assemble the molecular mechanism by which a muscle cell relaxes after a contraction Each protein will be used only once a Ca2 ATPase pump in the sarcolemma b Ca2 ATPase pump in the SR membrane c NaCaX an Na Ca2 exchanger d Parvalbumin e Actin f Voltage gated K channel 1 2 3 4 Efferent neuron stops firing and neurotransmitter is cleared from the synapse Sarcolemma is repolarized by the f Ca2 is pumped out of the cytosol by a and back into the SR by b Due to its electrochemical gradient Na will diffuse into the cell through c which trades Na for Ca2 5 As cytosolic Ca2 falls e binding proteins slide back into place preventing new actinmyosin cross bridges from forming 6 In skeletal muscles that contract and relax quickly there is d in the cytosol which can bind up Ca2 lowering free cytosolic Ca2 and act as a buffer Smooth muscle cells do not rely on the SR ATPase to relax Why do you think this is The SR is not a main source of Ca2 because it needs only small amounts over a long time 5 After a vertebrate animal dies its muscles become stiff and immovable a condition called rigor mortis Explain the molecular mechanism of rigor mortis No ATP available to fuel the Ca2 pumps so Ca2 remains abundant to allow actin myosin cross bridges However without ATP available these will not break and reform in cross bridge cycles they will just remain bound Many students will get the second part but not the first With only the second part the question arises how did they bind in the first place They are not bound while relaxed so this binding event had to be triggered somehow 3 6 Myofibrils are not individually membrane bound they exist as bundles within individual myocytes Use this to explain why T tubules are particularly extensive in large skeletal muscle fibers Because the action potential cannot travel into the myofibrils without these invaginations so voltage gated Ca2 release would be too far away Also in skeletal muscle cells the sarcolemmal channel is physically attached to the SR channel inside the cell 7 The force that any given sarcomere can exert is highest over a narrow range of sarcomere width about 2 2 2 m from Z line to Z line Why would the force a sarcomere is capable of decrease if it is too narrow or too wide Too narrow thin filaments start to overlap myosin heads cannot access actin Too wide too little overlap between thin and thick filaments so some myosin heads cannot bind any actin Force exerted by one sarcomere depends on number of cross bridges at any given time In a cross bridge cycle each myosin head is actually only bound about 5 of the time One contraction looks like an octopus crawling through a tube as multiple myosin heads about 600 per sarcomere bind and release over and over One myosin head generates about 5 pN of force but a sarcomere at any given moment generates only about 150 pN not 600x5 8 Tonic skeletal muscle fibers are innervated at multiple sites as opposed to phasic or twitch skeletal muscle fibers which are innervated only once However a single motor neuron may innervate multiple twitch fibers comprising a motor unit Each skeletal muscle as a whole is usually controlled by multiple motor neurons so it contains fibers from multiple motor units Do you think every motor unit for a given muscle is activated every time that muscle contracts What would be the advantage if not No this is recruitment By activating smaller motor units first animals can make more refined movements To generate more force larger motor units are recruited as well 9 The amount of force a muscle can generate is plastic For example bodybuilders train with increasingly larges amounts of weight to
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