Structure of Muscle Whole muscle Muscular System Made up of bundles of muscle cells called fascicles 10 100 cells fascicle not shown here Muscle cells also called muscle fibers Polynucleate Surrounded by plasma membrane just a cell membrane also called the sarcolemma sometimes Packed with mitochondria Packed with bundles of protein called myofibrils if myofibrils are ropes then the thread that makes the rope are called myofilaments two kinds of myofilaments thick filaments thin filaments thick and thin filaments arranged into sarcomeres Sarcomeres repeat through every myofibril in every muscle cell More details about muscle cells Muscular System Myofibrils surrounded my sarcoplasmic reticulum SR a type of smooth endoplasmic reticulum Stores Ca is the key that turns on the contraction protein machinery SR connect by a series of proteins to T tubules Transverse T tubules run up to surface of cell membrane Skeletal muscle cells connected to at least one neuron called a motor neuron Motor neuron tells the skeletal muscle cell when to contract and how long to contract This is called excitation contraction coupling Muscular System More details about excitation contraction coupling Electrical signal action potential travels down axon of motor neuron to synaptic terminal Causes the neurotransmitter acetylcholine Ach to be released from the neuron and into the synaptic cleft a synapse Ach binds to proteins on muscle cell membrane and triggers an electric signal action potential to travel along the muscle cell membrane And down the t tubules Which triggers the release of Ca from the sarcoplasmic reticulum Ca enters the cytoplasm Ca binds to proteins on sarcomere Which triggers the thick and thin filaments to slide past each other muscle cell contraction Demo time the mechanism explaining how thick and thin filaments slide past one another is called the sliding filament theory Muscular System More details about the sliding filament theory But first a little molecular anatomy Structure of thick filaments Myosin molecules shaped like little golf clubs The end portion of each molecule can ratchet move Many many myosin molecules form a thick filament Myosin heads can connect to thin filaments and pull them Structure of thin filaments Little balls of globular G actin Strung together into long filaments F actin Each G actin molecule has a binding site for myosin Myosin binding sites may be blocked by strand of tropomyosin Troponin connected to tropomyosin can change shape and pull tropomyosin off of binding sites Ca binding to troponin causes shape change Muscular System Muscular System The big idea now As soon as Ca binds to troponin Where did the Ca come from again Troponin pulls tropomyosin off myosin binding sites of thin filaments READY FOR SLIDING FILAMENT THEORY Myosin heads in ready position immediately form cross bridges Then immediately enter work power stroke pull actin toward middle of sarcomere ATP binds to myosin causes cross bridge detachment ATP hydrolyzed to ADP P Energy used to cock back myosin head to back into ready position If actin still has exposed myosin binding sites process will continue sarcomere gets narrower and narrower as this happens Muscular System The big idea now As soon as Ca binds to troponin Where did the Ca come from again Troponin pulls tropomyosin off myosin binding sites of thin filaments READY FOR SLIDING FILAMENT THEORY Myosin heads in ready position immediately form cross bridges Then immediately enter work power stroke pull actin toward middle of sarcomere ATP binds to myosin causes cross bridge detachment ATP hydrolyzed to ADP P Energy used to cock back myosin head to back into ready position If actin still has exposed myosin binding sites process will continue sarcomere gets narrower and narrower as this happens So then how does a muscle relax Muscular System
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