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Muscle contraction Students participating in the presentation: 1- naif aljabri 430101612 2- yousif alessa 430105885 3- faris abalkheel 430101692 4- Ali Moshaba AL-Ahmary 430103532 5-abdulelah bin numay 430104473 S KELETAL MUSCLE S KELETAL MUSCLE S KELETAL MUSCLE S KELETAL MUSCLE & M USCLE FILAMENTS Skeletal muscle Contraction of skeletal muscle is under voluntary control. each skeletal muscle cell is innervated by a branch of a motoneurons . Muscle filaments each muscle fiber behaves as a single unit is multinucleate and contains myofibrils the myofibrils are surround by sarcoplasmic reticulum are invaginated by transverse tubules ( t tubules) each myofibril contains interdigitating thich and thin filaments. T HICH FILAMENTS & T HIN FILAMENTS Thich filaments are comprised of a large molecular weight protein called myosin Thin filaments are composed of 3 proteins: 1- actin.2-tropomyosin.3-troponin. Arrangement of thick and thin filaments in sarcomeres CYTOSKELTAL PROTEINS T RANSVERSE TUBULE AND THE SARCOPLASMIC RETICULUM Transverse tubule and the sarcoplasmic reticulum are continuous with the sarcolemmal membrane and invaginated deep into the muscle fiber, making contact with terminalcisternae of the sarcoplsmic reticulum. E XCITATION - CONTRACTION COUPLING Excitation-contraction coupling In skeletal muscle the method of excitation contraction coupling relies on the ryanodine receptor being activated by a domain spanning the space between the T tubules and the sarcoplasmic reticulum to produce the calcium transient responsible for allowing contraction. The motor neuron produces an action potential that propagates down its axon to the neuromuscular junction. The action potential is sensed by a voltage-dependent calcium channel which causes an influx of Ca2+ ions which causes exocytosis of synaptic vesicles containing acetylcholine. Acetylcholine diffuses across the synapse and binds to nicotinic acetylcholine receptors on the myocyte, which causes an influx of Na+ and an efflux of K+ and generation of an end-plate potential. E XCITATION - CONTRACTION COUPLING The end-plate potential propagates throughout the myocyte's sarcolemma and into the T-tubule system. The T-tubule contains dihydropyridine receptors which are voltage-dependent calcium channels and are activated by the action potential. Opening of the Ryanodine receptors causes and flow of Ca2+ from the sarcoplasmic reticulum into the cytoplasm. Ca2+ released from the sarcoplasmic reticulum binds to Troponin C on actin filaments, which subsequently leads to the troponin complex being physically moved aside to uncover cross-bridge binding sites on the actin filament. E XCITATION - CONTRACTION COUPLING By hydrolyzing ATP, myosin forms a cross bridges with the actin filaments, and pulls the actin toward the center of the sarcomere resulting in contraction of the sarcomere. Simultaneously, the sarco/endoplasmic reticulum Ca2+-ATPase actively pumps Ca2+ back into the sarcoplasmic reticulum where Ca2+ rebinds to calsequestrin. With Ca2+ no longer bound to troponin C, the troponin complex slips back to its blocking position over the binding sites on actin. Since cross-bridge cycling is ceasing then the load on the muscle causes the inactive sarcomeres to lengthen. M ECHANISM OF TETANUS Mechanism of tetanus A single action potential result in the release of a fixed amount of ca+ from the sarcoplasmic reticulum which produce a single twist is terminated ( relaxation occurs ) when the sarcoplasmic Step 1: At the end of the previous round of movement and the start of the next cycle, the myosin head lacks a bound ATP and it is attached to the actin filament in a very short-lived conformation known as the 'rigor conformation'. Step 2: ATP-binding to the myosin head domain induces a small conformational shift in the actin-binding site that reduces its affinity for actin and causes the myosin head to release the actin filament. M ECHANISM OF TETANUS Step 3: ATP-binding also causes a large conformational shift in the 'lever arm' of myosin that 'cocks' the head into a position further along the filament. ATP is then hydrolysed, but the inorganic phosphate and ADP remain bound to myosin. Step 4: The myosin head makes weak contact with the actin filament and a slight conformational change occurs on myosin that promotes the release of inorganic phosphate. Step 5: The release of inorganic phosphate reinforces the binding interaction between myosin and actin and subsequently triggers the 'power stroke'. The power stroke is the key force-generating step used by myosin motor proteins; forces are generated on the actin filament as the myosin protein reverts back to its original conformation. Step 6: As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament at a new position from where it started, thereby bringing the cycle back to the beginning. M ECHANISM OF TETANUS