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6 Cards in this Set
- Front
- Back
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Describe the microanatomy of skeletal muscle fibers |
1. Fascicles: bundles of muscle cells 2. Muscle fibers: muscle cells 3. Myofibrils: cylindrical bundles of thick and thin filaments within the muscle fiber 4. Sarcomere: the functional unit of a muscle fiber--includes the thick filaments (myosin) and the thin filaments (actin, tropomyosin, and troponin) --Striations: A bands--overlapping myosin and actin; I bands--actin alone, area between thick filaments in 2 adjacent sarcomeres bisected by the Z-line |
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List the steps of the cross bridge cycle |
1. Energized cross bridge binds to actin 2. Pi is released from myosin head and cross bridge rotates (power stroke) and pulls actin filaments toward center of sarcomere. ADP is then released from the cross bridge. 3. ATP binds to myosin head and causes cross bridge to detach ("plasticizing" action of ATP) 4. ATP is hydrolyzed to Pi and ADP and "energizes" cross bridge [re-cocks cross bridge]. 5. The next cycle begins |
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Describe the functions of actin, troponin, and tropomyosin |
-Tropomyosin: intertwines with actin and covers myosin binding sites on actin -Troponin: binds to tropomyosin and holds tropomyosin over myosin binding site; inhibitory subunit (I), calcium binding subunit (C), and tropomyosin binding (T); binding of Ca2+ to the C subunit of troponin causes conformational change of troponin and causes tropomyosin to move aside and exposes the myosin binding sites on actin. |
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List the steps of excitation-contraction coupling |
1. Action potentials traveling down sarcolemma from the end plate enter the T-tubules and activate VG Ca2+ channels. 2. Ca2+ VG channels activate Ca2+ release channels, causing release of Ca2+ from the lateral sacs of the sarcoplasmic reticulum. 3. Ca2+ binds to troponin removing the blocking action of tropomyosin. 4. Myosin cross-bridges bind to actin and cycle at least 100 times. 5. Ca2+-ATPase pump Ca2+ from the cytosol into the sarcoplasmic reticulum. 6. Sequestration of Ca2+ into the SR decreases the cytosolic Ca2+ concentrations. 7. Ca2+ dissociates from troponin and cross-bridge cycling ceases. 8. The sarcomere extends to its resting length. |
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Describe how intracellular Ca2+ levels increase and are regulated |
Muscle AP propagated into T-tubules (activation of VG Ca2+ channels). VG Ca2+ channels in T-tubules are linked to and activate Ca2+ release channels (ryanodine receptors) in lateral sacs of the SR. Activation of these channels allows Ca2+ to flow out of the SR and into the cytoplasm (intracellular). The Ca2+ ATPase pumps cytosolic Ca2+ back into the SR. Low cytosolic Ca2+ means Ca2+ will dissociate from troponin and cross-bridge cycling ceases. |
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List the steps involved in producing a muscle contraction beginning with the excitation of a neuron in the cerebral cortex through sequestration of Ca2+ and relaxation of the muscle |
1. Integration of synaptic input (excitatory vs inhibitory) 2. Summation (temporal & spatial) 3. Depolarization of initial segment of axon to threshold 4. Generation of AP 5. Conductance of AP down an axon 6. Depolarization of nerve terminal 7. Excitation (secretion coupling by Ca2+ influx) 8. Release of NT 9. Activation of postsynaptic receptor 10. Summation of EPSPs & IPSPs, then repeat the process 11. Activation of motor neuron cell body leads to an AP in the motor neuron 12. AP at nerve terminal of motor neuron causes release of ACh at neuromuscular jxn 13. ACh activates nicotinic receptors in the endplate, producing an end plate potential 14. The EPP depolarizes the surrounding muscle membrane and produces an AP 15. The AP propagates to the end of the muscle fiber 16. APs enter transverse tubules and activate VG Ca2+ channels (DHP receptors) 17. Ca2+ is released from SR via Ca2+ release channels (ryanodine receptors) 18. Ca2+ binds to troponin 19. Tropomyosin moves to uncover myosin binding sites on actin 20. Cross-bridge cycling begins 21. Ca2+ ATPase pumps Ca2+ back into SR 22. Ca2+ dissociates from troponin 23. Tropomyosin covers myosin binding sites and cross bridge cycling ends (muscle relaxes) |
