How do calcium ions participate in muscle contraction

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21. How do calcium ions participate in muscle contraction? Why do both muscle contraction and muscle relaxation spend energy? In muscle cells, calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits a stimulus for muscle contraction, neurotransmitters called acetylcholine are released at the neuromuscular junction and the sarcolemma is excited. The excitation is transmitted to the sarcoplasmic reticulum, which then releases calcium ions into the sarcomeres. In the sarcomeres, the calcium ions bind to troponin molecules attached to actin, thus activating the myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The combined simultaneous contraction of sarcomeres and myofibrils constitutes a muscle contraction. During muscle relaxation, the calcium ions return to the sarcoplasmic reticulum. For myosin to bind to actin, and therefore for the contraction to occur, the hydrolysis of one ATP molecule is necessary. During relaxation, the return of calcium ions to the sarcoplasmic reticulum is also an active process that consumes ATP. Therefore, both muscle contraction and relaxation are energy- consuming processes. The Musculoskeletal System Review - Image Diversity: muscle contraction
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22. What is myoglobin? What is the function of this molecule in muscle tissue? Myoglobin is a pigment similar to hemoglobin which is present in muscle fibers. Myoglobin has a large affinity to oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. Therefore, myoglobin acts as an oxygen reserve for muscle cells. 23. How is phosphocreatine involved in muscle contraction and relaxation? Phosphocreatine is the main means of energy storage of muscle cells. During periods of relaxation, ATP molecules produced by aerobic cellular respiration transfer highly energized phosphate groups to creatine molecules, thus forming phosphocreatine. During periods of exercise, phosphocreatine and ADP resynthesize ATP to release energy for muscle contraction. 24. What happens when the oxygen supply is insufficient to maintain aerobic cellular respiration during muscle exercise? If oxygen from hemoglobin or myoglobin is not enough to supply energy to the muscle cells, the cell begins to use lactic fermentation in an attempt to compensate for that deficiency.
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Lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps. 25. What is the neurotransmitter of the neuromuscular junction? How does the nervous system trigger muscle contraction? The nervous cells that trigger muscle contraction are motor neurons. The neurotransmitter of the motor neurons is acetylcholine. When a motor neuron is excited, a depolarizing current flows along the membrane of its axon until reaching the synapse at the neuromuscular junction (the neural impulse passage zone between the axon extremity and the sarcolemma). Near the extremity of the axon, the depolarization
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