release channel has four projections called foot processesor junctional feet.Each foot process comes in contact with one of the four DHP receptors in a tetrad. When the T tubule is at resting membrane potential, the part of the Ca2+release channel that extends into the sarcoplasm is blocked by a given cluster of DHP receptors, which prevents Ca2+from leaving the SR. When an action potential travels along the T tubule, the DHP receptors detect the change in voltage and undergo a conformational change that ultimately causes the Ca2+release channels to open. Once these channels open, large amounts of Ca2+flow out of the SR into the sarcoplasm around the thick and thin filaments.
Skeletal Muscle Relaxes in Response to a Decrease in the Sarcoplasmic Ca2+ Concentration.
Skeletal Muscles have three ways of producing ATP:
Creatine Phosphate Is the First Source of ATP During Muscle Contraction. creatine phosphate: energy-rich molecule that is found in muscle fibers. Synthesized from the excess ATP muscles produce when at rest. creatine kinase (CK): catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP. Creatine is a small, amino acid–like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers. When contraction begins and the ADP level starts to rise, CK catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP. This direct phosphorylation reaction quickly regenerates new ATP molecules. Because the formation of ATP from creatine phosphate occurs very rapidly, creatine phosphate is the first source of energy when muscle contraction begins. Because of this process muscles can contract about 15 s without the need of aerobic or anaerobic respiration. Aerobic Respiration in the muscle: Glucose gets into the muscle by facilitated difusion through the blood vessels and by breaking down glycogen stored in the muscle. This glucose undergoes glycolysis where two molecules of pyruvate are created and 2 ATP. The pyruvate goes to the mitochondria and undergoes the aerobic cellular respiration. Under anaerobic conditions, the pyruvic acid generated from glycolysis is converted to lactic acid anaerobic glycolysis: The entire process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration. It provides about 2 minutes of energy.
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- Fall '11