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Muscles and Muscle Tissue Part C1 Prepared by Janice Meeking, W. Rose, and Jarvis Smith. Figures from Marieb & Hoehn 8th ed. Portions copyright Pearson Education Skeletal Muscle Contraction Cross Bridge Cycle reminder Actin ADP Pi 1 Cross bridge formation ADP ADP Pi Pi 4 2 Cocking of myosin head Power (working) stroke ATP ATP 3 Cross bridge detachment Figure 9.12 Muscle Metabolism: Energy for Contraction ATP is the only direct source of energy for muscle contraction Available stores of ATP depleted in 4–6 seconds ATP is regenerated by: – Direct phosphorylation of ADP by creatine phosphate (CP) – Anaerobic pathway (glycolysis) – Aerobic respiration (a) Direct phosphorylation Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP CP ADP Creatine kinase Creatine ATP Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Copyright © 2010 Pearson Education, Inc. Figure 9.19a Anaerobic Pathway (glycolysis) • Occurs when O2 delivery cannot keep up with O2 use • As contractile activity increases, O2 consumption may increase above O2 delivery capability, so anaerobic metabolism begins • Pyruvic acid lactic acid when not enough O2 • Lactic acid (lactate) • Makes muscle cells acidic, less efficient • Diffuses into bloodstream • Liver (with O2) can convert it back into pyruvic acid (b) Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol 2 O2 ATP Pyruvic acid net gain O2 Released to blood Lactic acid Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more Copyright © 2010 Pearson Education, Inc. Figure 9.19b Aerobic Pathway • Produces 95% of ATP during rest and light to moderate exercise • Occurs when O2 delivery can keep up with O2 use: • Krebs cycle • Electron transport chain • Fuels: stored glycogen, then glucose (blood), pyruvic acid from glycolysis, and free fatty acids and amino acids (c) Aerobic pathway Aerobic cellular respiration Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Glucose (from glycogen breakdown or delivered from blood) O2 Pyruvic acid Fatty acids O2 Aerobic respiration Aerobic respiration in mitochondria mitochondria Amino acids 32 CO2 H2O ATP net gain per glucose Oxygen use: Required Products: 32 ATP per glucose, CO2, H2O Duration of energy provision: Hours Copyright © 2010 Pearson Education, Inc. Figure 9.19c Anaerobic Short-duration exercise ATP stored in muscles is used first. Figure 9.20 ATP is formed from creatine Phosphate and ADP. Glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP. Aerobic Prolonged-duration exercise ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway. This pathway uses oxygen released from myoglobin or delivered in the blood by hemoglobin. When it ends, the oxygen deficit is paid back. Muscle Fatigue Muscle Fatigue Physiological fatigue: muscle doesn’t respond to nerve impulses • ATP deficit? Surprisingly, no. ATP drops very little. • Intracellular acidity, due to lactate? Maybe not, since pH hardly drops. • Ionic imbalances interfere with E-C coupling: K+ accumulation in T tubules • Pi may accumulate, inhibiting release of Pi from myosin • Disrupted storage & release of Ca due to SR damage (causes slow-developing fatigue during submaximal exercise) Central (“psychological”) fatigue: CNS doesn’t produce the neural commands • Cause unclear. Maybe partly a response to elevated body pH due to lactate. “Central governor” theory (disputed): brain won’t let the body hurt itself. Oxygen Deficit During intense exercise, O2 demand > supply. O2 deficit develops; deficit must be paid back later. Extra O2 needed after exercise to •Replenish O2 reserves (attached to myoglobin) •Replenish glycogen stores •Replenish ATP and CP reserves •Convert lactic acid to pyruvic acid, glucose, glycogen O2 needed afterward = difference between O2 that was actually used during exercise & what would been needed to do it aerobically. Force of muscle contraction affected by: • Number of muscle fibers stimulated (recruitment) • Muscle cross-sectional area: hypertrophy of cells increases strength • Frequency of stimulation: stimulation rate allows time for more effective transfer of tension to noncontractile components • Length of muscle (length-tension relation): a muscle contracts most strongly when its fibers are 80–120% of their normal resting length Large number of muscle fibers activated Large muscle fibers High frequency of stimulation Muscle and sarcomere stretched to slightly over 100% of resting length Contractile force Copyright © 2010 Pearson Education, Inc. Figure 9.21 Length-tension relationship in skeletal muscle Observable in whole muscle Cellular basis: • At short muscle length, force because thin filaments overlap each other • At long muscle length, force because # of potential crossbridges that can form 70% Sarcomeres greatly shortened 100% Sarcomeres at resting length 170% Sarcomeres excessively stretched Optimal sarcomere operating length (80%–120% of resting length) Figure 9.22 Type of Muscle Fibers Classified according to two characteristics: 1. Speed of contraction: slow or fast, according to: – Speed at which myosin ATPases split ATP • ATPases affected by pH1 2. Metabolic pathways for ATP synthesis: – Oxidative fibers—use mainly aerobic pathways – Glycolytic fibers—use mainly anaerobic glycolysis McArdle et al. Exercise Physiology. 4th ed1 Three types of muscle fibers: • • • Slow oxidative fibers: low contraction speed and low force capability (small fibers, slow ATPases, high mitochondria, myoglobin and capillary) Fast oxidative fibers: intermediate contraction speed and force capability (intermediate fibers, ATPases, mitochondria, myoglobin and capillary) Fast glycolytic fibers: high contraction speed and force capability (large fibers, ATPases, mitochondria, myoglobin and capillary) McArdle et al. Exercise Physiology. 4th ed Effects of Exercise Aerobic (endurance) exercise leads to: • muscle capillary density • number of mitochondria • myoglobin synthesis • endurance, strength, fatigue resistance • May convert fast glycolytic fibers into fast oxidative fibers Grete Waitz & Ingrid Chrisiansen mujeresriot.webcindario.com/Grete_Waitz.htm Effects of Exercise Resistance exercise (typically anaerobic) leads to: • Muscle hypertrophy (due mostly to cross sectional area of each fiber) • mitochondria • myofilaments • glycogen stores • connective tissue Arnold + exercise Arnold – exercise connect.in.com/arnold-schwarzenegger/photos-26045-368846.html Muscular Dystrophy • Group of inherited muscle-destroying diseases • Muscles enlarge due to fat and connective tissue deposits • Muscle fibers atrophy Copyright © 2010 Pearson Education, Inc. Muscular Dystrophy Duchenne muscular dystrophy (DMD): • Most common and severe type • Inherited, sex-linked, carried by females and expressed in males (1/3500) as lack of dystrophin • Victims become clumsy and fall frequently; usually die of respiratory failure in their 20s • No cure, but viral gene therapy or infusion of stem cells with correct dystrophin genes show promise Copyright © 2010 Pearson Education, Inc.