Muscles and Muscle Tissue Part C1 Prepared by Janice Meeking, W. Rose, and Jarvis Smith. Figures from Marieb & Hoehn 8 th ed. Portions copyright Pearson Education

1 Actin Cross bridge formation Cocking of myosin headPower (working) stroke Cross bridge detachment 24 3 ADP PiPi ATP ADP PiPi PiPi Figure 9.12 Cross Bridge Cycle reminder Skeletal Muscle Contraction

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

Copyright © 2010 Pearson Education, Inc. Figure 9.19a Coupled reaction of creatine phosphate (CP) and ADP Energy source: CP (a) Direct phosphorylation Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds Creatine kinase ADPCP Creatine ATP

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

Copyright © 2010 Pearson Education, Inc. Figure 9.19b Energy source: glucose Glycolysis and lactic acid formation (b) Anaerobic pathway Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 60 seconds, or slightly more Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol Pyruvic acid Released to blood net gain 2 Lactic acid O2O2 O2O2 ATP

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

Copyright © 2010 Pearson Education, Inc. Figure 9.19c Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism (c) Aerobic pathway Aerobic cellular respiration Oxygen use: Required Products: 32 ATP per glucose, CO 2, H 2 O Duration of energy provision: Hours Glucose (from glycogen breakdown or delivered from blood) 32 O2O2 O2O2 H2OH2O CO 2 Pyruvic acid Fatty acids Amino acids Aerobic respiration in mitochondria Aerobic respiration in mitochondria ATP net gain per glucose

Short-duration exercise Prolonged-duration exercise ATP stored in muscles is used first. ATP is formed from creatine Phosphate and ADP. Glycogen stored in muscles is broken down to glucose, which is oxidized to generate ATP. 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. Figure 9.20 AnaerobicAerobic

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 P i may accumulate, inhibiting release of P i 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, O 2 demand > supply. O 2 deficit develops; deficit must be paid back later. Extra O 2 needed after exercise to Replenish O 2 reserves (attached to myoglobin) Replenish glycogen stores Replenish ATP and CP reserves Convert lactic acid to pyruvic acid, glucose, glycogen O 2 needed afterward = difference between O 2 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

Copyright © 2010 Pearson Education, Inc. Figure 9.21 Large number of muscle fibers activated Contractile force High frequency of stimulation Large muscle fibers Muscle and sarcomere stretched to slightly over 100% of resting length

Sarcomeres greatly shortened Sarcomeres at resting length Sarcomeres excessively stretched 170% Optimal sarcomere operating length (80%–120% of resting length) 100%70% 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  Figure 9.22 Length-tension relationship in skeletal muscle

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 pH 1 2.Metabolic pathways for ATP synthesis: – Oxidative fibers—use mainly aerobic pathways – Glycolytic fibers—use mainly anaerobic glycolysis McArdle et al. Exercise Physiology. 4 th ed 1

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. 4 th ed

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 Effects of Exercise Grete Waitz & Ingrid Chrisiansen mujeresriot.webcindario.com/Grete_Waitz.htm

Resistance exercise (typically anaerobic) leads to: Muscle hypertrophy (due mostly to  cross sectional area of each fiber)  mitochondria  myofilaments  glycogen stores  connective tissue Effects of Exercise Arnold + exercise Arnold – exercise connect.in.com/arnold-schwarzenegger/photos html

Copyright © 2010 Pearson Education, Inc. 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