1. Microscopic Anatomy of the Skeletal Muscles Taking a look at the individual muscle fiber and how it works with other fibers

2. I. Microscopic Anatomy - organelles  A. Multinucleated The first nucleus (circular) can be seen just under the sarcolemma (plasma membrane) The first nucleus (circular) can be seen just under the sarcolemma (plasma membrane)  B. Myofibrils 1. Ribbon-like 1. Ribbon-like 2. Almost fill the cytoplasm 2. Almost fill the cytoplasm 3. Have alternating light and dark bands, give striation look to the fiber 3. Have alternating light and dark bands, give striation look to the fiber

3. I. Microscopic Anatomy - organelles  B. Myofibrils (cont.) 4. Myofibrils are made up of myofilaments 4. Myofibrils are made up of myofilaments 5. Myofibrils are chains of sarcomeres 5. Myofibrils are chains of sarcomeres a. Formation is from z-disc to z-disca. Formation is from z-disc to z-disc b. Arranged like boxcarsb. Arranged like boxcars c. The lighter/thinner bands are thin filamentsc. The lighter/thinner bands are thin filaments Light/thinner bands have a mid-line interruption called the Z-disc Light/thinner bands have a mid-line interruption called the Z-disc Z-discs connect the light/thinner bands of adjacent sarcomeres (brings the sarcomeres closer together during a contraction) Z-discs connect the light/thinner bands of adjacent sarcomeres (brings the sarcomeres closer together during a contraction)

4. I. Microscopic Anatomy - organelles  B. Myofibrils (cont) d. The darker bands are thick filaments (myosin filaments)d. The darker bands are thick filaments (myosin filaments) Has a light central area called the H-zone Has a light central area called the H-zone M-line in the H-zone has proteins that hold together adjacent filamentsM-line in the H-zone has proteins that hold together adjacent filaments e. The I-bande. The I-band Doesn’t polarize light Doesn’t polarize light Includes the z-disc Includes the z-disc f. The A-bandf. The A-band Polarizes light Polarizes light Includes the sarcomere Includes the sarcomere

6. I. Microscopic Anatomy - organelles  B. Myofibrils (cont.) g. Thick filaments run the entire length of the A-bandg. Thick filaments run the entire length of the A-band The middle of the thick filaments are smooth (around the M-line) The middle of the thick filaments are smooth (around the M-line) The ends of the thick filaments are studded with small projections (myosin heads) The ends of the thick filaments are studded with small projections (myosin heads) The myosin heads are called cross bridges, when they link the thick and thin filaments together during contractionThe myosin heads are called cross bridges, when they link the thick and thin filaments together during contraction

8. I. Microscopic Anatomy - organelles  B. Myofibrils (cont) h. Thin filaments are composed of contractile proteins called actinh. Thin filaments are composed of contractile proteins called actin Also contain other proteins to stop myosin heads from becoming bound Also contain other proteins to stop myosin heads from becoming bound Are anchored to the Z-disc Are anchored to the Z-disc  C. Sarcoplasmic Reticulum is a specialized ER 1. Surrounds the myofibril1. Surrounds the myofibril 2. Stores calcium for contractions2. Stores calcium for contractions

9. Sarcoplasmic Reticulum in Muscle Fibers

10. II. Microscopic Anatomy – Contraction of a single cell ► A. General Information  1. Muscle cells must be stimulated by nerve impulses  2. One motor neuron and all of the skeletal muscle it stimulates is called a motor unit  3. Nerve extensions (or axons) branch into axon terminals ► Form junctions with sarcolemma of different muscle cells (called neuromuscular junctions )

11. II. Microscopic Anatomy – Contraction of a single cell ► A. General Information (cont.)  4. The nerves never touch the muscle cells  5. The gap is called the synaptic cleft ► Filled with interstitial fluid  6. When an impulse reaches a nerve, a neurotransmitter is released into the interstitial fluid ► a. Acetylcholine or ACh is the chemical released

12. Synaptic Cleft

14. II. Microscopic Anatomy – Contraction of a single cell  6. (cont) ► b. When enough ACh is in the gap, the sarcolemma increase permeability to Na + ions and K + ions ► c. Na + ions rush into the cells ► d. K + ions rush out of the cells  This is why physical activity requires salt and bananas! ► e. The increased sodium ions creates a greater positive charge  Called an Action Potential  It is unstoppable  Travels over the entire surface of the sarcolemma  This conducts the electrical impulse over the entire surface of the muscle cell – resulting in a contraction

16. II. Microscopic Anatomy – Contraction of a single cell  7. Acetylcholine is broken down by enzymes after the action potential starts acetylcholinase  8. Since ACh breaks down after the start of the action potential, only one nerve impulse can cross the gap at a time

17. II. Microscopic Anatomy – Contraction of a single cell ► B. The Sliding Filament Theory  1. Contraction causes the shortening of each individual fiber  2. The fibers are shortened by the shortening of each individual myofibril  3. Myofibrils are shortened by reducing the length between the Z-discs  4. The A-bands do not shorten but appear closer together

18. II. Microscopic Anatomy – Contraction of a single cell ► B. The Sliding Filament Theory (cont.)  5. I-bands (which represent the distance between the A-bands) decrease in length  6. Both the thick and thin filaments remain the same length  7. Shortening of the sarcomeres is produced by sliding of the thin filaments over the thick filaments  8. The thin filaments extend deeper toward the center

20. II. Microscopic Anatomy – Contraction of a single cell ► B. The Sliding Filament Theory (cont.)  9.The thin fibers increase the amount of overlap, decreasing the H-zone ► C. What causes the filaments to slide?  1. The process is energized by ATP  2. The myosin heads attach to binding sites on the thin filaments  3. The cross bridges (myosin heads) attach and detach several times during contraction

21. II. Microscopic Anatomy – Contraction of a single cell ► C. What causes the filaments to slide? (cont)  4. Cross bridge attachment requires Ca 2+ ions  5. After contraction, the Ca 2+ ions are reabsorbed by the sarcoplasmic reticulum

23. III. Macroscopic Contraction of the Muscle ► A. “All or none” law of muscle physiology applies to the muscle cell  1. The muscle cell will contract completely when properly stimulated ► B. Muscle cells within the “muscle”, reacts with a graded response  1. Two ways of graded muscle reaction ► Changing the frequency ► Changing the number of muscle cells being stimulated

24. III. Macroscopic Contraction of the Muscle ► C. Response to increasingly rapid stimulation  1. A muscle twitch is a brief, single, jerky contraction ► May be a result of a nervous condition ► Not a normal muscle operation  2. Stimuli occur very rapidly, muscle cells do not have a chance to fully relax between stimuli  3. Successive contractions are ‘summed’ together  4. When stimulated where there is no relaxation, the muscle is fused or complete tetanus

25. III. Macroscopic Contraction of the Muscle ► C. Response to increasingly rapid stimulation (cont.)  5. As the stimuli increases, if there still is relaxation, the muscle is unfused or incomplete tetanus

26. IV. The Energy for Contraction A. General Information 1. Muscle cells only store 4-6 seconds worth of ATP Enough to start the contraction 2. 3 pathways for ATP generation a. Direct phosphorylation of ADP by creatin phosphate b. Aerobic respiration c. Anaerobic glycolysis and lactic acid formation

27. IV. The Energy for Contraction a. Direct Phosphorylation Creatin phosphate is a high energy molecule (called CP) Only found in muscle fibers Energy is transferred from ADP to CP after ATP reduction This regenerates ATP rapidly CP is exhausted in about 20 seconds

28. IV. The Energy for Contraction b. Aerobic Respiration During light activity, 95% of ATP is created by aerobic respiration Occurs in the mitochondria Oxygen is necessary The pathways are called oxidative phosphorylation Glucose is broken down into ATP molecules Slower than the other 2 forms

29. IV. The Energy for Contraction c. Anaerobic Glycolysis Breakdown of glucose into pyruvic acid in the absence of oxygen Pyruvic acid is converted into lactic acid (with 4 ATP molecules) Occurs in the cytosol of the mitochondria 2 1/2 times faster than aerobic respiration

30. V. Muscle fatigue and Oxygen Debt A. General Information 1. The muscle is unable to contract 2. Still being stimulated 3. Contraction becomes weaker without rest 4. Oxygen debt is the cause Occurs during prolonged muscle activity Glucose is converted into energy and then lactic acid acid build up causes the muscles to cramp

31. V. Muscle fatigue and Oxygen Debt A. General Information (cont) 5. Oxygen debt is recovered after the activity is concluded with rapid breathing and high overall blood pressure and rapid heart beat

32. VI. Types of Muscle Contractions  A. General Information 1. Isotonic Contractions  a. Myofilaments are successful in sliding and the muscle shortens 2. Isometric contractions  a. Myofilaments are “skidding their wheels” and the tension in the muscle increases  b. Muscles are trying to contract, but are moving against an immovable object