Muscle Physiology

Terminology

1. Skeletal muscle - voluntary control
   1. striated (dark, light bands)
2. Muscle cell = fiber (multi-nucleated)
   1. no mitosis except satellite cells
3. hypertrophy vs. Hyperplasia

Gross Anatomy of Skeletal Muscle

1. muscle belly - surrounded by epimysium (fascia)
2. Fasciculus = bundle of muscle fibers surrounded by perimysium
3. Muscle fiber surrounded by endomysium
   1. Fascia sheaths -> nerves, blood vessels
4. Skeletal Muscle shapes
   1. Fusiform - fasciculi long, parallel fibers
      1. Allow movement, little power (sartorius)
   2. Pennate- short fibers, fasciculi inserting
      1. oblique w/ central tendon
      2. less movement, greater power
      3. uni-, multi-, or bi pennate

Microscopic Anatomy of Skeletal Muscle

1. Functional unit - sarcomere
   1. separated by 2 Z lines
2. Thick myofilaments = Myosin
   1. Dark bands (anisotropic)
   2. contain lollypop heads (crossbridges)
   3. anchored by M line
   4. H zone contains only thick filaments
   5. 1 myosin= 2 heavy, 4 light chains bundled with cross bridges facing outward
   6. 200 myosin molecules per thick filament
3. Thin myofilaments = Actin (+)
   1. Light bands (isotropic)
      1. F actin -2 helical strands of G actin
   2. Tropomyosin
      1. threads lying on actin surface
      2. normally inhibits actin-myosin interaction
   3. Troponin
      1. small protein on tropomyosin
      2. binds Calcium
4. Tubule system
   1. Transverse tubules or T tubules
      1. plasma membrane invaginations deep into muscle
      2. conducts electrical impulse to sarcoplasmic reticulum (parallel to fibers)
      3. Calcium sequestered in terminal cisternae
      4. found at A - I band junctions of sarcomere
      5. Calsequestrin (re-sequesters Calcium)

Physiology of Skeletal muscle contraction

1. Nerve impulse reaches neuromuscular junction (acetycholine)
2. Causes depolarization wave (large influx of positive ions)
3. Action potential spreads along plasma membrane, down T tubules
4. Calcium released from sarcoplasmic reticulum
5. Calcium binds to troponin
6. Conformational change in Tropomyosin and moves off "hot spot" on actin
7. Actin myosin complex together following splitting of ATP on myosin head
8. High energy myosin head swivels, pulling actin inward (power stroke)
9. ADP released from myosin head
10. myosin and actin remain complexed
11. ATP binds to myosin, releasing actin
12. REPEATS until calcium returns to sarcoplasmic reticulum
13. Rigor Mortis
    1. rigidity due to lack of ATP upon death
14. Sliding Filament Theory
    1. contraction (muscle shortens) due to sliding of myofilaments
    2. No change inlength of myofilaments
    3. reduced width of sarcomere is end result
    4. requires energy (ATP split by myosin ATPase)
       1. stored ATP
       2. Creatine phosphate,
       3. anaerobic glycolysis
       4. aerobic metabolism