NOTES: Muscle Physiology
- Muscle fibers: modified cell of protein fibers; multiple nuclei in skeletal muscle cells (fusion of many cells); striated appearance.
Myofibrils: muscle fiber composed of many long thin units arranged in parallel rows; contain filaments
- Sarcolemma: cell membrane of a muscle fiber (muscle cell); has invaginations
Called transverse tubues (T-tubules)
- Function of T tubules: help conduct impulses to the sarcomeres
- Sarcoplasmic reticulua: role: these are modified smooth endoplasmic reticula; they store Calcium ions needed for muscle contraction
- Terminal cisternae: end of the T tubules sections; storehouse for Ca+2 ions
- Sarcomeres: functional contracting unit in myofibrils
- Sections of
- A band: dark, thick region of sarcomere; made up of the protein myosin
- H band: lighter strip in the center of the A band
- I band: light colored; extends from edge of one A band to an adjacent A
Band; made of thin filaments of protein actin
- Actin filaments: lie above the myosin filaments; extends from A band to
middle of myosin area
- Z disc: thin dark line down the center of the I bands; attachment point
for the thin filaments on either side of the Z disc
8. Muscle contraction: sarcomere will shorten in length as Z discs come closer together and size of the H band decreases; the thin filaments are overlapping the thick filaments.
9. Sliding filament hypothesis:
(1). Myosin heads bind ATP and split it forming ADP and Pi
(2). Myosin heads bind to sites on the actin filaments
(3). Myosin heads release the Pi allowing the head to bend toward the A band and
Pulling the actin filaments with it; this is the power stroke
(4).At end of power stroke, myosin head binds a new ATP and releases ADP; myosin head breaks away from the actin filament
10. Cross bridges: temporary union between the myosin and actin filaments
11. Troponin: protein that binds tropomyosin to actin; also is attracted to Ca+2 ions
Tropomyosin: double strand of protein; blocks actin binding sites preventing myosin attachment
12. Neuromuscular junction:
(1). Neuron releases acetylcholine which binds to Ach receptors on motor end Plate of muscle cell sarcolemma
(2). Ach opens Na+ channels which generates action potential in sarcolemma
(3) Action potential travels into T- tubules
(4). Action potential in T-tubules causes terminal cisternae to release Ca+2 into cytoplasm
(5) Ca+2 causes contraction
(6). After contraction, Ca+2 is pumped back into sarcoplasmic reticulum
13. Motor units: motor neuron and its fibers; when motor neuron is activated, it stimulates all its muscle fibers to contract
14. Precision movements: the fewer muscle fibers activated in a motor unit , the more precise the movement will be. Example: eye muscles innervation ratio is l:23; the calf muscle: 1:1000.
15. Lactic acid fermentation: when not enough oxygen can supplied to stressed muscles, lactic acid is produced; limited ATP production (2ATP from Glycolysis)
16. Phosphocreatine: cells can obtain extra ATP when phosphocreatine donates a phosphate group to ADP; high quantity in muscle cells
17. Muscle fatigue: (sustained muscle contraction) sustained muscle contraction can occur due to the accumulation of K+ ions outside the cell; it alters the membrane potential and makes it hard for the cell to continue generating action potentials; after one minute of rest, the Na+/K+ pump can return K+ to the inside of the membrane, reducing fatigue.
18. Skeletal muscle cells: striated, syncytial (multinucleate), voluntary; contracts as described above
Smooth muscle cells: non striated, one nucleus per cell; involuntary; similar to skeletal muscle in contraction but filaments not stacked; fewer think filaments compared to thin filaments; have longitudinal layer (contraction results in organ dilates and shortens) and circular layer (constricts the cavity of the organ; causes organ to elongate)
19. Intercalated discs: area of interlocking cells in cardiac tissue; allows behavior as a single unit (syncytium).
20. Spontaneous action potential: cardiac muscle producing action potentials without neural stimulation; nervous system acts to control rate of contraction
21. See #18 above.