Ch. 11 Muscles b (lecture)
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What is a synapse? And what is this called on a muscle fiber?
The point where a branched axon meets its target cell. A Neuromuscular Junction on muscle cells.
What are these and what do they contain? What is the release of its contents called?
Synaptic vesicles, contains acetylcholine (ACh). called exocytosis when released
What is voltage?
The difference in electrical charge between two points, such as outside the Sarcolemma versus inside the sarcolemma
What is the ion that is abundant outside the cell and what is the ion that is abundant inside the cell?
Na in ECF, K in ICF
What is depolarization?
Na channels open, it moves down the conc. gradient into the cell, making it briefly positive.
What is repolarization?
K is repelled out of the cell by the positive charge, making the cell negative again
What is an action potential?
The quick up-and-down voltage shift of the cell
Name and describe the first phase of muscle contraction
- 1 Nerve signal triggers opening of calcium channels on synaptic knob, calcium enters
- 2 Calcium stimulates exocytosis, release of ACh from synaptic vesicles.
- 3 ACh diffuses across knob into cleft and binds to the ACh receptors on the sarcolemma.
- 4 The binding of the ACh opens the channels, allowing Na to enter the cell and cause an end plate potential (same as an action potential)
- 5 The shift causes other nearby voltage-gated channels to open, results in a wave of action potentials along the sarcolemma.
Name and describe the second phase of muscle contraction
- Excitation-Contraction Coupling
- 6 Wave of action potential travels down the T tubules.
- 7 Causes calcium channels to open on SR, and allows calcium to diffuse into cytosol
- 8 Calcium binds to troponin molecules
- 9 This causes the tropomyosin to roll away and expose the active actin sites.
Name and describe the third phase of muscle contraction
- 10 ATP on the myosin head is hydrolyzed (into ADP and Pi), this causes the myosin head to extend/”cock”
- 11 The myosin head forms a cross-bridge to the thin filament by binding to the actin site
- 12 Head does a power stroke (also releases ADP Pi), but stays bound
- 13 Head unbinds once new ATP binds to it. Now the process can repeat.
Name and describe the fourth and final phase of muscle contraction
- 14 Nerve signals stop at NMJ, ceasing the release of ACh
- 15 This stops the opening of the ACh receptors
- 16 Back near the SR, calcium is pumped back into SR from the cytosol
- 17 basically above step
- 18 Now not bound to calcium, Troponin allows tropomyosin to roll back over onto the active actin sites. The myosin heads cannot grab the sites anymore and the sarcomere extends to its original length.
Name and describe the two diseases listed in the powerpoint
- Men: Muscular dystrophy; the anchor to the sarcolemma that thin filaments pull on when contracting (dystrophin) is lost and the sliding of the filaments causes scars=degeneration of muscle
- Women: Myasthenia Gravis; ACh receptors are blocked, causing muscles to get weak by the end of the day=droopy eyelids/slurred speech
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