Explain musclecontraction based on neuroelectrical factors, chemical interactions, and energy sources.

Neuroelectrical Factor

1. Potassium ions (K+) are greater inside the muscle cell than outside, whereas sodium ions (Na+) are greater outside than inside.

2. The inside of the cell is negatively charged electrically, whereas the outside is postively charged.

3. Nerve impulse reaching a neuromuscular junction causes axon endings to release the neurotransmitter acetylcholine

4. This makes the cell membrane permeable to Na+. They rush in creating an electrical potential. K+ begin to move out to try to restore the resting potential but cannot do so because so much Na+ rushes in.

5. This causes the muscle cell to generate its own impulse called the action potential, which penetrates deep into the muscle fiber via the T tubules

6. This action potential causes the sarcoplasmic reticulum to release Ca+ into the fluids surrounding the myofibrils

7. The Ca++ stops the action of the inhibitor substances troponin and tropomyosin (which kept actin and myosin apart), thus causing the contractile process to occur.

Chemical Factor

1. Ca++ combines loosely with myosin to form activated myosin

2. Activated myosin reacts with ATP, attached to the myosin, and releases energy from the ATP to form actomyosin

3. Myosin filaments have cross bridges that now connect with the actin and pull the actin filaments in among the myosin filaments

4. A sliding process occurs: the width of the A bands remain constant but Z lines move close. By now the sodium-potassium pump kicks in to restore the resting potential. Ca++ gets reabsorbed and contraction ceases

Energy Sources ATP

1. Glycolysis: glucose to pyruvic acid, net gain eight ATP if O2 present, if no O2 then lactic acid forms with net gain of two ATP.

2. Krebs citric acid cycle: pyruvic to CO2 + H2O + 30 ATP (28 ATP + 2 GTP)

3. Phosphocreatine in muscle cells: phosphocreatine --- creatine + PO4 ---ATP

4. Free fatty acids: fatty acids --- CO2 + H2O + ATP

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