Steps to Muscle Contraction Quiz

How a Muscle Contracts and Relaxes!!!!!
1. An electrical signal (action potential) travels down a nerve cell, causing it to release a chemical message (neurotransmitter) into a small gap between the nerve cell and muscle cell. This gap is called the synapse or synaptic cleft.

2. The neurotransmitter (Ach) crosses the gap, binds to a protein (receptor) on the muscle-cell membrane and causes an action potential inside the muscle cell.

3. The action potential rapidly spreads along the muscle cell and enters the cell through the T-tubule.

4. The action potential causes gates to open in the muscle's calcium storage warehouse known as the (sarcoplasmic reticulum).

5. Calcium ions flow into the cytoplasm, which is where the actin and myosin filaments are.

6. Calcium ions bind to troponin-tropomyosin molecules located in the grooves of the actin filaments. Normally, the rod-like tropomyosin molecule covers the sites on actin where myosin can form crossbridges.

7. Upon binding calcium ions, troponin changes shape and slide tropomyosin out of the groove, exposing the actin-myosin binding sites.

8. Myosin interacts with actin by cycling crossbridges, as described previously. The muscle thereby creates force, and shortens. This uses energy….

9. After the action potential has passed, the calcium gates close, and calcium pumps located on the sarcoplasmic reticulum remove calcium from the cytoplasm via active transport.

10. As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions come off the troponin.
11. The troponin returns to its normal shape and allows tropomyosin to cover the actin-myosin binding sites on the actin filament.

12. Because no binding sites are available now, no crossbridges can form, and the muscle relaxes.