1. Shortening of sarcomeres = muscle shortening
“The Sliding Filament Theory”
Shortening of sarcomeres = muscle shortening

2. Nervous system tells a muscle to contract by releasing a chemical messenger acetylcholine (Ach) into the synaptic cleft

3. 2. This causes sodium ion channels to open and sodium rushes into the muscle fiber and excite the muscle producing muscle action potential.
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ACh is quickly broken down by acetylcholinesterase AChE

5. 3. Increased sodium levels inside the muscle fiber causes the Sarcoplasmic Reticulum (SR) to release calcium into the sarcoplasm.
SR is a specialized organelle in muscle cells used to store calcium.

6. 4. The presence of calcium causes ATP to interact with the actin and myosin filaments by changing their shape.
5. Myosin heads make crossbridges to actin filaments which pulls the sarcomeres together, contracting the muscle.
In the sarcoplasm is a molecule called myoglobin – similar to hemoglobin with coloration but its job is to hold/store oxygen so that ATP can easily be generated.
Heads of myosin contain ATP synthase which will help convert ATP into ADP. As the head swivels it will release ADP and bind tightly to the actin binding sight without letting go.
Calcium changes the binding site on the thin filament (actin) which allows the myosin heads to attach to the actin. The power stroke will take place. In order to release the myosin from the actin you will need ATP to help you break that bond. 6

7. 6. When an enzyme removes the chemcial messanger ACh from receptors on the sarcolemma, ATP detaches myosin heads and calcium is pumped back into the SR and the muscle relaxes
In the sarcoplasm is a molecule called myoglobin – similar to hemoglobin with coloration but its job is to hold/store oxygen so that ATP can easily be generated.
Calcium changes the binding site on the thin filament (actin) which allows the myosin heads to attach to the actin. The power stroke will take place. In order to release the myosin from the actin you will need ATP to help you break that bond. 7

8. What happens to our muscles when we die?
Begins 3-4 hours after death and can last up to 24 hours and disappear after lysozymes break down crossbridges.
Ca+2 leaks out and muscle contract – no ATP to release contraction = Rigor Mortis 8