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“The Sliding Filament Theory” Shortening of sarcomeres = muscle shortening 1. Nervous system tells a muscle to contract by releasing a chemical messenger acetylcholine (Ach) into the synaptic cleft 2. This causes sodium ion channels to open and sodium rushes into the muscle fiber and excite the muscle producing muscle action potential. 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. 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. 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 What happens to our muscles when we die? Ca+2 leaks out and muscle contract – no ATP to release contraction = Rigor Mortis