2. Warm-Up 4/8/15 1.What is an autoimmune disorder? 2.What how are LEMS and MG similar? 3.How are LEMS and MG different? Test - Tuesday

3. Z line Thin myofilaments M line Thick myofilaments Z line Myofilaments of Sarcomere

4. Muscle contraction involves myosin binding to actin and exerting a pulling force. A myosin molecules is composed of a twisted protein strand with globular parts called heads projecting outward along their lengths. Many of these molecules together compose a myosin (thick) filament

5. An actin molecule is a globular structure with a binding site to which the myosin heads can attach. Many actin molecules twist into a double strand (helix) forming an actin (thin) filament. The proteins troponin and tropomyosin are also part of the thin filament.

6. At rest, actin’s binding sites are covered and blocked by tropomyosin. Tropomyosin is held in place by troponin

7. Within the sarcoplasm of a muscle fiber, is a network of membranous channels that surrounds each myofibril and runs parallel to it. These membranes form the sarcoplasmic reticulum (SR) which corresponds to the endoplasmic reticulum of other cells.

8. The SR is a reservoir of calcium. When a muscle is stimulated the SR releases calcium into the sarcoplasm and the sarcomere.

9. Once released calcium binds with troponin Troponin changes shape and moves tropomyosin Binding sites in the actin filament are exposed This initiates the contraction cycle

10. The Contraction Cycle Overview Myosin will attach to actin binding site, forming a cross- bridge and bend slightly pulling on the actin filament, then the myosin head can release straighten, combine with another binding site further down the actin filament and pull again.

11. Contraction Cycle Steps The myosin heads contain an enzyme ATPase which catalyzes the breakdown of ATP to ADP and phosphate. This reaction provides energy that puts the myosin head into a “cocked” position.

12. Myosin works similar to a mousetrap

13. When a cocked myosin head binds to the actin, forming a cross bridge, it pulls on the thin filament. In a step known as the “power stroke”

14. Powerstroke = trigger trap

15. After the powerstroke, another ATP binding to the myosin head breaks the cross-bridge, releasing the myosin head from the actin, even before the ATP splits.

16. The ATPase then catalyses the breakdown of ATP to ADP and phosphate, putting myosin head in a “cocked” position again.

17. This cycle repeats as long as ATP is available as an energy source and as long as the muscle fiber is stimulated to contract.

18. Once Ach is broken down and the stimulus to the muscle fiber ceases. Calcium ions are actively transported back into the sarcoplasmic reticulum. Its like mopping up all the spilled calcium.

19. END. https://www.youtube.com/watch?v=Ct8AbZn_A8A