Sliding Filament Theory, Slides of Physiology

Download Sliding Filament Theory and more Slides Physiology in PDF only on Docsity! Interactive Physiology Sliding Filament Theory Graphics are used with permission of: adam.com (http://www.adam.com/) Benjamin Cummings Publishing Co (http://www.awl.com/bc) Page 1. Introduction • When a muscle cell contracts, the thin filaments slide past the thick filaments, and the sarcomere shortens. Page 2. Goals • To explore the molecular structure and functional features of the thin and thick filaments. • To understand the sequence of events in a single cross bridge cycle. • To examine the sequence of events in multiple cross bridge cycling. Page 3. Molecular Participants • The chemical players in muscle contraction are: 1. myosin (protein) 2. actin (protein) 3. tropomyosin (protein) 4. troponin (protein) 5. ATP (nucleotide) 6. calcium ions Page 4. Sarcomere • The next several pages explain how each of these chemicals participate in the contraction of a sarcomere. Page 5. Myosin • Myosin is a protein molecule found in the thick filaments. Page 6. Myosin Molecule with Hinged Head • Myosin has a tail and two heads (called cross bridges) which will move back and forth, providing the power stroke for muscle contraction. Page 7. Myosin Molecule with Hinged Head and Tail • The tail of myosin has a hinge which allows vertical movement so that the cross-bridge can bind to actin. Page 8. Myosin ATP Binding Site • The cross bridge (head) of myosin has a binding site for ATP. • Myosin is in its low energy conformation when the cross bridge is in this position: ** Note: The term "conformation" is often used to indicate the shape of a protein. Proteins often change their shape or conformation as they function. Page 9. Energized Cross Bridge • ATP is a molecule with a high chemical energy. ATP binds to myosin heads when they are tilted back in their low energy position. When ATP is hydrolyzed into ADP and phosphate, the energy is released and transferred to the myosin head. • Note that the ATP is glowing yellow indicating that it's in a high energy state. After the ATP has been hydrolyzed to ADP and phosphate, the energy is transferred to the myosin head. Now the head is glowing to show that it's high energy. When the myosin head is pointing up, it is in a high energy state. Interactive Physiology 2 • As myosin functions within muscle cells, it undergoes the following four steps: ** This may seem like a lot of detail at this point, but later on this will be very important. At that time you may want to come back to this to review. Page 10. Actin Binding Site on Myosin • There are two binding sites on each myosin head, one for ATP and one for actin. Page 11. Thin Filaments of the Sarcomere • Thin filaments are made of these three protein molecules: 1. actin 2. tropomyosin 3. troponin Page 12. Actin • The major component of the thin filament, actin is composed of a double strand of actin subunits each of which contain myosin binding sites. Page 13. Tropomyosin • The regulatory protein, tropomyosin, is also part of the thin filament. Tropomyosin twists around the actin. When the sarcomere is not shortening, the position of the tropomyosin covers the binding sites on the actin subunits and prevents myosin cross bridge binding. Page 14. Troponin • Troponin, which is found periodically along the tropomyosin strand, functions to move the tropomyosin aside, exposing the myosin binding sites. Page 15. Calcium Ions • Role of Calcium in Muscle Contraction: Action Potential Occurs ↓ Calcium Ions are Released from the Terminal Cisternae ↓ Calcium Ions then Bind to Troponin ↓ Tropomyosin Moves Away from the Myosin Binding Sites on Actin Page 16. Review of Molecular Participants • Review of participants in the Cross Bridge Cycle: Participant Will bind to: 1. Myosin ATP, Actin 2. Actin Myosin, Troponin Interactive Physiology 5 themselves over and over again, before step 6 occurs. This allows the thin filament to slide all the way inward. Steps 2-5 repeat themselves over and over as long as both ATP and calcium ions are present. • Try to pick out these six steps: Step 6. The transport of calcium ions back into the sarcoplasmic reticulum. Step 1. The infux of calcium, triggering the exposure of binding sites on actin. Step 5. The hydrolysis of ATP, whichleads to the re-energizing and repositioning of the cross bridge. Step 2. The binding of myosin to actin. Steps 2-5 repeat over and over before step 6 occurs. Step 4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin. Step 3. The power stroke of the cross bridge that causes the sliding of the thin filaments. ** Now is a good time to go to quiz questions 10-13, and 15: • Click the Quiz button on the left side of the screen. • Click on the scrolling page list at the top of the screen and choose "10. What step just occurred?". • Answer questions 10, 11, 12, 13, skip over question 14, then answer question 15. • After answering question 13, click the Back to Topic button on the left side of the screen. • To get back to where you left off, click on the scrolling page list at the top of the screen and choose "27. Multiple Cross Bridge Cycles". Page 27. Multiple Cross Bridge Cycles Interactive Physiology 6 • During the contraction of a sarcomere about half of the cross bridges are attached to actin and about half are bound at any given time. If all the cross bridges detached at the same time, then the thin filament would slide back on the thick filament. ** Note: Many myosins (about 200) make up each thick filament - so there are many myosin heads (cross bridges) available to bind the thin filament. In this animation you see only four of these cross bridges, but keep in mind there are many more on each thick filament. Page 28. Multiple Myofilaments • Many power strokes occur to bring the Z lines of the sarcomere closer together during the contraction of a muscle cell. During relaxation, the myosin heads detach from the actin and the thin filaments slide back to their resting position. • The width of the H zone decreases during a contraction and increases during relaxation. • The length of the sarcomere shortens during a contraction, but the thin and thick filaments do not shorten, they just slide by each other. ** This is an important animation. View it several times to make sure to understand the theory here. Remember as long as calcium ions and ATP are present, the thin filaments slide toward the center of the sarcomere. When the calcium is taken back up into the SR, then all the cross bridges let go and the thin filaments passively slide back to their resting position. Page 29. Review of the Role of ATP • Summary of the role that ATP plays in the contraction of muscle: 1. ATP transfers its energy to the myosin cross bridge, which in turn energizes the power stroke. 2. ATP disconnects the myosin cross bridge from the binding site on actin. 3. ATP fuels the pump that actively transports calcium ions back into the sarcoplasmic reticulum. Page 30. Summary • The sequence of events in a single cross bride cycle includes: 1. In influx of calcium, triggering the exposure of binding sites on actin. 2. The binding of myosin to actin. 3. The power stroke of the cross bridge that causes the sliding of the thin filaments. 4. The binding of ATP to the cross bridge, which results in the cross bridge disconnecting from actin. 5. The hydrolysis of ATP, which leads to the re-energizing and repositioning of the cross bridge. 6. The transport of calcium ions back into the sarcoplasmic reticulum. • Multiple cross bridge cycling is coordinated sequentially to prevent all cross bridges from either being connected or disconnected at the same time. ** Now is a good time to go to quiz questions 7 and 14: • Click the Quiz button on the left side of the screen. • Click on the scrolling page list at the top of the screen and choose "7. Disconnects the Cross Bridge". • Answer question 7 then click on the scrolling page list at the top of the screen and choose "14. Effect of ATP Depletion". Notes on Quiz Questions: Quiz Question #1. Participants in Sliding Filament Theory • This question allows you to name the six chemicals which participate in the sliding filament theory of muscle contraction. Quiz Question #2. Binding Site for Calcium Ions • This question allows you to chose the participant that binds calcium. Quiz Question #3. Connected to the Z Line • This question allows you to choose the participant that is connected to the Z line. ** You may need to review: Anatomy Review: Skeletal Muscle Tissue p. 9. Quiz Question #4. Composes the Thick Filament • This question allows you to choose the participant that the thick filament is made of. Quiz Question #5. Makes up the Thin Filament • This question allows you to choose the participants that the thin filament is made of. Quiz Question #6. Binding Site for ATP • This question allows you to choose the participant that has a binding site for ATP. Quiz Question #7. Disconnects the Cross Bridge Interactive Physiology 7 • This question allows you to choose the participant that is responsible for disconnecting the cross bridge. Quiz Question #8. Covers Binding Sites on Actin • This question allows you to choose the participant that covers the binding sites on actin. Quiz Question #9. Stored in the Terminal Cisternae • This question allows you to choose the participant that is stored in the terminal cisternae. Quiz Question #10. What Step Just Occurred? • In this question, you are shown a step of cross bridge cycle and asked what just occurred. Quiz Question #11. What Step Will Come Next? • In this question, you are shown a step of cross bridge cycle and asked what will happen next. Quiz Question #12. What Has Just Occurred? • In this question, you are shown a step of cross bridge cycle and asked what just occurred. Quiz Question #13. What Step Will Come Next? • In this question, you are shown a step of cross bridge cycle and asked what will happen next. Quiz Question #14. Effect of ATP Depletion • This question will ask you what will happen if ATP is depleted in the muscle. • An animation shows what happens in rigor mortis. Upon death, the body can't make more ATP, so ATP is unavailable to detach the cross bridges. The muscle contracts and can't relax. Quiz Question #15. Events in a Single Cross Bridge Cycle • This question asks you to list the steps in a single cross bridge cycle. Start at the top "gold bar". Hold down the mouse button and a list of options will appear. Drag the cursor to the first step in a single cross bridge cycle. Continue down the list - then check your answer. Study Questions on the Sliding Filament Theory: 1. (Page 1.) According to the sliding filament theory, when a muscle cell contracts, the _ _ _ _ _ _ _ _ _ _ filaments slide past the ___________ filaments and the ____________ shortens. 2. (Page 3.) List the six most important chemicals involved in muscle contraction. 3. (Page 5.) Where is myosin found in skeletal muscle cells? a. in the thin filaments b. in the thick filaments c. in the sarcoplasmic reticulum d. in the terminal cisternae e. in the T tubules 4. (Page 6,7) a. What are the two parts to a myosin molecule? b. Which part moves providing the power stroke for muscle contraction? c. Which part of the myosin molecule has a hinge which allows vertical movement so that the cross- bridge can bind to actin? 5. (Page 7.) In this diagram of actin, which hinge allows for vertical movement necessary for actin binding and which hinge allows for the power stroke cycle? 6. (Page 9.) Which of these are high energy conformations of myosin? Why? 7. (Page 8,10.) What two important binding sites are found on the cross bridges (heads) of myosin? 8. (Page 10.) What binds at each of these sites on myosin?