4.1 Sliding Filament Theory Flashcards
Explain how skeletal muscle contracts by the sliding filament theory in short.
4.1.3
Thick and thin filaments within a sarcomere slide past each other, leading to muscle contraction. This process involves the binding of myosin to actin, the pivoting of the myosin head, and the use of ATP to power these movements. The regulation of this process is dependent on calcium ions and the proteins troponin and tropomyosin.
Microanatomy of a Muscle Fiber. Describe the following structures:
Transverse tubules (T tubules)
Myofibrils
Sarcomeres
Sarcolemma
Sarcoplasm
Sarcoplasmic reticulum (SR)
Transverse tubules (T tubules): ->
Myofibrils (contraction organelle): A bundle of Myofilaments
Sarcomere: Repeating structural unit of the myofibril
Sarcolemma: Muscle cell membrane surrounding the whole muscle fiber
Sarcoplasm: Muscle cell cytoplasm that contains organelles
- Ca2+ is released from the SR into the Sarcoplasm which initiates muscle contraction
Sarcoplasmic reticulum (SR) : Network of membranes surrounding each myofibril, storing Calcium (Ca2+)
How could you define the sliding filament theory? What is this theory based on?
The series of events involved in causing a skeletal muscle contraction. The theory is based on the structure of myofibrils.
What are myofibrils? What are myofibrils composed of?
They’re long, thread-like structures found in muscle fibers. Myofibrils are composed of repeating units called sarcomeres (smallest functional unit of a muscle fiber)
How are sarcomeres involved in muscle contractions?
Sarcomeres have parallel, overlapping proteins (called filaments) that slide past each other when contracting or relaxing a muscle.
Describe the components of a sarcomere unit.
-Sarcomeres are made up of 2 types of protein fibers: thick & thin myofilaments
thick filament: Myosin
thin filaments: Actin, Troponin, Tropomyosin:
- Z-disk: is the border of each sarcomere
- I-band: extends from myosin to myosin across 2 different sarcomeres.
- A-band: runs length of myosin filament & overlaps on some actin
- H-zone: spans middle of sarcomere (only has myosin). These filaments slide past each other to contract muscle.
Describe what thick filaments are made up of.
Thick filaments are made up of many bonded units of the protein myosin. Myosin is the protein that causes muscles to contract.
Myosin itself has a long tail & head which bind on specific sites on actin filaments.
Describe what thin filaments are made up of.
Thin filaments are made up of 3 proteins.
1. Actin- forms a helical structure which is the majority of the thin filament mass. Actin contains binding sites that allow myosin to connect to and move actin during muscle contraction.
- Tropomyosin- is a long protein fiber that wraps around actin & covers the myosin binding sites on actin. Essentially it prevents myosin from binding to actin in resting muscles.
- Troponin- is bound very tightly to tropomyosin. Troponin moves tropomyosin away from the myosin binding sites during muscle contraction.
In short, outline the contraction process.
- Actin active site & myosin cross-bridge interact
- Thin filaments slide past thick filament
- cross-bridges undergo a cycle of movement
- attach, pivot, detach, return - Troponin-tropomyosin control interaction
- prevent interaction at rest
Describe this resting sarcomere.
- The actin & myosin filaments aren’t straight lines but complex 3D shapes.
- Tropomyosin covers active site on actin, hindering myosin to form a cross-bridge
ATP hydrolysis prior to cross-bridge:
- ATP binds to myosin and ‘recharges’ it -> moving it to a high-energy state
- ATP is hydrolyzed into ADP & inorganic phosphate
- ATP hydrolysis releases energy to change angle of myosin head into ‘cocked’ position -> prepares to bind to actin if sites are available.
Explain Step 1 of the sliding filament theory
Hint: It’s called “Active-_____ _______”
Step 1: Active-site exposure
When muscle fiber receives the signal to contract, the sarcoplasmic reticulum, a network of membranes surrounding each myofibril, releases calcium ions. The calcium ions bind to troponin, causing troponin to change it’s shape & move tropomyosin away from the myosin-binding sites on actin.
Explain Step 2 of the sliding filament theory
Hint: It’s called the “_____-_____ formation”
Step 2: Cross-bridge formation
Once tropomyosin is moved away (by troponin) from the myosin-binding site on actin, the high-energy myosin head bridges to form a cross-bridge. Once myosin is attached to actin, it releases phosphate.
(As seen on the picture, the cross-bridge is not straight but diagonal. This enables the the filament to be flung forward -> called a power stroke)
Explain Step 3 of the sliding filament theory
Hint: It’s called the “Pivoting of the ______ ____”
Step 3: The pivoting of the myosin head, the power stroke
Once a cross-bridge is formed, the myosin head pivots, pulling the actin filament toward the center of the sarcomere, the M-line. This action shortens the sarcomere, causing muscle contraction. As myosin’s head bends during the power stroke, ADP is released,
Explain Step 4 of the sliding filament theory.
Hint: It’s called “Cross-______ __________”
Step 4: Cross-bridge detachment
After the myosin head pivots, the cross-bridge remains. ADP and phosphate rejoin to form an ATP molecule which will bind to myosin. This allows the myosin head to detach from actin.
Explain Step 5 of the sliding filament theory.
Hint: It’s called “Myosin __________”
Step 5: Myosin reactivation
After myosin detaches from actin through ATP, the ATP is hydrolysed to ADP & inorganic phosphate, providing the needed energy to re-cock myosin head in preparation for another cycle of contraction.
What is the role of ATP the sliding filament theory?
ATP provides the energy for the contraction.
ATP hydrolysis prior to cross-bridge:
- ATP binds to myosin and ‘recharges’ it -> moving it to a high-energy state
- ATP is hydrolyzed into ADP & inorganic phosphate
- ATP hydrolysis releases energy to change angle of myosin head into ‘cocked’ position -> prepares to bind to actin if sites are available.
Cross-Bridge Muscle Contraction Cycle:
- Once myosin-binding site exposed, high-energy myosin head bridges forms cross-bridge, releasing phosphate
- After power stroke towards M-line, myosin head bends -> releases ADP
Cross-bridge detachment:
- After release of ADP, cross-bridge remains
- ADP & phosphate rejoin to reform ATP
- ATP binds to myosin -> moving myosin to high-energy state
- This releases the myosin head from actin active site
