1. Explain the Sliding Filament Theory of Muscle Contraction.
Answer:
The Sliding Filament Theory proposes that muscle contraction occurs when thin (actin) and thick (myosin) filaments slide past each other within the muscle fiber, shortening the sarcomere. During contraction, the myosin heads attach to the binding sites on actin filaments, forming cross-bridges. As the myosin heads pivot, they pull the actin filaments toward the center of the sarcomere, which shortens the muscle. This movement continues as long as calcium ions are present and ATP is available to detach the myosin heads and re-cock them for another power stroke.
2. What is the role of calcium ions in the sliding filament theory?
Answer:
Calcium ions play a critical role in muscle contraction. When a nerve impulse reaches the muscle, calcium is released from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber. Calcium ions bind to the troponin complex, which causes a conformational change in tropomyosin, exposing the myosin-binding sites on the actin filaments. This allows the myosin heads to attach to actin, initiating the power stroke and thus muscle contraction.
3. Describe the process of excitation-contraction coupling.
Answer:
Excitation-contraction coupling is the process by which an electrical signal (action potential) is translated into a mechanical response (muscle contraction). The action potential travels along the motor neuron and reaches the neuromuscular junction, releasing acetylcholine. This neurotransmitter binds to receptors on the muscle fiber’s sarcolemma, causing an action potential in the muscle fiber. The action potential travels through the T-tubules to the sarcoplasmic reticulum, triggering the release of calcium ions. These ions enable the sliding filament mechanism, resulting in muscle contraction.
4. What is the role of ATP in muscle contraction?
Answer:
ATP is essential for muscle contraction and relaxation. During contraction, ATP binds to the myosin head, enabling it to detach from the actin filament after the power stroke. ATP is also necessary for the re-cocking of the myosin heads so they can perform another power stroke. During muscle relaxation, ATP is used to actively pump calcium ions back into the sarcoplasmic reticulum, which prevents further contraction by binding to troponin and moving tropomyosin back over the actin-binding sites.
5. How do the sarcomere and its components contribute to muscle contraction?
Answer:
The sarcomere is the basic functional unit of a muscle, composed of actin (thin) and myosin (thick) filaments. The myosin filaments contain cross-bridges (myosin heads) that attach to actin during contraction. The actin filaments slide past the myosin filaments as the myosin heads pivot, shortening the sarcomere. Other components of the sarcomere, such as the Z-disc, M-line, A-band, I-band, and H-zone, help to maintain the structure of the sarcomere and facilitate the sliding process during contraction.
6. What happens to the sarcomere during muscle contraction?
Answer:
During muscle contraction, the sarcomere shortens. This occurs because the actin filaments slide past the myosin filaments, pulling the Z-discs closer together. As a result, the I-band (which contains only actin) and the H-zone (which contains only myosin) become narrower. The A-band (which contains both actin and myosin) remains the same length, but the overlap between actin and myosin increases, leading to the overall shortening of the sarcomere.
7. What are the roles of troponin and tropomyosin in muscle contraction?
Answer:
Troponin and tropomyosin work together to regulate muscle contraction. Tropomyosin is a protein that lies along the actin filament and blocks the myosin-binding sites in a relaxed muscle. Troponin is a protein complex attached to tropomyosin. When calcium ions are released into the cytoplasm, they bind to troponin, causing it to change shape. This movement shifts tropomyosin, exposing the myosin-binding sites on actin and allowing myosin heads to form cross-bridges, leading to muscle contraction.
8. Explain the role of myosin in the sliding filament theory.
Answer:
Myosin is a thick filament protein that plays a central role in muscle contraction. Each myosin molecule has a head region capable of binding to actin and forming cross-bridges. During contraction, the myosin heads attach to exposed binding sites on the actin filaments, and through a process called the power stroke, they pull the actin filaments toward the center of the sarcomere. The energy for this movement comes from the hydrolysis of ATP. The myosin heads then detach from actin, re-cock, and bind to a new site to repeat the process.
9. What is the function of the sarcoplasmic reticulum in muscle contraction?
Answer:
The sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum in muscle fibers that stores calcium ions. When an action potential reaches the muscle fiber, it triggers the release of calcium ions from the SR into the cytoplasm. These calcium ions bind to troponin, initiating the contraction process. After contraction, the calcium ions are actively pumped back into the SR, ending the contraction and allowing the muscle to relax.
10. What is the sliding filament theory’s explanation for muscle relaxation?
Answer:
Muscle relaxation occurs when the neural signal stops, leading to the cessation of acetylcholine release and the termination of the action potential in the muscle fiber. Calcium ions are actively pumped back into the sarcoplasmic reticulum, lowering their concentration in the cytoplasm. As the calcium ions dissociate from troponin, tropomyosin moves back to block the actin-binding sites, preventing further interaction between actin and myosin. This process leads to the cessation of cross-bridge formation, allowing the muscle to relax.
11. Describe the structure of actin and myosin filaments and their role in muscle contraction.
Answer:
Actin filaments are thin, helical structures composed of globular actin monomers. They have binding sites for myosin heads, which are crucial for muscle contraction. Myosin filaments are thick, made up of myosin molecules with long tails and globular heads. The myosin heads interact with the actin filaments during contraction. The myosin heads pivot and pull the actin filaments towards the center of the sarcomere, generating force and shortening the muscle. This interaction between actin and myosin is central to the sliding filament mechanism.
12. What is the role of ATP in the detachment of myosin heads from actin?
Answer:
ATP plays a crucial role in muscle contraction by enabling the detachment of myosin heads from actin. After a power stroke, the myosin head is bound to actin. ATP binds to the myosin head, causing a conformational change that weakens the bond between myosin and actin, leading to detachment. ATP is then hydrolyzed to ADP and inorganic phosphate, which re-energizes the myosin head, preparing it for another cycle of contraction.
13. What is the significance of the T-tubules in muscle contraction?
Answer:
T-tubules (transverse tubules) are extensions of the sarcolemma that penetrate deep into the muscle fibers. They play a key role in conducting action potentials from the surface of the muscle fiber to the interior, ensuring that the signal reaches the sarcoplasmic reticulum. This allows for a synchronized release of calcium ions throughout the muscle fiber, enabling uniform contraction of the entire muscle.
14. How do the Z-discs contribute to muscle contraction?
Answer:
The Z-discs are the boundaries of a sarcomere, marking the ends of actin filaments. During muscle contraction, the actin filaments are pulled toward the center of the sarcomere, causing the Z-discs to move closer together. This shortening of the sarcomere is responsible for the overall shortening of the muscle fiber and, in turn, the contraction of the muscle.
15. Explain the power stroke in muscle contraction.
Answer:
The power stroke is the process by which the myosin heads pull the actin filaments toward the center of the sarcomere during muscle contraction. After myosin heads bind to actin, they undergo a conformational change, powered by the hydrolysis of ATP. This causes the myosin heads to pivot, pulling the actin filaments and causing the sarcomere to shorten. The power stroke is repeated as long as ATP and calcium ions are available, resulting in continuous contraction.
16. What are the steps involved in the cycle of cross-bridge formation?
Answer:
The cross-bridge cycle involves several steps:
- ATP binding: ATP binds to the myosin head, causing it to detach from the actin filament.
- Hydrolysis of ATP: The ATP is hydrolyzed into ADP and inorganic phosphate, which energizes the myosin head.
- Cross-bridge formation: The myosin head binds to an exposed site on the actin filament.
- Power stroke: The myosin head pivots, pulling the actin filament toward the center of the sarcomere.
- Detachment: The myosin head releases ADP and inorganic phosphate and detaches from actin when a new ATP molecule binds.
17. What is the role of creatine phosphate in muscle contraction?
Answer:
Creatine phosphate is a high-energy molecule stored in muscle cells. It serves as a quick source of energy for muscle contraction by donating a phosphate group to ADP, forming ATP. This reaction is catalyzed by creatine kinase. Creatine phosphate helps replenish ATP during short bursts of intense activity, enabling the muscle to sustain contraction until ATP production from glycolysis or oxidative phosphorylation takes over.
18. What is the significance of the A-band in the sarcomere during contraction?
Answer:
The A-band is the region of the sarcomere that contains the full length of the thick myosin filaments. It appears unchanged during muscle contraction because the length of the myosin filaments remains constant. The A-band does not shorten; instead, the actin filaments slide past the myosin filaments, causing a relative increase in overlap and thus shortening the sarcomere.
19. What is the difference between isotonic and isometric contractions?
Answer:
Isotonic contractions occur when the muscle changes length while generating constant tension. An example is lifting a weight, where the muscle shortens (concentric contraction) or lengthens (eccentric contraction). Isometric contractions occur when the muscle generates tension without changing its length, such as holding a weight in place without moving it.
20. How does muscle fatigue relate to the sliding filament theory?
Answer:
Muscle fatigue occurs when the muscle can no longer maintain the force of contraction. This can result from the depletion of ATP, accumulation of metabolic byproducts like lactic acid, or disturbances in the calcium ion regulation. As ATP becomes scarce, the myosin heads are unable to detach from actin, leading to a reduction in cross-bridge formation and thus muscle force. The muscle becomes unable to contract effectively, resulting in fatigue.
