Sliding Filament Theory (Muscle Contraction)

Question 1

Explains how actin and myosin filaments interact during skeletal muscle contraction.

Answer 1

During skeletal muscle contraction, actin (thin) and myosin (thick) filaments interact via a “sliding filament” mechanism, where myosin heads, powered by ATP, bind to actin, pull it inward, and then detach, repeating this cycle to shorten the muscle fiber

Question 2

1st Steps in Muscle Contraction

Answer 2

1. Excitation: An action potential reaches the neuromuscular junction.
   
   • Acetylcholine (ACh) is released from the presynaptic terminal.
   • ACh binds to nicotinic receptors on the sarcolemma (muscle membrane).
   • This triggers depolarization, initiating an action potential that propagates along the sarcolemma and down the T-tubules.

Question 3

2nd Steps in Muscle Contraction

Answer 3

Calcium Release: The action potential activates voltage-sensitive dihydropyridine receptors (DHPR) on T-tubules, mechanically linked to ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR).
* This causes calcium ions (Ca²⁺) to be released from the SR into the sarcoplasm.

Question 4

3rd Steps in Muscle Contraction

Answer 4

Cross-Bridge Formation:
Ca²⁺ binds to troponin C, a component of the troponin complex on the actin filament.
* This induces a conformational change that shifts tropomyosin, exposing myosin-binding sites on actin.
* Energized myosin heads (with ADP + Pi bound) attach to the exposed sites on actin, forming cross-bridges.

Question 5

4th Steps in Muscle Contraction

Answer 5

Power Stroke: ADP and Pi are released from the myosin head.
* This triggers the power stroke, where the myosin head pivots, pulling the actin filament toward the center of the sarcomere (M-line).
* ATP binds to the myosin head, causing it to detach from actin.

Question 6

5th Steps in Muscle Contraction

Answer 6

Relaxation: Ca²⁺ is actively pumped back into the SR by SERCA (sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase) pumps.
* The drop in Ca²⁺ levels leads to tropomyosin covering the myosin-binding sites on actin.
* Without cross-bridge cycling, the muscle relaxes and returns to its resting length.

Question 7

Role of ATP in Muscle Contraction

Answer 7

Required to detach myosin from actin after the power stroke.
* Hydrolysis of ATP re-cocks the myosin head for another cycle.
* Powers calcium reuptake into the SR via SERCA pumps during relaxation.

Question 8

Muscular Dystrophy:

Answer 8

A group of genetic disorders, including Duchenne Muscular Dystrophy, caused by mutations in the dystrophin gene.
* Dystrophin stabilizes the sarcolemma during contraction.
* Lack of dystrophin leads to muscle fibre damage, weakness, and progressive wasting due to impaired transmission of force and calcium handling.