Muscle Contraction — Revision Notes

⚡ 30-Second Revision

Sliding Filament Theory: — Actin slides over myosin.
Sarcomere: — Basic contractile unit (Z-line to Z-line).
Key Proteins:

- Actin (Thin): Contains myosin-binding sites. - Myosin (Thick): Heads form cross-bridges, has ATPase activity. - Troponin: Binds $Ca^{2+}$, moves tropomyosin. - Tropomyosin: Blocks myosin-binding sites on actin in relaxed state.

Ions:

- **$Ca^{2+}$: Binds to troponin, initiating contraction. - $Na^+$:** Influx causes depolarization at NMJ.

Energy: — ATP required for:

1. Myosin head re-cocking (hydrolysis). 2. Myosin detachment from actin (binding). 3. $Ca^{2+}$ reuptake into SR (SERCA pump).

Sarcomere Changes: — I-band and H-zone shorten; A-band length remains constant.

2-Minute Revision

Muscle contraction is explained by the Sliding Filament Theory, where thin actin filaments slide past thick myosin filaments, shortening the sarcomere. The process begins with a neural signal at the neuromuscular junction, releasing acetylcholine, which generates an action potential in the muscle fiber.

This action potential travels via T-tubules, triggering the release of calcium ions ($Ca^{2+}$) from the sarcoplasmic reticulum. $Ca^{2+}$ binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on actin.

Myosin heads then bind to actin, forming cross-bridges. ATP hydrolysis energizes the myosin head for the power stroke, pulling actin. A new ATP molecule causes myosin to detach. This cycle repeats as long as $Ca^{2+}$ and ATP are available.

Relaxation occurs when the neural signal stops, $Ca^{2+}$ is pumped back into the SR, and tropomyosin re-covers the binding sites. ATP is crucial for myosin detachment, re-cocking, and calcium reuptake.

5-Minute Revision

Muscle contraction is a highly coordinated process driven by the Sliding Filament Theory. Each muscle fiber contains myofibrils, which are composed of repeating units called sarcomeres. A sarcomere, bounded by Z-lines, consists of thin actin filaments and thick myosin filaments. In a relaxed state, tropomyosin, stabilized by troponin, covers the myosin-binding sites on actin.

Contraction initiates with a nerve impulse reaching the neuromuscular junction, releasing acetylcholine (ACh). ACh binding to receptors on the muscle fiber generates an action potential that propagates along the sarcolemma and into the T-tubules.

This electrical signal triggers the sarcoplasmic reticulum (SR) to release stored calcium ions ($Ca^{2+}$) into the sarcoplasm. These $Ca^{2+}$ ions bind to Troponin C, causing a conformational change that moves tropomyosin away from the actin-binding sites, making them available for myosin.

The cross-bridge cycle then begins: Myosin heads, already energized by ATP hydrolysis (ATP $\rightarrow$ ADP + Pi), bind to actin, forming cross-bridges. The release of Pi initiates the power stroke, where the myosin head pivots, pulling the actin filament towards the M-line.

ADP is then released. A new ATP molecule binds to the myosin head, causing it to detach from actin. This ATP is then hydrolyzed, re-energizing and re-cocking the myosin head for the next cycle. This continuous cycling shortens the sarcomere, and thus the muscle.

Relaxation occurs when the nerve signal ceases. ACh is broken down, $Ca^{2+}$ is actively pumped back into the SR by SERCA pumps (requiring ATP), and $Ca^{2+}$ detaches from troponin. Tropomyosin then moves back to block the actin-binding sites, preventing further cross-bridge formation. The muscle passively returns to its resting length. ATP is vital for all active processes: myosin re-cocking, detachment, and $Ca^{2+}$ reuptake.

Prelims Revision Notes

Muscle Contraction: NEET Essentials

1. Basic Unit: Sarcomere (Z-line to Z-line). * A-band: Length of myosin (constant during contraction). * I-band: Only actin (shortens). * H-zone: Only myosin (shortens/disappears).

2. Key Proteins:

* Actin (Thin Filament): Globular (G-actin) polymerizes into fibrous (F-actin). Contains myosin-binding sites. * Myosin (Thick Filament): Has globular heads (cross-bridges) with actin-binding and ATP-binding sites (ATPase activity). * Tropomyosin: Covers myosin-binding sites on actin in relaxed state. * Troponin: Complex of three proteins (TnC, TnI, TnT). TnC binds $Ca^{2+}$.

3. Sequence of Events (Excitation-Contraction Coupling & Cross-Bridge Cycle):

1. Neural Signal: Action potential arrives at motor neuron terminal. 2. ACh Release: Acetylcholine (ACh) released into neuromuscular junction (NMJ) synaptic cleft. 3. Muscle Action Potential: ACh binds to receptors on sarcolemma, causing $Na^+$ influx and muscle depolarization (action potential).

4. T-tubule Propagation: Action potential travels along sarcolemma and into T-tubules. 5. **$Ca^{2+}$ Release:** T-tubule depolarization triggers release of $Ca^{2+}$ from sarcoplasmic reticulum (SR) via RyR channels.

6. **$Ca^{2+}$ Binds to Troponin:** $Ca^{2+}$ binds to Troponin C. 7. Tropomyosin Shift: Troponin-Ca$^{2+}$ complex pulls tropomyosin away from actin's myosin-binding sites. 8. Cross-Bridge Formation: Energized myosin heads bind to exposed actin sites.

9. Power Stroke: Myosin head pivots, pulling actin towards M-line (releases Pi, then ADP). 10. ATP Binding & Detachment: New ATP binds to myosin head, causing detachment from actin. 11. Myosin Re-cocking: ATP hydrolyzed (ATP $\rightarrow$ ADP + Pi), re-energizing myosin head.

4. Muscle Relaxation:

1. Neural signal stops, ACh broken down by acetylcholinesterase. 2. $Ca^{2+}$ actively pumped back into SR by SERCA pumps (requires ATP). 3. $Ca^{2+}$ detaches from troponin. 4. Tropomyosin re-covers actin-binding sites. 5. Muscle returns to resting length.

5. Energy (ATP) Requirements:

* Myosin head re-cocking (hydrolysis). * Myosin detachment from actin (binding). * $Ca^{2+}$ reuptake into SR (SERCA pump).

6. Energy Sources: Creatine phosphate (immediate), Anaerobic glycolysis (short-term), Aerobic respiration (long-term).

7. Types of Contraction:

* Isotonic: Muscle length changes (concentric: shortens; eccentric: lengthens). * Isometric: Muscle length constant, tension changes.

Vyyuha Quick Recall

Calcium Triggers The Actin-Myosin Pull.

Calcium: Released from SR.
Triggers: Binds to Troponin.
The: Moves Tropomyosin.
Actin-Myosin: Allows Actin and Myosin to bind (cross-bridge).
Pull: Myosin performs Power stroke (pulls actin).