MUSCLE CONTRACTION- KEY NOTES(CLASS 12 CBSE BIOLOGY)

  

Mechanism of Muscle Contraction

   

1. Types of Muscles

• Skeletal muscles – Voluntary, striated, attached to bones.

• Smooth muscles – Involuntary, non-striated, found in internal organs.

• Cardiac muscles – Involuntary, striated, found only in the heart.

2. Structure of Skeletal Muscle

Muscle → Fascicles → Muscle fibers (cells) → Myofibrils → Sarcomeres

Sarcomere: Structural and functional unit (between two Z-lines)

Bands in Sarcomere:

• A-band: Dark band; both actin and myosin.

• I-band: Light band; only actin.

• H-zone: Only myosin; center of A-band.

• M-line: Middle line of H-zone.

• Z-line: Boundaries of sarcomere.

3. Types of Proteins

• Contractile proteins:

  Actin (thin filament): Two F-actins + tropomyosin + troponin.
• Myosin (thick filament): Heavy chains (tail) + light chains (head).
• Regulatory proteins: Tropomyosin and Troponin.
• Accessory proteins: Help in alignment and elasticity (e.g., titin).

3. Mechanism of Muscle Contraction

Sliding Filament Theory (by Huxley and Niedergerke):

"Actin filaments slide over myosin filaments, shortening the sarcomere."

Steps:

• Nerve impulse → reaches neuromuscular junction.

• Release of Acetylcholine (ACh) → depolarizes sarcolemma.

• Ca²⁺ released from sarcoplasmic reticulum.

• Ca²⁺ binds to troponin, exposing myosin-binding sites on actin.

• Myosin head binds to actin → cross-bridge formation.

• ATP hydrolysis → myosin head pulls actin (power stroke).

• New ATP binds → myosin head detaches → recocks → cycle repeats.

4. Role of ATP and Calcium

• ATP: Energy source for detachment of myosin head and resetting it.

• Calcium ions: Initiate contraction by binding to troponin.

5. Muscle Relaxation

• Calcium ions pumped back into SR.

• Troponin-tropomyosin complex blocks binding sites.

• Muscle returns to resting state.

6.Types of Muscle Contractions

• Isotonic: Muscle changes length (e.g., lifting weight).

• Isometric: Muscle length constant, tension changes (e.g., holding posture).

7.Disorders Related to Muscles (for extras)

• Myasthenia Gravis: Autoimmune disorder; affects neuromuscular junction.

• Muscular Dystrophy: Genetic disease; progressive muscle degeneration.

• Fatigue: Due to lactic acid buildup after anaerobic respiration.