Quick Summary
Movement is a fundamental characteristic of animals. Muscle contraction (powered by ATP and triggered by Ca²⁺) is the basis of locomotion. The sliding filament theory explains how actin and myosin interact. The skeletal system provides the framework, and joints allow movement. Disorders like osteoporosis and arthritis are also tested.
Types of Movement
| Type | Description | Example |
|---|---|---|
| Amoeboid | Pseudopodia formation | WBCs (phagocytosis), Amoeba |
| Ciliary | Cilia beating | Respiratory tract (mucus movement), oviduct (egg movement) |
| Muscular | Muscle contraction | Limb movement, heartbeat, peristalsis |
Skeletal System
Axial skeleton (80 bones):
- Skull (22): cranium (8) + facial (14)
- Vertebral column: 33 vertebrae (cervical 7, thoracic 12, lumbar 5, sacral 5 fused, coccygeal 4 fused)
- Sternum + 12 pairs of ribs (true 7, false 3, floating 2)
Appendicular skeleton (126 bones):
- Pectoral girdle: clavicle + scapula (each side)
- Upper limb: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges
- Pelvic girdle: ilium, ischium, pubis (fused = os coxa / hip bone)
- Lower limb: femur (largest/strongest bone), tibia, fibula, patella, 7 tarsals, 5 metatarsals, 14 phalanges
Joints
| Type | Movement | Example |
|---|---|---|
| Fibrous (synarthrosis) | None (immovable) | Skull sutures |
| Cartilaginous (amphiarthrosis) | Slight | Intervertebral discs, pubic symphysis |
| Synovial (diarthrosis) | Freely movable | Most limb joints |
Synovial Joint Types
| Type | Movement | Example |
|---|---|---|
| Ball and socket | All directions | Shoulder, hip |
| Hinge | Flexion/extension | Knee, elbow, ankle |
| Pivot | Rotation | Atlas-axis (head turning) |
| Gliding | Sliding | Wrist carpals |
| Saddle | Two axes | Thumb carpometacarpal |
| Condyloid (ellipsoid) | Two axes | Wrist joint |
Structure of Skeletal Muscle
Levels: Muscle → fascicle → muscle fibre → myofibril → sarcomere (functional unit)
Sarcomere:
- A band (dark): Myosin + actin overlap region
- I band (light): Actin only (no myosin)
- H zone: Myosin only (centre of A band)
- Z line (disc): Boundary of sarcomere; actin attached
- M line: Centre of A band; myosin attached
Thin filaments: Actin + Tropomyosin + Troponin Thick filaments: Myosin (with globular head — ATPase activity)
Sliding Filament Theory of Muscle Contraction
1. Nerve impulse → end plate potential → Ca²⁺ released from sarcoplasmic reticulum (SR) 2. Ca²⁺ binds to troponin → removes tropomyosin block → actin binding sites exposed 3. Myosin head binds actin → forms cross-bridge; ATP hydrolysis (ATPase) → power stroke → actin slides inward 4. New ATP binds → cross-bridge detaches → myosin head re-cocks 5. Cycle repeats → I band and H zone shorten; A band length unchanged 6. When nerve impulse stops: Ca²⁺ pumped back to SR → troponin covers binding sites → muscle relaxes
Changes During Contraction
| Band/Zone | Change |
|---|---|
| A band | No change (myosin length constant) |
| I band | Shortens (actin slides in) |
| H zone | Shortens (actin overlaps more of myosin) |
| Sarcomere length | Shortens |
Disorders
| Disorder | Description |
|---|---|
| Myasthenia gravis | Autoimmune; acetylcholine receptors attacked → muscle weakness |
| Muscular dystrophy | Genetic; progressive muscle degeneration |
| Tetany | Hypocalcaemia → rapid muscle spasms |
| Arthritis | Joint inflammation; osteoarthritis (wear and tear), rheumatoid (autoimmune) |
| Osteoporosis | Reduced bone density; common in post-menopausal women (low oestrogen) |
| Gout | Uric acid crystals in joints → pain |
Important Questions
One Mark
- Name the functional unit of a muscle.
- What is the role of Ca²⁺ in muscle contraction?
- Which is the largest bone in the human body?
- What is the total number of bones in the adult human body?
Short Answer (3 marks)
- Describe the changes in I band, A band, and H zone during muscle contraction.
- What is a synovial joint? Give two examples with the type of movement.
- Distinguish between rigor mortis and tetany.
Long Answer (5 marks)
- Explain the sliding filament theory of muscle contraction with the role of Ca²⁺, actin, and myosin.
- Describe the axial skeleton of humans. How many bones are in each region?
Common Mistakes
- A band does NOT change length during contraction — only I band and H zone shorten.
- Ca²⁺ is released from sarcoplasmic reticulum (NOT mitochondria) during contraction.
- Tropomyosin BLOCKS actin-myosin binding at rest; troponin-Ca²⁺ complex REMOVES this block.
- Femur is the longest/strongest bone; stapes (in ear) is the smallest bone.
Board Exam Tips
- Sliding filament theory (step by step) is a standard 5-mark question — include all components.
- Sarcomere diagram with A band, I band, H zone, Z line, M line labels is a regular question.
- "Changes in bands during contraction" is a classic 2-mark or 3-mark question.
- NEET: Sliding filament theory, joint types, and skeletal numbers (206 bones, axial 80, appendicular 126) appear frequently.