Chapter 8 — Locomotion and Movement
Key Concepts Quick Recap
| Term | Definition |
|---|
| Sarcomere | Functional unit of skeletal muscle; from Z line to Z line |
| Actin (thin filament) | Composed of F-actin, tropomyosin, and troponin |
| Myosin (thick filament) | Has globular heads (cross-bridges) that bind to actin; ATPase activity |
| Cross-bridge | Myosin head attached to actin during contraction |
| ATP | Energy for muscle contraction; ATPase on myosin heads hydrolyses it |
| Ca²⁺ | Released from SR; binds troponin → removes tropomyosin inhibition → allows cross-bridge |
| Axial skeleton | Skull (22) + Vertebral column (26) + Sternum (1) + Ribs (24) = 80 bones |
| Appendicular skeleton | Limbs (60) + Pectoral girdle (4) + Pelvic girdle (2) = 126 bones |
| Total human bones | 206 bones (adult) |
I. Choose the Best Answer (MCQs)
1. The number of bones in an adult human body is: (a) 300 (b) 270 (c) 206 (d) 180
Answer(c) 206 Adult human has 206 bones. Newborns have ~300-350 bones; many fuse as the body matures (e.g. skull fontanelles close; vertebral column bones fuse in sacrum and coccyx).
2. The functional unit of a skeletal muscle is: (a) Myofibril (b) Sarcomere (c) Muscle fibre (d) Fascicle
Answer(b) Sarcomere Sarcomere = unit from Z line to Z line. Contains A band (dark: myosin + overlap with actin), I band (light: actin only), H zone (myosin only), M line (centre of A band). During contraction, sarcomeres shorten.
3. Calcium ions bind to which protein during muscle contraction? (a) Myosin (b) Tropomyosin (c) Troponin (d) Titin
Answer(c) Troponin At rest: tropomyosin covers actin's myosin-binding sites. When Ca²⁺ is released from sarcoplasmic reticulum, it binds to troponin (part of troponin-tropomyosin complex) → troponin changes shape → tropomyosin moves → actin's binding sites exposed → myosin heads bind to actin → cross-bridge formed → contraction begins.
4. What is the main source of energy for muscle contraction? (a) Glycogen (b) ATP (c) Creatine phosphate (d) Glucose directly
Answer(b) ATP ATP directly powers muscle contraction. Myosin ATPase hydrolyses ATP → ADP + Pi → releases energy → myosin head moves (power stroke). Creatine phosphate is an immediate ATP reserve. Cellular respiration (aerobic and anaerobic) regenerates ATP.
5. Rigor mortis (stiffness of body after death) occurs because: (a) Muscles produce excess ATP (b) Ca²⁺ levels drop suddenly (c) ATP is no longer produced — myosin heads remain attached to actin (d) All muscles become flaccid
Answer(c) ATP is no longer produced — myosin heads remain attached to actin After death, ATP synthesis stops. Without ATP, myosin heads cannot detach from actin (detachment requires ATP binding). Cross-bridges remain locked → muscles rigid = rigor mortis. Lasts 12–36 hours until muscle proteins begin to decompose.
6. Synovial joints allow: (a) No movement (b) Slight gliding movement (c) Free movement in multiple directions (d) Movement in one plane only
Answer(c) Free movement in multiple directions Synovial joints are the most common and movable joints. Features: synovial cavity (with synovial fluid for lubrication), articular cartilage, joint capsule. Examples: ball-and-socket (shoulder, hip — all directions), hinge (knee, elbow — one plane), pivot (neck — rotation).
7. Osteoporosis is caused by: (a) Excess calcium (b) Deficiency of calcium and Vitamin D (c) Excess Vitamin D (d) Excess potassium
Answer(b) Deficiency of calcium and Vitamin D Osteoporosis: loss of bone density and mass. Common in post-menopausal women (reduced estrogen → less Ca²⁺ retention). Risk factors: Vitamin D deficiency (poor Ca²⁺ absorption), low calcium diet, sedentary lifestyle, smoking. Bones become brittle and prone to fractures.
II. Short Answer Questions
Q1. Describe the sliding filament theory of muscle contraction.
Answer
Proposed by Hugh Huxley and Jean Hanson (1954).
Steps of muscle contraction:
1. Nerve impulse arrives at neuromuscular junction:
Answer
- Motor neuron releases acetylcholine → binds to sarcolemma receptors → action potential in muscle fibre.
2. Ca²⁺ release:
Answer
- Action potential spreads through T-tubules → sarcoplasmic reticulum releases Ca²⁺.
3. Cross-bridge formation (active state):
Answer
- Ca²⁺ binds to troponin → tropomyosin moves → actin binding sites exposed.
- Myosin heads (charged with ATP → ADP + Pi) bind to actin → cross-bridges form.
4. Power stroke:
Answer
- ADP + Pi released → myosin head pivots → pulls actin toward M line → sarcomere shortens.
5. Cross-bridge detachment:
Answer
- Fresh ATP binds to myosin head → head detaches from actin.
- ATP hydrolysed → myosin head cocked (ready for next cycle).
6. Relaxation:
Answer
- Nerve impulse stops → Ca²⁺ pumped back into SR → tropomyosin covers actin → cross-bridges cannot form → muscle relaxes.
Changes during contraction:
- I band: shortens (actin slides in)
- H zone: shortens (actin covers more of myosin)
- A band: remains CONSTANT (myosin length unchanged)
- Sarcomere: shortens
Q2. Classify joints in the human body with examples.
Answer
Joints are points of articulation between bones.
1. Fibrous joints (Synarthroses — immovable):
Answer
- Bones held together by fibrous connective tissue.
- Example: Sutures in the skull, between teeth and jaw (gomphosis).
2. Cartilaginous joints (Amphiarthroses — slightly movable):
Answer
- Bones connected by cartilage.
- Example: Pubic symphysis, intervertebral discs.
3. Synovial joints (Diarthroses — freely movable):
Answer
- Synovial cavity with synovial fluid (lubrication).
- Types:
- Ball and socket: Shoulder (humerus + scapula), hip — maximum movement.
- Hinge: Elbow, knee, ankle — movement in one plane.
- Pivot: Atlas-axis joint (neck) — rotation.
- Gliding (Plane): Between carpals — sliding movement.
- Saddle: Carpometacarpal joint of thumb.
- Condyloid (Ellipsoid): Between radius and wrist bones — biaxial.
Common Mistakes
- Saying the A band shortens during contraction — the A band stays CONSTANT; only I band and H zone shorten.
- Forgetting that ATP is needed for DETACHMENT of myosin from actin (not just for the power stroke).
- Confusing axial skeleton (80 bones: skull + vertebral column + ribs) and appendicular (126 bones: limbs + girdles).
- Total bones: 206 (adult); NOT 300 (that is infants).
Exam Checklist
- Total bones: 206 (adult); axial (80) + appendicular (126).
- Sarcomere: Z-Z; I band (actin), A band (actin+myosin), H zone (myosin only).
- During contraction: I band ↓, H zone ↓, A band constant.
- Ca²⁺ binds troponin → moves tropomyosin → exposes actin binding sites.
- ATP required for: cross-bridge formation (power stroke) AND detachment.
- Rigor mortis = no ATP → myosin locked onto actin.
- Synovial joints: ball-and-socket (shoulder), hinge (knee/elbow), pivot (neck).
