Unit 9 Skeleton, muscle and movement
Key unit competence: Relate features and characteristics of various types of muscles to their functioning mechanisms.
Key ideas
- Types of skeleton: hydrostatic — fluid under pressure in a body cavity, with circular and longitudinal muscles acting antagonistically (earthworm, jellyfish); exoskeleton — hard outer chitin cuticle with muscles attached inside (arthropods); protects, prevents water loss, but must be moulted (ecdysis) to grow and is heavy in large animals; endoskeleton — internal bone and cartilage (vertebrates); grows with the body, gives leverage and allows large size.
- Functions of the skeleton/bones: support and shape, protection (skull — brain, ribs — heart and lungs), movement (levers for muscles), blood cell production (red bone marrow), mineral storage (calcium, phosphate), fat storage (yellow marrow). Bone is living tissue: osteocytes in lacunae within a matrix of collagen (flexibility) and calcium phosphate (hardness), arranged in Haversian systems. Cartilage: chondrocytes in a rubbery matrix; hyaline cartilage covers joint surfaces.
- Human skeleton: axial skeleton — skull, vertebral column (7 cervical including atlas and axis, 12 thoracic, 5 lumbar, sacrum, coccyx), ribs and sternum; appendicular skeleton — pectoral girdle (scapula, clavicle), pelvic girdle, and limbs (humerus, radius, ulna; femur, tibia, fibula; carpals, tarsals, phalanges). Features of vertebrae relate to function: cervical (vertebrarterial canals, small body), thoracic (long spine, facets for ribs), lumbar (large body for weight, large processes for back muscles).
- Types of muscle: skeletal (striated, voluntary) — long multinucleate fibres, striped, fast and powerful contraction, fatigues quickly, attached to bones by tendons; smooth (unstriated, involuntary) — spindle-shaped, one nucleus, slow sustained contraction, in walls of gut, blood vessels, uterus; cardiac — striated, branched, uninucleate cells joined by intercalated discs, myogenic, does not fatigue.
- Ultrastructure of skeletal muscle: muscle → bundles → muscle fibres (cells) → myofibrils → repeating units called sarcomeres between Z lines. Thick filaments of myosin (with heads) form the A band (dark); thin filaments of actin (with tropomyosin and troponin) form the I band (light); the H zone is the centre of the A band with only myosin; the M line is the middle. The fibre has a sarcolemma, sarcoplasm, many mitochondria, sarcoplasmic reticulum (stores Ca²⁺) and T-tubules.
- Sliding filament theory: an impulse at the neuromuscular junction spreads along the sarcolemma and down T-tubules; the sarcoplasmic reticulum releases Ca²⁺; Ca²⁺ binds troponin, which moves tropomyosin and exposes binding sites on actin; myosin heads (with ADP + Pᵢ) bind to actin forming cross-bridges; the heads tilt (power stroke), pulling actin towards the M line, and ADP + Pᵢ are released; a new ATP binds, causing the head to detach; ATP is hydrolysed, re-cocking the head; the cycle repeats. Ca²⁺ is pumped back (active transport) when stimulation stops, and the muscle relaxes. During contraction the I band and H zone shorten, the sarcomere shortens, but the A band stays the same. Rigor mortis occurs after death because no ATP is available to detach myosin.
- Energy for contraction: ATP from aerobic respiration, anaerobic respiration (lactate) and creatine phosphate (rapidly regenerates ATP: CP + ADP → creatine + ATP). Slow-twitch fibres: red (much myoglobin), many mitochondria and capillaries, aerobic, fatigue-resistant, for endurance (long-distance running, posture). Fast-twitch fibres: white, fewer mitochondria, more glycogen and creatine phosphate, anaerobic, powerful, fatigue quickly (sprinting, weightlifting).
- Joints: fibrous (immovable) — sutures of the skull; cartilaginous (slightly movable) — between vertebrae (intervertebral discs), pubic symphysis; synovial (freely movable) — capsule, synovial membrane secreting synovial fluid (lubricant), articular cartilage (reduces friction, absorbs shock), ligaments (bone to bone, stability). Synovial types: ball and socket (shoulder, hip; movement in all planes), hinge (elbow, knee; one plane), pivot (atlas–axis; rotation), gliding (wrist, ankle bones), saddle (thumb), condyloid.
- Movement: skeletal muscles work in antagonistic pairs because muscles can only pull (contract), not push. At the elbow, the biceps (flexor) contracts to bend the arm while the triceps (extensor) relaxes; to straighten the arm the triceps contracts and the biceps relaxes. Bones act as levers, joints as fulcra; origin (fixed end) and insertion (moving end). The nervous system coordinates muscles; the cerebellum smooths movement.
- Exercise and injury: regular exercise increases muscle size (hypertrophy), strength, bone density, joint flexibility, more mitochondria and capillaries. Injuries: fractures (simple, compound), dislocation, sprain (ligament), strain (muscle/tendon), torn cartilage. Disorders: arthritis (osteoarthritis — worn cartilage; rheumatoid — autoimmune), osteoporosis (low bone density, especially after menopause; prevent with calcium, vitamin D and weight-bearing exercise), rickets (vitamin D deficiency in children). First aid: RICE — Rest, Ice, Compression, Elevation.
Common misconceptions
- "Muscles push bones back into place." No: muscles can only pull; another muscle (the antagonist) pulls the bone back.
- "Actin and myosin filaments shorten during contraction." No: the filaments stay the same length; they slide past each other.
- "Ligaments join muscles to bones." No: tendons join muscle to bone; ligaments join bone to bone.
- "Bone is dead material." No: bone contains living osteocytes, blood vessels and nerves, and constantly remodels.
Worked examples
Example 1. A sarcomere is 2.4 µm at rest and 1.8 µm when contracted. The A band is 1.6 µm. Calculate the I band length in each state.
Solution
I band total = sarcomere − A band. At rest: 2.4 − 1.6 = 0.8 µm (0.4 µm each side). Contracted: 1.8 − 1.6 = 0.2 µm. The A band stays at 1.6 µm because myosin filaments do not change length.
Example 2. Explain why a marathon runner has more slow-twitch fibres than a sprinter.
Solution
Slow-twitch fibres have many mitochondria, myoglobin and capillaries, so they respire aerobically for long periods without fatigue — ideal for endurance. Sprinters need fast, powerful contraction from fast-twitch fibres using creatine phosphate and anaerobic respiration.
Practice questions
Compare the three types of skeleton.
Show answer
Hydrostatic: fluid pressure, earthworm, soft body; exoskeleton: chitin outside, insects, protects, must moult; endoskeleton: bone inside, vertebrates, grows with body, levers.
Common mistake: Saying insects have bones.
State five functions of the human skeleton.
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Support; protection; movement; blood cell production; storage of calcium and phosphate; attachment of muscles.
Common mistake: Including digestion.
Compare skeletal, smooth and cardiac muscle.
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Skeletal: striated, multinucleate, voluntary, fast, fatigues. Smooth: spindle, uninucleate, involuntary, slow, sustained. Cardiac: striated, branched, intercalated discs, myogenic, no fatigue.
Common mistake: Calling cardiac muscle voluntary.
Describe the structure of a sarcomere.
Show answer
Between two Z lines; thin actin filaments attached to Z lines (I band); thick myosin filaments in centre (A band); H zone with only myosin; M line in middle.
Common mistake: Saying actin forms the A band only.
Explain the sliding filament theory.
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Impulse causes Ca²⁺ release from SR; Ca²⁺ binds troponin; tropomyosin moves; myosin heads bind actin; power stroke pulls actin; ATP binds, heads detach; ATP hydrolysed, heads recock; sarcomere shortens.
Common mistake: Saying filaments shorten.
Compare slow-twitch and fast-twitch fibres.
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Slow: red, myoglobin, many mitochondria and capillaries, aerobic, slow fatigue, endurance. Fast: white, less myoglobin, more glycogen and CP, anaerobic, powerful, quick fatigue.
Common mistake: Swapping the respiration types.
Describe a synovial joint.
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Bones covered by articular cartilage; joint capsule; synovial membrane secretes synovial fluid; ligaments hold bones; allows free movement.
Common mistake: Saying tendons hold the joint together.
Explain how the arm is bent and straightened.
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Biceps contracts, triceps relaxes, forearm raised (flexion); triceps contracts, biceps relaxes, arm straightened (extension); elbow acts as fulcrum; origin on scapula, insertion on radius/ulna.
Common mistake: Saying the biceps pushes the arm straight.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. During contraction, which band stays the same length? a. calcium phosphate 2. The number of cervical vertebrae in humans is: b. A band 3. The hard part of bone matrix is: c. myosin 4. Thick filaments are made of: d. 7 Show answer
1 → b (A band) · 2 → d (7) · 3 → a (calcium phosphate) · 4 → c (myosin)
- Length of myosin.
- Neck vertebrae.
- Gives hardness.
- With heads.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
fibrous joints · cardiac muscle · thoracic · hinge joints- Intercalated discs are found in .
- Vertebrae with facets for ribs are .
- Joints in the skull are .
Show answer
1: cardiac muscle · 2: thoracic · 3: fibrous joints
- Link heart cells.
- 12 in humans.
- Sutures, immovable.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Bones are joined to bones by: a. ATP binds to them 2. Calcium ions for contraction are stored in the: b. ball and socket joint 3. Myosin heads detach from actin when: c. sarcoplasmic reticulum 4. The hip joint is a: d. ligaments Show answer
1 → d (ligaments) · 2 → c (sarcoplasmic reticulum) · 3 → a (ATP binds to them) · 4 → b (ball and socket joint)
- Stability.
- Released by impulse.
- No ATP: rigor mortis.
- Movement in all planes.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
deltoid · biceps · red bone marrow · sarcomere- Red blood cells are made in the .
- Bending the arm at the elbow is done by the .
- The repeating unit of a myofibril is the .
Show answer
1: red bone marrow · 2: biceps · 3: sarcomere
- In spongy bone.
- Flexor.
- Between Z lines.