Support & Movement: MDCAT Notes and MCQs
Cartilage
- Flexible connective tissue; cells = chondrocytes in spaces called lacunae; matrix of chondrin (protein + chondroitin). No blood vessels (nutrients diffuse from perichondrium) → heals slowly.
- Types: Hyaline (most common; ends of bones, nose, trachea rings), Elastic (pinna of ear, epiglottis), Fibrocartilage (intervertebral discs, pubic symphysis — strongest).
Bone
- Hard connective tissue; matrix = collagen fibres + calcium phosphate (hydroxyapatite).
- Cells: osteoblasts (form bone), osteocytes (mature cells in lacunae), osteoclasts (resorb bone).
- Compact bone organised into Haversian systems (osteons); spongy bone contains red marrow (blood cell formation).
- Functions: support, protection, movement (levers), mineral storage (Ca, P), blood cell production.
Three muscle types
| Skeletal | Cardiac | Smooth | |
|---|---|---|---|
| Striations | Yes | Yes | No |
| Control | Voluntary | Involuntary | Involuntary |
| Shape / nuclei | Long cylindrical, multinucleate | Branched, 1–2 nuclei, intercalated discs | Spindle-shaped, 1 nucleus |
| Fatigue | Fatigues quickly | Never fatigues | Slow, sustained |
| Location | Attached to bones | Heart wall | Gut, blood vessels, uterus |
Ultrastructure of skeletal muscle
- Muscle → fascicles → muscle fibres (cells; membrane = sarcolemma, cytoplasm = sarcoplasm) → myofibrils → sarcomeres.
- Sarcomere = Z-line to Z-line (functional unit). A-band (dark, anisotropic) = thick myosin filaments; I-band (light, isotropic) = thin actin only; H-zone = centre of A-band with myosin only; M-line in middle.
- Thin filament: actin + tropomyosin + troponin.
Contraction — sliding filament theory (Huxley & Hanson)
- Impulse at neuromuscular junction → acetylcholine → action potential spreads via T-tubules.
- Ca2+ released from sarcoplasmic reticulum → binds troponin → tropomyosin shifts, exposing binding sites on actin.
- Myosin heads (with ADP + Pi) form cross-bridges, power stroke pulls actin towards M-line.
- ATP binds → myosin detaches → ATP hydrolysed to re-cock the head. Lack of ATP after death → rigor mortis.
During contraction: sarcomere, I-band and H-zone shorten; A-band length stays the same.
Joints
- Fibrous / immovable — sutures of skull. Cartilaginous / slightly movable — between vertebrae, pubic symphysis. Synovial / freely movable — have synovial fluid: ball-and-socket (shoulder, hip), hinge (elbow, knee), pivot (atlas–axis), gliding (wrist), saddle (thumb).
Arthritis
Inflammation of joints causing pain and stiffness. Osteoarthritis — degeneration of articular cartilage (ageing). Rheumatoid arthritis — autoimmune inflammation of synovial membrane. Gout — deposition of uric acid crystals.
PMDC learning outcomes for this unit
- Human skeleton: 9.1 Describe cartilage, muscle and bone
- Human skeleton: 9.2 Explain the main characteristics of cartilage and bone along with functions.
- Muscles: 9.3 Compare characteristics of smooth muscles, cardiac muscles and skeletal muscles
- Skeletal muscles: 9.4 Explain the ultra-structure of skeletal muscles
- Muscle contraction: 9.5 Describe in brief the process of skeletal muscle contraction
- Joints: 9.6 Classify joints
- Arthritis: 9.7 Define arthritis