Skeletal System

Bone Blueprint - Foundation Blocks

• Composition:
  • Organic (~30%): Collagen Type I (major), proteoglycans, glycoproteins.
  • Inorganic (~70%): Hydroxyapatite crystals $Ca_{10}(PO_4)_6(OH)_2$.
• Bone Cells:
  • Osteoblasts: Bone formation; synthesize osteoid; ↑Alkaline Phosphatase.
  • Osteocytes: Mature cells in lacunae; mechanosensation, maintain matrix.
  • Osteoclasts: Bone resorption; multinucleated; Howship's lacunae; ↑TRAP (Tartrate-Resistant Acid Phosphatase). 📌 Mnemonic: OsteoBlasts Build, OsteoClasts Carve.
• Microscopic Types:
  • Lamellar: Mature, organized (compact & spongy bone).
  • Woven: Immature, disorganized (fetal development, fracture repair).
• Macroscopic Classification: Long, short, flat, irregular, sesamoid.

⭐ Volkmann's canals connect Haversian systems (osteons) and run perpendicular to them, transmitting blood vessels from the periosteum into the bone.

Axial Skeleton - Core Framework

• 80 bones: Skull (22), Vertebral Column (26), Thoracic Cage (25), Hyoid (1), Auditory Ossicles (6).
• Skull:
  • Cranium (8): Frontal, Parietal(2), Temporal(2), Occipital, Sphenoid, Ethmoid.
  • Facial (14): Mandible, Maxilla(2), Zygomatic(2), Nasal(2), Lacrimal(2), Palatine(2), Inf. Nasal Concha(2), Vomer.
• Vertebral Column:
  • Cervical (C1-C7; Atlas C1, Axis C2), Thoracic (T1-T12; articulate ribs), Lumbar (L1-L5; largest).
  • Sacrum (5 fused), Coccyx (4 fused).
  • Primary curves (Thoracic, Sacral), Secondary (Cervical, Lumbar).
• Thoracic Cage:
  • Sternum: Manubrium, Body, Xiphoid.
  • Ribs (12 pairs): True (1-7), False (8-10), Floating (11-12).
  • Protects organs, aids respiration.

⭐ Atlas (C1) lacks body & spinous process; allows "yes" head motion via occipital condyle articulation.

Appendicular Skeleton - Limb Levers

Limbs function as levers. Components: Fulcrum (F) joint, Effort (E) muscle insertion, Resistance (R) load.

• Lever Classes: 📌 FRE 123: Fulcrum (1st), Resistance (2nd), Effort (3rd) in middle.
  • Class 1 (EFR/RFE): F between E & R. E.g., Triceps (elbow ext.), atlanto-occipital jt. MA varies.
  • Class 2 (ERF/FRE): R between F & E. E.g., Plantarflexion (tiptoes). MA > 1 (Force adv.).
  • Class 3 (FER/REF): E between F & R. E.g., Biceps (elbow flex.). MA < 1 (Speed/ROM adv.). Most common.
• Mechanical Advantage (MA): $MA = L_E / L_R$
  • $L_E$: Effort Arm length (Fulcrum to Effort).
  • $L_R$: Resistance Arm length (Fulcrum to Resistance).

⭐ Most body levers are Class 3: for speed & ROM, not force.

Joint Junctions - Motion & More

• Classification:
  • Structural: Based on connecting tissues.
    • Fibrous: Dense connective tissue. E.g., Sutures, Syndesmoses, Gomphoses. (Mostly Synarthrosis)
    • Cartilaginous: Cartilage.
      • Primary (Synchondrosis): Hyaline. E.g., Epiphyseal plate. (Synarthrosis)
      • Secondary (Symphysis): Fibrocartilage. E.g., Pubic symphysis. (Amphiarthrosis)
    • Synovial: Joint cavity. E.g., Knee, Shoulder. (Diarthrosis)
  • Functional: Based on degree of movement.
    • Synarthrosis: Immovable.
    • Amphiarthrosis: Slightly movable.
    • Diarthrosis: Freely movable.

• Synovial Joints:
  • Key Features: Articular (hyaline) cartilage, joint capsule, synovial membrane, synovial fluid.
  • Types: Hinge, Pivot, Ball & Socket, Saddle, Condylar, Plane.
  • Movements: Flexion/Extension, Abduction/Adduction, Rotation, Circumduction.

⭐ Hilton's Law: The nerve innervating a joint also typically innervates the muscles moving that joint and the skin overlying the insertions of these muscles.

High‑Yield Points - ⚡ Biggest Takeaways

• Patella is the largest sesamoid bone; Stapes is the smallest bone.
• Pneumatic bones include maxilla, sphenoid, ethmoid, frontal and mastoid.
• Clavicle and skull vault undergo intramembranous ossification; most long bones use endochondral.
• Osteoclasts, multinucleated cells in Howship's lacunae, are for bone resorption.
• Haversian system (Osteon) is compact bone's structural unit; Volkmann's canals connect Haversian canals.
• Primary cartilaginous joints (synchondrosis) have hyaline cartilage; secondary (symphysis) have fibrocartilage.