The Skeletal System

Study Unit 3: The Skeletal System

3.1 Osseous Tissue and Bone Structure

3.1.1 Study Outcomes: Primary Functions of the Skeletal System
  • Support: Provides structural support for the body; serves as a framework for soft tissue and organ attachment.
  • Storage of Minerals and Lipids: Stores inorganic ions (minerals) and lipids (energy) in yellow bone marrow.
  • Blood Cell Production: Produces red and white blood cells in red bone marrow.
  • Protection: Protects internal organs (e.g., skull protects brain, ribs protect thoracic organs).
  • Leverage: Bones act as levers to enhance muscle force and movement efficiency.
Classification of Bones by Shape and Internal Organization
  1. Sutural Bones (Wormian Bones):
    • Small, flat, oddly shaped bones between cranial bones.
    • Size ranges from a grain of sand to a quarter.
  2. Irregular Bones:
    • Complex shapes; e.g., vertebrae, bones of the pelvis, some bones in the skull.
  3. Short Bones:
    • Boxlike appearance; e.g., carpal (wrist) and tarsal (ankle) bones.
  4. Flat Bones:
    • Thin and parallel surfaces; protect soft tissues and provide muscle attachment sites; e.g., skull roof, sternum, ribs, scapulae.
  5. Long Bones:
    • Long and slender; e.g., femur, arm/forearm bones, palms, soles, fingers and toes.
  6. Sesamoid Bones:
    • Small, round, flat bones found near joints; e.g., patella (kneecaps).
Cell Types in Bone and Their Major Functions
  • Osteogenic Cells: Maintain populations of osteoblasts; important in fracture repair.
  • Osteoblasts: Produce new bone matrix and help deposit calcium salts; convert osteoid to bone.
  • Osteocytes: Maintain matrix mineral content; participate in repairing damaged bone.
  • Osteoclasts: Absorb and remove bone matrix.
Bone Markings
  • Projections: Sites where muscles, tendons, and ligaments attach and where joints form.
  • Openings and Depressions: Sites for blood vessels or nerves alongside/through bone.
Compact vs. Spongy Bone
  • Compact Bone:
    • Densest where stresses come from one direction.
    • Contains osteons, concentric lamellae surrounding a central canal, perforating canals.
  • Spongy Bone:
    • Less dense; forms a meshwork of trabeculae.
    • Provides support and houses bone marrow.
Mechanisms of Bone Formation
  • Endochondral Ossification:
    • Bone develops from hyaline cartilage models, beginning in the second month of development.
  • Intramembranous Ossification:
    • Bone develops within mesenchymal tissue, starting around the 8th week of embryonic development.

3.1.2 How Fractures Heal

  1. Hematoma Formation: Blood clots at fracture site following extensive bleeding.
  2. Callus Formation: Internal callus of spongy bone and external callus of cartilage stabilize the fracture area.
  3. Bone Replacement: Cartilage in external callus is replaced by bone, and spongy bone connects fractured ends.
  4. Remodelling: Swelling reduces, and new compact bone forms.
Types of Fractures
  1. Displaced vs. Nondisplaced - alignment of bone ends.
  2. Complete vs. Incomplete - completeness of break.
  3. Linear vs. Transverse - orientation of break.
  4. Open vs. Closed - penetration of skin.
    • Greenstick Fracture (children): One side of bone bends, opposite side bends.
Aging and the Skeletal System
  • Bone density decreases with age; up to 8% loss in women, 3% in men post-30.
  • Osteoblast activity decreases; leads to osteoporosis risks.

3.1.3 Hormones in Bone Growth and Maintenance

  • Calcitriol: Increases calcium/phosphate absorption.
  • Growth Hormone: Stimulates osteoblast activity.
  • Thyroxine: Supports osteoblast activity with growth hormone.
  • Sex Hormones (estrogen/testosterone): Stimulate bone matrix synthesis; estrogen promotes earlier closure of epiphyseal plates.
  • Parathyroid Hormone: Stimulates osteoclast and osteoblast activity, increasing blood calcium levels.
  • Calcitonin: Lowers blood calcium levels during pregnancy/nursing.
Key Questions
  1. Define osteopenia: Inadequate ossification due to decreased osteoblast activity.
  2. Compare compact and spongy bone structure and functions.
  3. Impact of osteoclast/osteoblast activity imbalance on bone mass.

3.2 The Axial Skeleton

3.2.1 Bones and Functions
  • Cranial Bones (8): Protect brain, support muscles.
  • Facial Bones (14): Provide structure for face, protect cavities.
  • Auditory Ossicles (6): Contribute to hearing.
  • Hyoid Bone: Supports voice box.
  • Vertebrae (24): Transfer weight along vertebral column.
  • Sacrum (1): Protects various organs.
  • Coccyx (1): Supports pelvic organs.
  • Thoracic Cage: Protects thoracic organs, attaches to muscles.
Key Bone Markings for Skull
  • Occipital Bone:
    • Foramen Magnum: connects cranial cavity with the vertebral canal.
    • Occipital condyles: articulate with the first cervical vertebra.
    • Jugular foramen: pathway for internal jugular vein.
Aging Effects on the Axial Skeleton
  • Skull structure changes throughout life, bones fuse.
  • Vertebral health can reduce, leading to postural complications with age.

3.3 The Appendicular Skeleton

Types of Limbs and Girdles
  • Pectoral Girdle: Clavicle and scapula provide arm support.
  • Upper Limb: Humerus, radius, and ulna with carpal bones (8), metacarpals (5) and phalanges (14).
    • Major joints: Glenohumeral (shoulder) and elbow articulate with surrounding bones in complex movements.
  • Pelvic Girdle: Hip bones (ilium, ischium, pubis) protect reproductive organs and support weight.
  • Lower Limb: Femur, patella, tibia, fibula, tarsals (14), metatarsals (10) and phalanges (28).
Key Joint Movements
  • Flexion: Decreases angle between joints (e.g., elbow).
  • Extension: Increases angle between joints.
  • Abduction: Movement away from midline.
  • Adduction: Movement towards midline.
  • Rotation: Inward and outward movement of limbs.

3.4 Joints

Types and Classifications
  • Synovial Joints: High mobility due to joint capsules, articular cartilage, synovial fluid (lubrication, nutrient distribution).
  • Accessory Structures: Menisci, fat pads, ligaments, tendons, bursae stabilize joints, allowing motion without damaging tissues.
Common Issues and Age-Related Problems
  • Rheumatism: General term for joint pain/stiffness.
  • Arthritis: Joint inflammation, especially in older adults. Osteoarthritis most common.
Conclusion
  • The skeletal system's structure, function, and related interdependence with other body systems highlights its critical role in overall human physiology.