Skeletal System: Structure, Function, and Disorders

Chapter: Skeletal System

1. Functions of the Skeletal System

The skeletal system provides the framework for the body, supporting and protecting organs, enabling movement, and serving as a reservoir for minerals and blood cell production.

• Support: Bones form the body's support structure.

• Protection: Protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Muscles attach to bones, enabling movement at joints.

• Mineral Storage: Stores minerals such as calcium and phosphorus.

• Blood Cell Production: Houses bone marrow, which produces red and white blood cells and platelets.

• Energy Storage: Yellow bone marrow stores fat.

Example: The femur supports body weight and contains marrow for blood cell production.

2. Composition of the Skeleton

The skeleton is made up of bones, cartilage, ligaments, and joints. Bones are rigid organs, while cartilage is more flexible and found at joints.

• Bones: Provide structure and protection.

• Cartilage: Reduces friction and absorbs shock at joints.

• Ligaments: Connect bones to other bones.

• Tendons: Connect muscles to bones.

3. Subdivisions of the Skeletal System

The human skeleton is divided into two main parts:

• Axial Skeleton: Skull, vertebral column, and rib cage.

• Appendicular Skeleton: Limbs and girdles (shoulder and pelvic girdles).

Example: The clavicle is part of the appendicular skeleton.

4. Types of Bone Tissue

Bone tissue is classified as either compact or spongy bone.

• Compact Bone: Dense and strong, forms the outer layer of bones.

• Spongy Bone: Porous, contains spaces for marrow, found at the ends of long bones.

5. Bone Shapes

Bones are classified by shape:

• Flat Bones: Thin and broad (e.g., skull bones).

• Long Bones: Longer than they are wide (e.g., femur, phalanges).

• Short Bones: Cube-shaped (e.g., carpals).

• Irregular Bones: Complex shapes (e.g., vertebrae).

6. Gross Anatomy of Long Bones

Long bones have distinct regions and structures:

• Diaphysis: Shaft, composed of compact bone.

• Epiphysis: Ends of the bone, spongy bone covered by compact bone.

• Periosteum: Membrane covering the outer surface of bone.

• Endosteum: Membrane lining the medullary cavity.

• Medullary Cavity: Central cavity containing bone marrow.

• Articular Cartilage: Covers joint surfaces, reduces friction.

Example: In children, the medullary cavity contains red marrow; in adults, it contains yellow marrow.

7. Microscopic Anatomy of Bone

Compact bone is organized into structural units called osteons or Haversian systems.

• Osteon: Cylindrical structure containing a central canal surrounded by concentric lamellae.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces containing osteocytes (bone cells).

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange.

8. Bone Cells

Bone tissue contains several types of cells:

• Osteoblasts: Build new bone matrix (bone formation).

• Osteocytes: Mature bone cells, maintain bone tissue.

• Osteoclasts: Break down bone matrix (bone resorption).

9. Bone Growth and Ossification

Bone growth occurs in length and width through two main processes:

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (most bones).

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

Growth in Length: Occurs at the epiphyseal plate (growth plate) until adulthood.

Growth in Width: Appositional growth, where osteoblasts add new bone to the surface.

10. Bone Physiology and Calcium Homeostasis

Bone acts as a reservoir for calcium, which is regulated by hormones:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts to break down bone.

• Calcitonin: Lowers blood calcium by stimulating osteoblasts to build bone.

$\text{Ca}^{2+}_{\text{blood}} \uparrow \Rightarrow \text{Osteoclast activity} \uparrow$

$\text{Ca}^{2+}_{\text{blood}} \downarrow \Rightarrow \text{Osteoblast activity} \uparrow$

11. Fractures

Fractures are breaks in bones, classified by their characteristics:

• Simple (Closed): Bone breaks but does not pierce the skin.

• Compound (Open): Bone breaks through the skin.

• Comminuted: Bone shatters into several pieces.

• Depressed: Broken bone is pressed inward (e.g., skull fracture).

• Impacted: Ends of broken bone are forced into each other.

• Spiral: Ragged break due to twisting forces.

• Greenstick: Incomplete break, common in children.

12. Fracture Healing

Bone healing occurs in four main steps:

1. Hematoma formation (blood clot).

2. Fibrocartilage callus formation.

3. Bony callus formation.

4. Bone remodeling.

13. Bone Markings

Bones have specific markings for muscle attachment, passage of nerves and blood vessels, and articulation with other bones. Students should be able to identify these on diagrams.

14. Joints (Articulations)

Joints are sites where two or more bones meet, allowing for movement and flexibility.

• Types of Joints by Structure:

  • Fibrous: Immovable (e.g., skull sutures).

  • Cartilaginous: Slightly movable (e.g., intervertebral discs).

  • Synovial: Freely movable (e.g., shoulder, knee).

• Types of Joints by Function:

  • Synarthroses: Immovable.

  • Amphiarthroses: Slightly movable.

  • Diarthroses: Freely movable.

15. Synovial Joints

Synovial joints are the most common and movable type of joint in the body. They have a synovial cavity filled with fluid, articular cartilage, and a joint capsule.

• Parts: Synovial cavity, articular cartilage, synovial fluid, joint capsule, ligaments.

• Types: Ball-and-socket, hinge, pivot, saddle, condyloid, and plane joints.

16. Joint Disorders

Several disorders can affect joints, leading to pain and reduced mobility.

• Dislocation: Bone is forced out of its normal position.

• Inflammation: Redness, swelling, heat, and pain.

• Bursitis: Inflammation of the bursa (fluid-filled sac).

• Sprain: Injury to ligaments.

• Osteoarthritis: Degeneration of cartilage, common in elderly.

• Rheumatoid Arthritis: Autoimmune disease, affects synovial membrane.

• Gout: Uric acid crystals accumulate in joints, causing pain.

17. Abnormal Spinal Curvatures

Abnormal curvatures of the spine can affect posture and movement.

• Scoliosis: Lateral (side-to-side) curvature.

• Kyphosis: Exaggerated thoracic curvature (humpback).

• Lordosis: Exaggerated lumbar curvature (swayback).

Causes: Neuromuscular weakness, genetics, or other conditions.

18. Osteoporosis

Osteoporosis is a condition characterized by decreased bone mass and increased risk of fractures.

• Causes: Low estrogen, low calcium, low protein, low vitamin D, inactivity, old age, smoking, family history.

• Prevention: Adequate calcium and vitamin D intake, regular exercise.