The Musculoskeletal System and Skin: Organs of Movement and Protection

The Skeletal System

The adult human body contains 206 bones.

Functions of bone:
→ Support: maintains the shape of the body
→ Protection: brain (skull), heart and lungs (ribcage), spinal cord (vertebral column)
→ Movement: acts as a lever system for muscles
→ Hematopoiesis: blood cell production in red bone marrow
→ Storage: reservoir for calcium and phosphorus

Bone structure:
Cortical bone (compact bone): hard outer layer — provides strength
Cancellous bone (spongy bone): porous interior — reduces weight
Periosteum: covers the bone surface — sensation and regeneration
Medullary cavity: inner space — red marrow / yellow marrow

Fractures and Bone Healing

Types of fractures

Complete fracture: bone broken into two separate pieces
Incomplete fracture: cracked but not fully separated (greenstick in children)
Open fracture: bone exposed through the skin (high infection risk)
Compression fracture: vertebrae and similar bones crushed by compressive force

Four stages of healing

Hematoma formation (days 1–3): bleeding → blood clot forms
Fibrocartilaginous callus (weeks 2–3): cartilage bridges the break temporarily
Bony callus (weeks 3–4): calcium deposits → callus hardens
Remodeling (months to years): original bone shape restored

Joints and Arthritis

Types of joints:

Synarthroses: cranial sutures — no movement
Amphiarthroses: intervertebral joints — limited movement
Diarthroses (synovial joints): shoulder, knee, fingers — free movement

Joint structure:

Synovial membrane: secretes synovial fluid → reduces friction
Cartilage: absorbs shock
Ligaments: bone-to-bone connection → joint stability
Tendons: muscle-to-bone connection

Major Joint Disorders

Osteoarthritis (degenerative arthritis):

Cartilage wears away → bone-on-bone friction
Caused by aging, obesity, and overuse
Primarily affects knees, hips, and finger joints

Rheumatoid arthritis:

Autoimmune disease — immune system attacks the synovial membrane
Symmetric bilateral involvement; morning stiffness is characteristic
Treated with DMARDs (disease-modifying antirheumatic drugs)

Gout:

Uric acid crystals deposit in joints
Typically begins in the big toe joint
Sudden, severe acute pain
Limit purine-rich foods (organ meats, beer)

Osteoporosis

Definition: decreased bone density → increased fracture risk

Risk factors

Women (estrogen decline → post-menopause)
Advanced age, family history
Calcium and vitamin D deficiency
Smoking, heavy alcohol use
Long-term corticosteroid use
Diagnosis: DEXA scan (bone mineral density measurement)
T-score: above -1 = normal, -1 to -2.5 = osteopenia, below -2.5 = osteoporosis

Prevention

1,000–1,200 mg calcium/day + 800–1,000 IU vitamin D/day
Regular weight-bearing exercise (walking, running)
Fall prevention (bathroom grab bars, anti-slip measures)

The Muscular System

Types of muscle:
Skeletal muscle: voluntary contraction, striated pattern
Cardiac muscle: involuntary, tireless
Smooth muscle: lines organs and blood vessels, involuntary

Mechanism of muscle contraction (sliding filament theory):
Actin (thin filaments) + myosin (thick filaments)
→ Myosin heads bind to actin → pull → contraction
→ Requires ATP energy and calcium ions

Muscle injuries:
Strain: microscopic tears in muscle fibers → DOMS (delayed-onset muscle soreness)
Tendinitis: tendon inflammation (tennis elbow, Achilles tendinitis)
Rupture: sudden severe force → may require surgery

The Integumentary System — The Body’s Largest Organ

Three-layer structure of skin:
Epidermis (outer barrier):
  → Keratinocytes → stratum corneum (waterproof layer)
  → Melanocytes: UV protection (determines skin color)
  → Thickness: 0.1 mm (eyelid) to 1.5 mm (sole of foot)

Dermis (support and nourishment):
  → Collagen and elastin: elasticity and strength
  → Hair follicles, sweat glands, sebaceous glands, nerves, blood vessels

Hypodermis (subcutaneous tissue):
  → Shock absorption, temperature regulation, energy storage

Major Skin Disorders

Atopic dermatitis:
→ Impaired skin barrier function + hypersensitive immune response
→ Intense itching, chronic and relapsing
→ Aggressive moisturization, avoid irritants

Psoriasis:
→ Hyperproliferation of skin cells (normal 28-day cycle → 3–4 days)
→ Erythematous plaques with silvery white scales
→ Autoimmune component; difficult to cure

Skin cancer:
Basal cell carcinoma: most common, rarely metastasizes
Squamous cell carcinoma: linked to UV exposure, smoking, HPV
Melanoma: most dangerous, malignant transformation of melanocytes
→ ABCDE rule: Asymmetry, Border, Color, Diameter, Evolution

Key Points to Remember

4 stages of bone healing: hematoma → fibrocartilaginous callus → bony callus → remodeling Osteoporosis T-score: below -2.5 = osteoporosis (calcium + vitamin D + exercise) 3 skin layers: epidermis (barrier) → dermis (collagen) → hypodermis (fat) Melanoma ABCDE: Asymmetry, irregular Border, multiple Colors, large Diameter, Evolution