Skeletal, Articulations, & Muscular System

🦴 SKELETAL SYSTEM

Functions of the Skeletal System

• Support – Framework that supports the body and cradles soft organs.

• Protection – Shields vital organs (e.g., skull protects brain, ribs protect heart/lungs).

• Movement – Muscles attach to bones via tendons; bones act as levers.

• Mineral Storage – Stores calcium and phosphate; released as needed.

• Blood Cell Production (Hematopoiesis) – Occurs in red bone marrow.

• Triglyceride (Fat) Storage – Stored in yellow bone marrow.

• Hormone Production – Osteocalcin regulates bone formation and insulin.

Bone Classification

1. Long Bones – Longer than wide (e.g., femur, humerus).

2. Short Bones – Cube-shaped (e.g., carpals, tarsals).

3. Flat Bones – Thin, flat, often curved (e.g., skull, ribs, sternum).

4. Irregular Bones – Complex shapes (e.g., vertebrae, hip bones).

5. Sesamoid Bones – Embedded in tendons (e.g., patella).

Structure of Osseous Tissue

• Osteogenic Cells – Stem cells that differentiate into osteoblasts.

• Osteoblasts – Bone-building cells; secrete osteoid.

• Osteocytes – Mature bone cells in lacunae; maintain matrix.

• Osteoclasts – Bone-resorbing cells; break down bone.

• Osteoprogenitor Cells - Found in Periosteum and Endosteum, they play their role in the formation of new osteoblasts, contributing to the repair and growth of bone tissue.

Matrix Composition:

• Organic (Osteoid) – Collagen fibers, ground substance → flexibility.

• Inorganic (Hydroxyapatite) – Calcium phosphate → hardness.

Compact vs. Spongy Bone

• Compact Bone (Cortical):

  • Dense outer layer.

  • Made of osteons (Haversian systems): concentric lamellae, central canal, perforating canals, lacunae with osteocytes.

• Spongy Bone (Cancellous):

  • Honeycomb-like network of trabeculae.

  • No osteons; spaces filled with red or yellow marrow.

  • Found in epiphyses and flat bones.

Gross Anatomy of a Long Bone

• Epiphysis – Ends of bone, spongy bone interior.

• Articular Cartilage – Hyaline cartilage covering joint surfaces.

• Epiphyseal Plate – Growth plate (hyaline cartilage).

• Epiphyseal Line – Remnant of plate in adults.

• Diaphysis – Shaft; compact bone surrounding medullary cavity.

• Medullary Cavity – Contains yellow marrow (fat).

• Periosteum – Outer membrane; contains osteoblasts & osteoclasts.

• Endosteum – Lines medullary cavity; bone remodeling site.

Bone Ossification

• Intramembranous Ossification:

  • Bone develops from fibrous membrane (flat bones: skull, clavicle).

  • Steps: Mesenchymal cells → osteoblasts → bone matrix → trabeculae → periosteum forms.

• Endochondral Ossification:

  • Bone replaces hyaline cartilage (most bones).

  • Steps: Bone collar forms → cartilage calcifies → cavities form → blood vessels invade → diaphysis ossifies → epiphyses ossify.

Bone Growth

• Longitudinal Growth (Length):

  • Occurs at epiphyseal plate.

  • Zones: resting → proliferation → hypertrophic → calcification → ossification.

• Appositional Growth (Width):

  • Osteoblasts add bone to periosteum; osteoclasts remove from endosteum.

Bone Remodeling & Osteoporosis

• Remodeling = Bone resorption (osteoclasts) + bone deposition (osteoblasts).

• Regulated by:

  • Hormones (PTH increases Ca release; Calcitonin lowers Ca levels).

  • Mechanical Stress (Wolff’s Law: bone grows/remodels in response to stress).

• Osteoporosis – Bone resorption > bone formation → brittle bones.

Types of Fractures

• Simple (Closed) – Doesn’t break skin.

• Compound (Open) – Breaks skin.

• Comminuted – Bone shatters into pieces.

• Compression – Bone crushed.

• Spiral – Twisting forces.

• Greenstick – Partial break (common in children).

Fracture Repair

1. Hematoma Formation – Blood clot forms at fracture site.

2. Fibrocartilaginous Callus – Collagen + cartilage stabilize break.

3. Bony Callus – Spongy bone replaces callus.

4. Bone Remodeling – Compact bone replaces spongy bone; shape restored.

🤸‍♀ ARTICULATIONS (JOINTS)

Synovial Joint Structure

• Articular Cartilage – Hyaline cartilage prevents friction.

• Joint (Articular) Cavity – Space with synovial fluid.

• Articular Capsule – Fibrous outer + synovial inner layer.

• Synovial Fluid – Lubricates, nourishes cartilage.

• Reinforcing Ligaments – Strengthen joint.

• Nerves & Blood Vessels – Sensation and nourishment.

Synovial Joint Movements

• Flexion/Extension – Decrease/increase angle.

• Abduction/Adduction – Move away/toward midline.

• Rotation – Movement around axis.

• Circumduction – Cone-shaped motion.

• Supination/Pronation – Palm up/down.

• Elevation/Depression, Protraction/Retraction, Opposition (thumb).

Synovial Joint Types & Examples

Comparing Major Joints

• Knee vs. Elbow:

  • Knee: Hinge + some rotation; contains menisci & many ligaments.

  • Elbow: True hinge; strong ligaments, radius + ulna articulation.

• Shoulder vs. Hip:

  • Shoulder: Shallow socket → high mobility, low stability.

  • Hip: Deep socket → low mobility, high stability.

Joint Disorders

• Arthritis – General joint inflammation.

• Osteoarthritis – Wear-and-tear; cartilage breaks down.

• Rheumatoid Arthritis – Autoimmune; synovial membrane inflamed.

• Gout – Uric acid crystal buildup in joints.

💪 MUSCULAR SYSTEM

Functions

• Movement

• Posture maintenance

• Stabilization of joints

• Heat production (shivering)

• Protect internal organs

Muscle Types

Sarcomere Structure

• Z line – Boundary.

• A band – Dark band (thick filaments).

• I band – Light band (thin only).

• H zone – Only thick filaments.

• M line – Middle of sarcomere.

• Actin (thin) + Myosin (thick) filaments slide during contraction.

Neuromuscular Junction (NMJ)

• Motor neuron meets muscle fiber.

• Steps:

  1. Nerve impulse → ACh released.

  2. ACh binds receptors on sarcolemma → Na⁺ channels open.

  3. Depolarization → action potential travels down T-tubules.

  4. Ca²⁺ released from sarcoplasmic reticulum → contraction starts.

Excitation-Contraction Coupling

• AP on sarcolemma → Ca²⁺ release → Ca²⁺ binds troponin → tropomyosin shifts → myosin binds actin → contraction.

Crossbridge Cycle

1. Myosin binds actin (crossbridge forms).

2. Power stroke (ADP + Pi released → myosin pulls actin).

3. New ATP binds → myosin detaches.

4. ATP hydrolyzed → myosin re-cocks.

ATP Generation in Muscles

1. Direct Phosphorylation: Creatine phosphate + ADP → ATP (fast, short).

2. Anaerobic Respiration: Glucose → lactic acid + 2 ATP (no O₂, short).

3. Aerobic Respiration: Glucose + O₂ → CO₂ + H₂O + 36 ATP (long-term).

Muscle Fiber Types

Exercise Effects:

• Aerobic: Increases endurance, mitochondria, capillaries.

• Resistance: Increases strength, hypertrophy (fiber size).

Terms:

• Fatigue: Decreased ability to contract.

• Atrophy: Muscle shrinkage (disuse).

• Sarcopenia: Age-related muscle loss.

• Hypertrophy: Increase in fiber size.

• Hyperplasia: Increase in fiber number (rare in humans).

Motor Units & Muscle Tension

• Motor Unit: Motor neuron + all fibers it controls.

• Muscle Tension Factors:

  • Number of motor units recruited.

  • Frequency of stimulation.

  • Muscle fiber size & length.

Isometric vs. Isotonic Contractions

• Isometric: Muscle tension increases, no shortening (e.g., plank).

• Isotonic: Muscle shortens, moves load (e.g., bicep curl).

Smooth Muscle

• Innervation: Involuntary (ANS).

• Structure: No striations; actin/myosin in diagonal pattern.

• Contraction: Slow, sustained, can stretch more.

• Single-Unit: Contracts as one (digestive organs).

• Multi-Unit: Independent fibers (iris, large arteries).