Chapter 6: Skeletal System

Cartilage Types: Location & Function

Cartilage is a flexible connective tissue found in various locations throughout the body. It serves structural, supportive, and cushioning roles in the skeletal system.

• Key Point: Cartilage is avascular—nutrients diffuse through the matrix, resulting in slow healing.

Functions of Bones & Calcium Regulation

Bones are dynamic organs with multiple functions essential for body structure and homeostasis.

• Support: Framework for body and soft tissues

• Protection: Vital organs (e.g., skull, rib cage)

• Movement: Levers for muscles

• Mineral Storage: Especially calcium and phosphate

• Blood Cell Formation: Occurs in red bone marrow (hematopoiesis)

• Fat Storage: Yellow marrow stores fat

• Hormone Production: Osteocalcin regulates bone formation and insulin

Parathyroid Hormone (PTH) & Blood Calcium Regulation

• When blood Ca2+ levels drop, PTH is released.

• PTH:

  • Stimulates osteoclasts to break down bone, releasing Ca2+ into blood

  • Increases Ca2+ reabsorption in kidneys

• This restores blood calcium levels to normal.

Bone Classification by Shape

Gross Anatomy of Bone

Long Bone

• Diaphysis: Shaft; compact bone surrounding medullary cavity (contains yellow marrow in adults)

• Epiphyses: Ends; spongy bone inside; covered by articular cartilage

• Periosteum: Outer membrane; fibrous + osteogenic layers; anchoring point for tendons/ligaments

• Endosteum: Lines internal bone surfaces and canals; contains bone-forming cells

• Red Bone Marrow: In trabecular cavities of spongy bone (flat bones, epiphyses of long bones in adults)

• Yellow Marrow: Fat storage in medullary cavity; can revert to red marrow if needed

Flat Bone

• Structure: Two layers of compact bone with spongy bone (diploë) in between

Compact vs. Spongy Bone & Histology

Compact Bone

• Osteon (Haversian system): Structural unit

• Central canal: Contains blood vessels and nerves

• Lamellae: Concentric rings of bone matrix

• Lacunae: House osteocytes

• Canaliculi: Tiny canals connecting lacunae; allow nutrient exchange

Spongy Bone

• No osteons; consists of trabeculae aligned along stress lines

• Trabeculae contain lamellae, osteocytes, and canaliculi

Bone Cells

Bone Development (Ossification)

• Intramembranous Ossification: Bone develops directly from mesenchyme; forms flat bones (skull, clavicle)

• Endochondral Ossification: Bone replaces hyaline cartilage model; forms most bones below the skull

Steps of Endochondral Ossification

1. Bone collar forms around diaphysis

2. Cartilage calcifies & develops cavities

3. Periosteal bud invades – spongy bone forms

4. Diaphysis elongates, medullary cavity forms

5. Epiphyses ossify, leaving epiphyseal plates

Bone Remodeling & Growth

Epiphyseal Plate Growth

• Resting zone – cartilage cells inactive

• Proliferation zone – cartilage cells divide

• Hypertrophic zone – cells enlarge

• Calcification zone – matrix calcifies, chondrocytes die

• Ossification zone – new bone forms

• Bone growth ceases when epiphyseal plate closes and becomes epiphyseal line

Bone Remodeling

• Ongoing replacement of old bone by new bone through:

  • Bone deposition (osteoblasts)

  • Bone resorption (osteoclasts)

• Wolff’s Law: Bone grows/remodels in response to mechanical stresses placed upon it (e.g., dominant arm thicker bones)

Bone Repair

Steps in Bone Fracture Repair

1. Hematoma Formation – Blood clot forms; inflammation

2. Fibrocartilaginous Callus – Collagen + cartilage stabilize break

3. Bony Callus Formation – Spongy bone replaces callus

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

Fracture Types (Examples)

• Simple vs. Compound (closed vs. open)

• Comminuted (shattered), Compression, Spiral, Epiphyseal, Depressed, Greenstick

Bone Disorders

Joints (Articulations)

A joint (articulation) is any point where two or more bones meet. Joints allow for movement, support, and growth of the skeleton.

• Movement: Allows varying degrees of movement depending on joint type

• Support & Stability: Holds bones together, stabilizes skeleton

• Growth: Some joints (epiphyseal plates) allow bone growth

Classification of Joints

Structural Classification

Functional Classification

Synovial Joint Structure

• Articular Capsule: Encloses cavity; made of:

  • Fibrous layer (dense irregular CT) for strength

  • Synovial membrane (loose CT) that secretes synovial fluid

• Synovial Fluid: Lubricates, nourishes cartilage, reduces friction

• Reinforcing Ligaments: Support joint; may be capsular, extracapsular, or intracapsular

• Nerves & Blood Vessels: Provide sensation and nutrients

Accessory Structures

• Bursae: Flattened sacs with synovial fluid; reduce friction

• Tendon Sheaths: Elongated bursae around tendons (e.g., wrist, ankle)

Joint Stability Depends On:

1. Articular surfaces – Shape fits together (e.g., hip vs. shoulder)

2. Ligaments – More = more stability (but may reduce flexibility)

3. Muscle tone – Most important stabilizer; keeps tendons taut

Examples of Major Synovial Joints

Homeostatic Imbalances of Joints

• Lyme disease: Bacterial infection transmitted by tick bite; causes arthritis

• Dislocations: Bone forced out of alignment

• Bursitis/Tendonitis: Inflammation of bursae/tendon sheaths

• Arthritis: Joint inflammation; common types:

  • Osteoarthritis (OA) – Degenerative, wear and tear

  • Rheumatoid arthritis (RA) – Autoimmune destruction

  • Gouty arthritis – Uric acid crystal deposition

Chapter 9: Muscular System

Functions of Muscle Tissue

• Movement: Skeletal muscles move bones; smooth muscles move substances through organs; cardiac muscle pumps blood

• Posture & Stabilization: Constant small contractions maintain body position

• Joint Stability: Muscle tone reinforces joints

• Heat Production: Muscle contraction generates heat (important for temperature regulation)

• Protection: Muscles protect internal organs (e.g., abdominal wall)

Types of Muscle Tissue

Skeletal Muscle Structure

Gross Anatomy

• Muscle → Fascicles → Muscle fibers (cells) → Myofibrils → Sarcomeres

• Connective Tissue Coverings:

  • Epimysium – Surrounds entire muscle

  • Perimysium – Surrounds fascicles

  • Endomysium – Surrounds individual muscle fibers

• These layers extend to form tendons (attach muscle to bone) or aponeuroses (flat sheets)

Microscopic Anatomy

• Sarcolemma: Muscle fiber plasma membrane

• Sarcoplasm: Cytoplasm; contains glycosomes & myoglobin

• Myofibrils: Rod-like organelles containing sarcomeres

• Sarcoplasmic Reticulum (SR): Stores & releases Ca2+ for contraction

• T-tubules: Invaginations of sarcolemma that transmit action potentials deep into the cell

Sarcomere Structure & the Sliding Filament Model

• Sarcomere: Functional unit of skeletal muscle

• Z disc: Boundary

• Thin filaments (actin): Anchored to Z disc

• Thick filaments (myosin): In center (A band)

• M line: Center of sarcomere

Sliding Filament Theory

• Myosin heads attach to actin → pull thin filaments toward M line → sarcomere shortens → muscle contracts

• Filaments do not shorten, they slide

Neuromuscular Junction

• Neuromuscular Junction (NMJ): Motor neuron axon → synaptic terminal → releases acetylcholine (ACh) into synaptic cleft

• ACh binds to receptors on sarcolemma → triggers action potential in muscle fiber

Excitation-Contraction Coupling

1. Action potential travels along sarcolemma and T-tubules

2. SR releases Ca2+

3. Ca2+ binds to troponin → moves tropomyosin, exposing active sites on actin

4. Cross-bridge cycle occurs (myosin binds to actin → power stroke → detach – reset with ATP)

5. Ca2+ pumped back into SR → muscle relaxes

Motor Units & Muscle Contraction Types

Motor Unit

• One motor neuron + all the muscle fibers it innervates

• Small units = fine control (e.g., eye muscles)

• Large units = gross movements (e.g., quadriceps)

Types of Muscle Contractions

• Isotonic: Muscle changes length

  • Concentric: Muscle shortens (lifting)

  • Eccentric: Muscle lengthens (lowering a weight)

• Isometric: Muscle does not change length (holding a position)

Muscle Twitch & Graded Responses

• Muscle Twitch: Single, brief contraction in response to a single stimulus

• Graded Muscle Responses: Vary in strength depending on frequency and number of motor units activated

ATP Sources for Muscle Contraction

1. Direct phosphorylation (creatine phosphate) – quick, short-term (15 seconds)

2. Anaerobic glycolysis – 10-40 seconds, produces lactic acid

3. Aerobic respiration – prolonged activity

Smooth Muscle

• Controlled by autonomic nervous system, hormones, and local factors

• No sarcomeres; actin and myosin arranged diagonally

• Contracts slowly, can sustain contractions for long periods

Muscle Disorders

• Spasms: Involuntary contractions

• Cramp: Prolonged, painful muscle spasm