Chapter 6: Bones and Skeletal Tissue

Organization and Functions of the Skeletal System

The skeletal system is divided into the axial and appendicular skeletons, each with distinct roles and anatomical structures. Understanding their organization is essential for recognizing how bones support movement and physiological functions.

• Axial skeleton: Includes the skull, vertebral column, and thoracic cage. Provides support and protection for the brain, spinal cord, and vital organs.

• Appendicular skeleton: Comprises the limbs and girdles (pectoral and pelvic). Facilitates movement and interaction with the environment.

• Major functions: Support, protection, movement, mineral storage (especially calcium and phosphate), and blood cell production (hematopoiesis).

• Bone categories by shape: Long (femur), short (carpals), flat (sternum), irregular (vertebrae), sesamoid (patella).

• Bone markings: Projections (for muscle attachment) and depressions (for passage of nerves and vessels).

Gross Anatomy of Bone

Bones have complex structures that optimize strength and minimize weight. Key anatomical features include:

• Diaphysis: Shaft of a long bone; provides leverage and weight support.

• Epiphyses: Ends of long bones; site of articulation and red marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains the epiphyseal plate (growth area).

• Periosteum: Dense connective tissue covering bone; essential for growth and repair.

• Endosteum: Lines internal bone surfaces; involved in bone remodeling.

• Medullary cavity: Central cavity containing yellow marrow (fat storage).

• Compact bone: Dense, strong outer layer; resists stress.

• Spongy bone: Lighter, porous inner layer; supports marrow and reduces weight.

• Red marrow: Site of blood cell formation.

• Yellow marrow: Stores fat.

• Articular cartilage: Covers joint surfaces; reduces friction.

Why both compact and spongy bone? Compact bone provides strength, while spongy bone reduces weight and absorbs shock.

Microscopic Bone Structure

Bone tissue is composed of specialized cells and matrix structures that enable growth, repair, and mineral storage.

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize new bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

• Compact bone structure: Organized into osteons (Haversian systems) with central canals, concentric lamellae, lacunae (housing osteocytes), and canaliculi (channels for nutrient exchange).

• Spongy bone: Contains trabeculae; supports marrow and resists multidirectional stress.

Bone Growth, Formation, and Remodeling

Bones grow and remodel throughout life via two main processes and respond to mechanical stress.

• Intramembranous ossification: Direct formation of bone from mesenchyme (e.g., skull).

• Endochondral ossification: Bone replaces cartilage model (e.g., long bones).

• Epiphyseal growth plate: Site of longitudinal bone growth; closes after puberty.

• Bone remodeling: Osteoblasts build bone; osteoclasts resorb bone. Remodeling adapts bone to stress.

• Mechanical stress: Stimulates bone formation and strengthens bone.

Calcium Homeostasis and Bone Repair

Bone tissue regulates blood calcium levels and repairs itself after injury.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Calcium functions: Muscle contraction, nerve transmission, blood clotting.

• Fracture repair stages:

  1. Hematoma formation

  2. Soft callus formation

  3. Hard callus formation

  4. Remodeling

• Types of fractures: Simple, compound, comminuted, greenstick, etc.

Chapter 8: Articulations (Joints)

Classification of Joints

Joints are classified by their structure and function, which determines their mobility and stability.

• Structural classification:

  • Fibrous (e.g., sutures)

  • Cartilaginous (e.g., symphysis)

  • Synovial (e.g., knee)

• Functional classification:

  • Synarthrosis (immovable)

  • Amphiarthrosis (slightly movable)

  • Diarthrosis (freely movable)

Synovial Joint Anatomy

Synovial joints are highly mobile and contain specialized structures for movement and stability.

• Articular cartilage: Cushions and reduces friction.

• Joint cavity: Space containing synovial fluid.

• Articular capsule: Encloses joint; consists of fibrous capsule and synovial membrane.

• Synovial fluid: Lubricates and nourishes cartilage.

• Ligaments: Connect bone to bone; stabilize joints.

• Menisci/articular discs: Improve fit and absorb shock.

• Labra: Deepen socket in ball-and-socket joints.

• Bursae/tendon sheaths: Reduce friction between structures.

Distinction: Ligaments (bone to bone), tendons (muscle to bone), cartilage (cushion), menisci (shock absorption).

Types of Synovial Joints

Synovial joints are classified by shape and movement permitted.

Joint Movements

Joints permit a variety of movements essential for daily activities.

• Flexion/extension

• Hyperextension

• Abduction/adduction

• Circumduction

• Rotation

• Pronation/supination

• Dorsiflexion/plantar flexion

• Inversion/eversion

• Elevation/depression

• Protraction/retraction

• Opposition

Stability, Mobility, and Joint Injuries

Joint stability and mobility are influenced by anatomical structures and are relevant to injury and disease.

• Shoulder vs. hip: Shoulder is more mobile but less stable due to shallow socket.

• Ligaments: Provide stability; injury reduces joint integrity.

• Meniscal injuries: Affect shock absorption and joint congruence.

• Common ankle sprain: Involves lateral ligaments.

• Osteoarthritis: Degeneration of articular cartilage; impairs movement.

Chapter 9: Skeletal Muscle Tissue and Physiology

Muscle Tissue and Its Functions

Muscle tissue is classified by structure, function, and control.

• Functions: Movement, posture, joint stabilization, heat production, support of soft tissues.

Organization of Skeletal Muscle

Skeletal muscle is organized hierarchically, with connective tissue layers supporting function.

• Hierarchy: Whole muscle → Fascicle → Muscle fiber (cell) → Myofibril → Sarcomere → Myofilaments

• Connective tissue coverings: Epimysium (surrounds muscle), perimysium (fascicle), endomysium (fiber)

• Sarcolemma: Muscle cell membrane

• Sarcoplasmic reticulum: Stores calcium

• T tubules: Conduct action potentials

• Terminal cisternae: Enlarged SR regions; release calcium

Sarcomere Structure and Sliding Filament Theory

The sarcomere is the functional unit of muscle contraction, involving actin and myosin filaments.

• Actin: Thin filament

• Myosin: Thick filament

• Troponin/tropomyosin: Regulatory proteins

• Z discs: Sarcomere boundaries

• M line: Center of sarcomere

• A band: Length of thick filaments

• I band: Only thin filaments

• H zone: Only thick filaments

Sliding filament theory: Myosin heads bind to actin, pulling filaments together and shortening the sarcomere.

Neuromuscular Junction

The neuromuscular junction transmits signals from motor neurons to muscle fibers, initiating contraction.

• Sequence:

  1. Motor neuron action potential arrives

  2. Calcium enters axon terminal

  3. Acetylcholine (ACh) released

  4. ACh binds to receptors on motor end plate

  5. Muscle action potential generated

  6. Acetylcholinesterase breaks down ACh

• Signal termination: Prevents continuous contraction

Excitation–Contraction Coupling

This process links electrical stimulation to mechanical contraction.

• Muscle action potential travels along sarcolemma and T tubules

• Sarcoplasmic reticulum releases calcium

• Calcium binds to troponin

• Tropomyosin shifts, exposing actin binding sites

• Myosin binds to actin; cross-bridge cycling occurs

• Calcium pumped back into SR during relaxation

• ATP: Required for both contraction and relaxation

Cross-Bridge Cycling

Muscle contraction involves repeated cycles of myosin binding and releasing actin.

• Cross-bridge formation

• Power stroke

• Myosin detachment (requires ATP)

• Myosin reactivation

• Rigor mortis: Occurs when ATP is unavailable; myosin remains bound to actin

Muscle Tension and Motor Units

Muscle force depends on motor unit recruitment and stimulation frequency.

• Motor unit: One motor neuron and all muscle fibers it innervates

• Precision: Small motor units for fine control; large for strength

• Types of contraction:

  • Single twitch

  • Wave summation

  • Treppe

  • Incomplete tetanus

  • Complete tetanus

• Force increases: By higher frequency and more motor units

Types of Muscle Contraction

Muscle contractions are classified by whether muscle length changes.

• Isometric: Tension without length change (e.g., holding a weight steady)

• Isotonic concentric: Muscle shortens (e.g., lifting a weight)

• Isotonic eccentric: Muscle lengthens (e.g., lowering a weight)

Muscle Metabolism and Energy Systems

Muscles regenerate ATP via several pathways, depending on activity intensity and duration.

• Phosphocreatine system: Rapid ATP regeneration; short duration

• Anaerobic glycolysis: Produces ATP without oxygen; short, intense activity

• Aerobic metabolism: Uses oxygen; sustained activity

• Muscle fatigue: Caused by ATP depletion, lactic acid buildup, ion imbalances

Muscle Fiber Types and Adaptation

Muscle fibers differ in contraction speed, metabolism, and fatigue resistance.

• Hypertrophy: Muscle growth

• Atrophy: Muscle loss

• Endurance adaptations: Increased mitochondria, capillaries

• Strength adaptations: Increased fiber size

Chapter 10: Functional Organization of the Muscular System

Organization and Architecture

Muscles are grouped by location and fascicle arrangement, affecting their function.

• Axial muscles: Located on trunk and head

• Appendicular muscles: Located on limbs

• Fascicle arrangements:

  • Parallel

  • Convergent

  • Pennate (uni-, bi-, multi-)

  • Circular

• Arrangement influences: Force production and range of movement

Levers and Mechanical Advantage

Muscles act on bones as levers, classified by the relative positions of fulcrum, effort, and load.

• Third-class levers: Most common; favor speed and range over force

Muscle Attachments and Actions

Muscle contraction produces movement by pulling on bones at specific attachment points.

• Origin: Fixed attachment

• Insertion: Movable attachment

• Direction of pull: Determines movement

• Attachment location: Influences mechanical function

Functional Roles of Muscles

Muscles work together in coordinated roles to produce controlled movement.

• Agonist (prime mover): Main muscle responsible for movement

• Antagonist: Opposes agonist

• Synergist: Assists agonist

• Fixator: Stabilizes origin

• Naming conventions: Based on location, action, shape, size, number of origins, and attachments

Integrating the Chapters: Movement and Clinical Scenarios

Scenario Applications

• Lifting a weight: Elbow flexion (hinge joint), isotonic contraction, third-class lever, increased motor-unit recruitment with heavier weight.

• Ankle injury: Inversion movement, lateral ligaments and menisci at risk, stability compromised.

• Immobilization: Bone repair involves staged healing; muscle atrophy occurs with inactivity.

• Exercise and fatigue: Sprint uses phosphocreatine and fast fibers; walk uses aerobic metabolism and slow fibers; fatigue patterns differ.

• Joint degeneration: Osteoarthritis damages articular cartilage; movement impaired; muscle strength supports joint function.

Suggested Study Strategies

• Practice identifying anatomical structures on diagrams.

• Explain physiological processes aloud in sequence.

• Compare similar terms using charts (e.g., osteoblasts vs. osteoclasts, ligaments vs. tendons).

• Apply concepts to movement and clinical scenarios.

• Teach processes to others for deeper understanding.

Final Self-Assessment

• Can you identify major anatomical structures?

• Can you explain structure-function relationships?

• Can you describe physiological processes in order?

• Can you distinguish between similar concepts?

• Can you predict outcomes of structural or functional disruptions?

• Can you integrate concepts across chapters?

Additional info: This guide summarizes key concepts from chapters 6, 8, 9, and 10, focusing on the skeletal system, joints, and muscular system. It is intended for exam preparation and should be supplemented with lecture materials and assigned readings.