Chapter 6: Skeletal System

Functions of the Skeletal System

• Support: Provides structural framework for the body.

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

• Movement: Serves as levers for muscles to act upon.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood Cell Production: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Yellow bone marrow stores fat.

Bone Tissues

• Fibrous CT: Periosteum (outer fibrous covering of bone), ligaments.

• Cartilage: Types include hyaline, articular, fibrocartilage, and elastic cartilage.

Bone Matrix Components

• Osteoid (organic): Collagen fibers and ground substance; provides tensile strength and flexibility.

• Hydroxyapatites (inorganic): Mineral salts, primarily calcium phosphate; provides hardness and resistance to compression.

Architectural Anatomy & Location of Bone

• Spongy Bone: Porous, found at ends of long bones and inside flat bones.

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

Skeletal Tissue Cell Types

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells that maintain bone matrix.

• Osteoclasts: Bone-resorbing cells.

• Chondroblasts: Cartilage-forming cells.

• Fibroblasts: Produce collagen and other fibers.

Development of Bones (Osteogenesis)

• Intramembranous Ossification: Bone develops from fibrous membrane (e.g., flat bones of skull).

• Endochondral Ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

Growth & Remodeling of Bones

• Epiphyseal Plate (Growth Zone): Site of longitudinal bone growth in children and adolescents.

• Hormonal Regulation:

  • Growth hormone (GH), thyroid hormone, sex hormones regulate growth.

  • Parathyroid hormone (PTH) and calcitonin regulate calcium balance.

• Remodeling:

  • Positive: Response to mechanical stress (Wolff's Law).

  • Negative: Lack of stress leads to bone loss.

Healing Fractures

• Stages of Healing:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling (maturation)

Muscle Tissue

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, attached to bones.

• Cardiac Muscle: Involuntary, striated, found in heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

• Similarities: Both skeletal and cardiac muscle are striated and contract via sliding filament mechanism.

Gross Anatomy of Skeletal Muscle

• Connective Tissue Layers:

  • Endomysium: Surrounds individual muscle fibers.

  • Perimysium: Surrounds fascicles (bundles of fibers).

  • Epimysium: Surrounds entire muscle.

• Fascicles: Bundles of muscle fibers.

• Physical Characteristics: Muscle shape, size, and arrangement affect function.

Growth & Development of Skeletal Muscle

• Myoblasts: Embryonic cells that fuse to form muscle fibers.

• Satellite Cells: Aid in muscle repair and regeneration.

Cellular Anatomy of Skeletal Muscle Fibers

• Sarcolemma: Plasma membrane of muscle fiber.

• T-Tubules: Invaginations of sarcolemma; conduct action potentials.

• Motor End-Plate: Specialized region for neuromuscular junction.

• Myofibrils: Rod-like units containing contractile proteins.

• Sarcoplasmic Reticulum: Stores and releases calcium ions.

• Sarcomere: Functional unit of muscle contraction; contains A bands, I bands, Z lines, M lines, H zone.

• Myofilaments: Actin (thin) and myosin (thick) filaments.

• Regulatory Proteins: Troponin and tropomyosin control actin-myosin interaction.

Skeletal Muscle Fiber Types

• Slow Oxidative (Type I): Fatigue-resistant, use aerobic metabolism.

• Fast Glycolytic (Type II): Fatigue quickly, use anaerobic metabolism.

Excitation-Contraction Coupling

Events of Excitation

• Neuromuscular Junction: Site where motor neuron communicates with muscle fiber via acetylcholine.

• Depolarization and Repolarization: Changes in membrane potential initiate action potential.

• Role of Acetylcholinesterase: Breaks down acetylcholine to terminate signal.

• Triads: T-tubule and two terminal cisternae of sarcoplasmic reticulum; coordinate calcium release.

Events of Contraction

• Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin.

• Cross-bridge formation: Myosin heads bind to actin.

• Power stroke: Myosin head pivots, pulling actin filament.

• ATP binds to myosin, causing detachment from actin.

• ATP hydrolysis re-cocks myosin head.

• Cycle repeats as long as calcium and ATP are present.

• ATP splitting:

Muscle Physiology

All-or-None Principle

• At the level of an individual muscle fiber, contraction is all-or-none.

• At the motor unit level, all fibers in the unit contract together.

Muscle Twitch

• Latent Phase: Time between stimulus and contraction.

• Contraction Phase: Muscle fibers shorten and tension increases.

• Relaxation Phase: Tension decreases as muscle returns to resting state.

Increasing Muscle Force/Tension

• Wave Summation: Increased frequency of stimulation increases tension.

• Motor Unit Recruitment: More motor units activated for greater force.

• Cross-sectional Area: Larger muscles generate more force.

Sarcomere Length-Tension Relationship

• Optimal sarcomere length allows maximal cross-bridge formation and force production.

Types of Muscle Contraction

• Isometric: Muscle length does not change; tension increases.

• Isotonic: Muscle changes length; includes concentric (shortening) and eccentric (lengthening) contractions.

Muscle Metabolism

• Creatine Phosphate Mechanism: Rapid ATP regeneration.

• Glycolysis: Anaerobic breakdown of glucose; produces ATP and lactic acid.

• Aerobic Metabolism: Includes Krebs cycle and electron transport chain (ETC); requires oxygen and produces more ATP.