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Exam 5 Study Guide: Musculoskeletal System

Differences Between Bone Cells

  • Osteogenic Cell:

    • Definition: Stem cell that can differentiate into other types of bone cells.

  • Osteoblasts:

    • Role: Bone-forming cells, also known as "bone-builders".
    • Origin: Derived from mesenchymal cells.
    • Function:
    • Produce osteocalcin when stimulated by vitamin D.
    • Synthesize osteoid, leading to the formation of nonmineralized bone matrix.
    • Follow Wolfe’s law: Bone shape is determined by its function.
    • Mature into osteocytes when imbedded in the bone.

  • Osteoclasts:

    • Role: Bone-resorbing cells, known as "bone-crushers".
    • Characteristics:
    • Large, multinucleated cells.
    • Attach to integrins using podosomes to bind to bone.
    • Function:
    • Resorb bone by secreting hydrochloric acid and protease enzymes.
    • Undergo degeneration when not actively resorbing, reverting to parent cells or leaving the site.

  • Osteocytes:

    • Role: Bone maintenance.
    • Function:
    • Develop dendritic processes to communicate with bone surface and vascular space.
    • Signal osteoblasts and osteoclasts for bone formation and resorption, coordinating their functions.
    • Respond to parathyroid hormone (PTH).

Bone Remodeling and Repair Basics

  • Bone Formation Process:

    • First step: Develop bone matrix.
    • Final step: Calcification or mineralization.

  • Mineralization Phases:

    • Initial mineral deposit formation (initiation).
    • Growth through proliferation of additional mineral crystals.
    • End product: Hydroxyapatite, the predominant mineral content of bone.

  • Modeling: Bone formed at one site is resorbed at another, altering its shape and position.

  • Remodeling: Bone is resorbed and replaced at the same site, involving:

    • Regulatory Factors: Genes, environment (diet, physical activity), and hormones (PTH, calcitriol, calcitonin, sex hormones, and others).
    • Local regulation: Cytokines and growth factors.

  • RANK/RANKL/OPG System:

    • RANK: receptor on osteoclast precursor cells.
    • RANKL: ligand secreted by osteoblasts that activates osteoclasts.
    • OPG: decoy receptor produced by osteoblasts that inhibits osteoclast differentiation.
    • Involves repair of microscopic injuries and maintenance of bone integrity, following four phases: activation, resorption, formation of new bone, and takes 3-4 months.

  • Repair Process:

    • Hematoma formation, procallus formation, callus formation, callus replacement, and remodeling to restore size and shape.

Compact vs. Spongy Bone

  • Bone Matrix Composition:

    • Organic (35%): Collagen fibers, proteoglycans, glycoproteins, osteocalcin, osteonectin, osteopontin, cytokines, growth factors.
    • Inorganic (65%): Calcium and phosphate minerals.

  • Compact Bone:

    • Also known as cortical bone, making up 85% of the skeleton, solid and strong.
    • Contains the Haversian system as the basic structural unit.

  • Spongy Bone:

    • Cancellous or trabecular bone, making up 15% of the skeleton, porous and filled with red bone marrow.

  • Periosteum:

    • Double-layered connective tissue covering all bones.
    • Outer Layer: Contains blood vessels and nerves.
    • Inner Layer: Anchored to bone by collagenous fibers (Sharpey fibers).

Joint Classifications

  • Definition: Joints are where two or more bones meet, providing stability and mobility.

  • Movement Classifications:

    • Synarthrosis: Immovable joints
    • Amphiarthrosis: Slightly movable joints
    • Diarthrosis: Freely movable joints (most complex).

  • Structure Classifications:

    • Fibrous Joints: United by dense fibrous connective tissue (mostly synarthroses).
    • Example: Sutures in the skull.
    • Cartilaginous Joints: United by fibrocartilage or hyaline cartilage.
    • Example: Symphysis pubis and intervertebral disks.
    • Synovial Joints: Most movable joints, featuring a fibrous joint capsule, synovial membrane, joint cavity, synovial fluid, and articular cartilage.

Muscle Characteristics and Function

  • Skeletal Muscle:

    • Composed primarily of myoblasts, surrounded by connective tissue layers (fascia, epimysium, perimysium, endomysium).
    • Types of Muscle Fibers:
    • Red Muscle (Type 1 fibers): Slow-twitch fibers.
    • White Muscle (Type 2 fibers): Fast-twitch fibers.

  • Muscle Contraction Phases:

    • Excitation: Nerve impulse triggers muscle fiber action potential.
    • Coupling: Calcium release enables binding of actin and myosin.
    • Contraction: Shortening of fibers through sliding filament mechanism (cross-bridge theory).
    • Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, detaching cross-bridges.

  • Types of Muscle Contractions:

    • Isometric: Muscle contracts without changing length; tension increases but no movement.
    • Isotonic: Muscle changes length while maintaining tension; includes eccentric (lengthening) and concentric (shortening) contractions.

Aging Effects on the Musculoskeletal System

  • Bones:

    • Loss of bone density; increased fragility and risk of osteoporosis; prolonged remodeling time.

  • Joints:

    • Stiffening and decreased range of motion due to cartilage changes.

  • Muscles:

    • Loss of muscle mass (sarcopenia); decreased strength, oxygen intake, and metabolic rate.

  • Exercise for Older Adults:

    • Improves muscle strength and balance, increases bone mineral density, reduces risk of falls.