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.