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Importance of Skeletal Systems

  • Provide rigidity to the body
  • Bear body weight
  • Offer muscle attachment surfaces
  • Protect vulnerable organs

Types of Skeletons

Hydrostatic Skeletons

  • Found in many invertebrates
  • Body fluids form internal skeleton
  • Muscular force is developed against incompressible fluids

Rigid Skeletons

  • Composed of rigid, often jointed elements
  • Exoskeletons: Rigid shells (e.g., molluscs, arthropods)
  • Endoskeletons: Internal support (e.g., corals, echinoderms, vertebrates)

Cartilage

  • Makes up skeletons of jawless and cartilaginous fishes
  • Other vertebrates have bony skeletons with cartilage
  • Soft, pliable tissue with chondrocytes and collagen
  • Serves as articulating surfaces for bone joints

Morphology of Bone

  • Contains calcium salts in an extracellular matrix
  • Develops by replacing cartilage with calcium
  • Contains bone marrow (erythrocytes, platelets, white blood cells)
  • Consists of spongy bone and compact bone

Microscopic Structure of Bone

  • Calcified bone matrix in concentric rings
  • Lacunae filled with osteocytes, interconnected by canaliculi
  • Osteons interlinked with blood vessels and nerves
  • Osteoblasts build new bone; osteoclasts digest old bone

Muscular Systems

  • Movement depends on contractile proteins (actin, myosin)
  • Powered by ATP

Types of Invertebrate Muscle

  • Striated and smooth muscles in invertebrates

Vertebrate Muscle Types

Skeletal Muscle

  • Voluntary movements; fast, powerful contractions
  • Organized into bundles attached to skeleton
  • Multinucleate cells in fascicles

Cardiac Muscle

  • Present only in the heart; involuntary control
  • Striated; consists of uninucleate fibers with intercalated disks

Smooth Muscle

  • Small, tapering cells; involuntary contractions
  • Forms sheets around tubular structures

Structure of Striated Muscle

  • Surrounded by sarcolemma; contains myofibrils
  • Myofibrils made of actin and myosin filaments
  • Contractile element is the sarcomere

Molecular Structure of Filaments

  • Myosin: Composed of polypeptide chains with heads
  • Actin: Helical strands of actin molecules
  • Tropomyosin & Troponin: Proteins associated with actin

Sliding Filament Hypothesis

  • Myosin heads link to actin to pull filaments past each other
  • Z-lines pulled closer, sarcomere shortens during contraction

Control of Contraction at the Neuromuscular Junction

  • Skeletal muscle contraction initiated by nerve stimulation
  • Synaptic cleft separates nerve terminal and muscle fiber
  • Action potential releases acetylcholine (ACh)

Excitation-Contraction Coupling

  • Step 1: Calcium ions released from sarcoplasmic reticulum (SR)
  • Steps 2-5: Myosin binds to active sites on actin, swinging toward the sarcomere center, then releasing and rebinding
  • Muscle contraction continues with sustained calcium release and stops when impulses cease, leading to muscle relaxation.