Unit 7.10

• Skeletons


  • Serve as a support system for animals
  • Provide rigidity, protection, and surface for muscle attachment
• Variety of skeletal types in the animal kingdom


  • Hydrostatic
  • Exoskeleton
  • Endoskeleton

  
• Hydrostatic skeleton


  "Soft skeleton" that uses fluid pressure to offer support and resistance to the contraction of muscles so that mobility results
• Exoskeleton


  External skeleton, composed of calcium carbonate (mollusks) or chitin (arthropods), must molt as they grow
• Endoskeleton


  Internal skeleton, composed of mineralized bone and cartilage, grows as the animal grows, does not limit space for internal organs, supports greater weight, adapted to individual organisms' particular mode of locomotion

  
• Functions of the human skeletal system


  • Support of the body
  • Protection of the vital internal organs
  • Sites for muscle attachment
  • Storage reservoir for ions
  • Production of blood cells
• Bone growth and renewal


  1. Cartilage structures in early development act as models for future bones

  2. Calcium salts deposited in matrix by cartilage cells and later by osteoblasts

  3. Endochondral ossification

  4. Rates of growth controlled by growth and sex hormones

  5. Osteoclasts break down bone, remove worn cells, deposit calcium in the blood, work with osteoblasts to heal broken bones

  
• Anatomy of a long bone


  Thick outer cylinder of compact bone, medullary cavity in center with yellow bone marrow, spongy bone at ends, unit of structure called osteon with concentric lamellae arranged around central canal, osteocytes lie in lacunae at lamellar boundaries, spongy bone has numerous bars and plates separated by irregular spaces designed for strength, spaces filled with red bone marrow

  
• Components of the axial skeleton


  • The skull
  • The vertebral column
  • The sternum
  • The ribs

  
• The skull


  Formed by cranium and facial bones, major bones named after lobes of the brain and facial bones, foramen magnum at base of skull where spinal cord connects to brain, bones of cranium surround sinuses

  
• The vertebral column


  Supports the head and trunk, protects the spinal cord and roots of spinal nerves, segments include cervical, thoracic, lumbar, sacral, and coccyx, intervertebral disks of fibrocartilage act as padding
• The rib cage


  Protects the heart and lungs, and assists breathing, supported by the thoracic vertebrae, twelve pairs of ribs including seven pairs of true ribs that connect directly to the sternum and five pairs of "false" ribs that do not connect directly to the sternum
• Components of the appendicular skeleton


  • The bones within the pectoral and pelvic girdles
  • The attached limbs

  
• Classification of joints


  • Fibrous joints (immovable, between cranial bones)
  • Cartilaginous joints (slightly movable, between vertebrae)
  • Synovial joints (freely movable, bones separated by a cavity, ligaments bind bones together, knee & elbow - hinged, shoulders & hips - ball-and-socket)

  
• Skeletal muscles


  • Attached to the skeleton by cable-like fibrous connective tissue called tendons
  • Arranged in antagonistic pairs that can only contract, not push
  • At "rest" exhibit tone (minimal contraction)
  • In tetany are at maximum sustained contraction
• Microscopic anatomy of skeletal muscle

  Composed of fibers in bundles, each fiber has a sarcolemma (plasma membrane) that forms a transverse system with sarcoplasmic reticulum (modified endoplasmic reticulum) that stores calcium ions for contraction, myofibrils in sarcoplasm are the contractile structures
• Sliding filament model of muscle contraction


  1. Actin filaments at both ends of sarcomere, one end attached to Z-plate, other end suspended in sarcoplasm

  2. Myosin filaments suspended in between Z-plates, contain cross-bridges that pull actin filaments inward, causing Z-plates to move toward each other and sarcomere to shorten

  3. Sarcomeres stacked together in series cause myofiber to shorten

  4. Working muscles require ATP, myosin breaks down ATP, sustained exercise requires cellular respiration to regenerate ATP
• Neuromuscular junction


  Region containing nerve axon terminal, sarcolemma, and synaptic cleft where nerve impulses bring about the release of a neurotransmitter that crosses the synaptic cleft and signals the muscle fiber to contract, acetylcholinesterase breaks down acetylcholine to end contraction
• Role of Ca2+ in muscle contraction

  Ca2+ binds to troponin on actin, exposing myosin binding sites, myosin hydrolyzes ATP resulting in movement of myosin heads that pull actin filament, new ATP binds to heads causing release, process continues until nerve stimulation ends