GENBIO - Muscular System

• Types of Muscle Tissue

  • Skeletal Muscle
  • Smooth Muscle
  • Cardiac Muscle

  
• Skeletal Muscle


  • Attached to bones
  • Contraction makes possible locomotion, facial expressions, posture, and other voluntary movements
  • 40% of body mass
  • Generates heat as a byproduct of contraction
  • Multinucleated with nuclei located on the periphery

  
• Smooth Muscle


  • Most widely distributed type
  • Found in walls of hollow organs and tubes
  • Spindle shaped with single nucleus and no visible striations

  
• Cardiac Muscle


  • Found only in the heart
  • Contractions provide major force for moving blood
  • Cells have one nucleus centrally located
  • Cells physically and electrically connected
  • Extensively branched and connected by intercalated discs

  
• Skeletal Muscle Anatomy


  1. Muscle consists of fascicles

  2. Fascicles surrounded by perimysium

  3. Entire muscle surrounded by epimysium

  4. Muscle fibers encased in endomysium

  
• Muscle Fiber


  Long and cylindrical skeletal muscle cells

  
• Sarcolemma


  Plasma membrane of muscle fiber

  
• Sarcoplasm


  Cytoplasm of muscle fiber
• Sarcoplasmic Reticulum (SR)


  Specialized smooth endoplasmic reticulum that stores, releases, and retrieves calcium ions
• Sarcomere


  Functional unit of skeletal muscle fiber, highly organized arrangement of actin and myosin filaments

  
• Skeletal Muscle Contraction (Overview)


  1. Neuromuscular junction stimulates sarcoplasmic reticulum

  2. Sarcoplasmic reticulum releases calcium ions

  3. Calcium ions bind to troponin, exposing actin-myosin binding sites

  4. Actin and myosin form cross-bridges

  5. Myosin pulls actin, shortening muscle and producing tension

  
• Action Potentials at Neuromuscular Junction


  1. Action potential arrives at presynaptic terminal

  2. Calcium ions enter presynaptic terminal, initiating acetylcholine release

  3. Acetylcholine diffuses across synaptic cleft and binds to postsynaptic receptors

  4. Sodium ions enter postsynaptic cell, depolarizing membrane

  5. Acetylcholinesterase breaks down acetylcholine

  6. Choline recycled to reform acetylcholine in presynaptic terminal
• Muscle Contraction


  1. Action potential propagates along sarcolemma and T tubules

  2. Depolarization of T tubules causes calcium release from sarcoplasmic reticulum

  3. Calcium binds to troponin, exposing actin binding sites

  4. Myosin heads bind to actin, forming cross-bridges

  
• Cross-bridges form during muscle contraction
• Skeletal Muscle Contraction (Summary)

  1. Action potential travels to neuromuscular junction

  2. Calcium enters presynaptic terminal, triggering acetylcholine release

  3. Acetylcholine binds to postsynaptic receptors, initiating action potential

  4. Action potential in T tubules causes calcium release from sarcoplasmic reticulum

  5. Calcium binds to troponin, exposing myosin binding sites

  6. ATP hydrolysis provides energy for myosin heads to pull actin, causing contraction

  
• Reference: VanPutte, C. L., Regan, J. L., & Russo, A. F. (2017). Seeley's Anatomy & Physiology. McGraw-Hill.