Muscular

• Types of muscle tissue

  • Skeletal muscle
  • Smooth muscle
  • Cardiac muscle
• Skeletal muscle


  • Attached to bones
  • Responsible for most body movements
  • Maintain tone to keep us sitting or standing erect
• Smooth muscle


  • Found in walls of hollow organs and tubes
  • Each cell is spindle shaped with a single nucleus and no visible striations
• Cardiac muscle


  • Found only in the heart
  • Contractions provide the major force for moving blood through the circulatory system
  • Cells are physically and electrically connected to each other so the heart contracts as one unit
• Muscle fibers


  Long and cylindrical skeletal muscle cells
• Sarcolemma


  Plasma membrane of muscle fibers
• Sarcoplasm


  Cytoplasm of muscle fibers
• Sarcoplasmic reticulum (SR)


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


  Functional unit of a skeletal muscle fiber, highly organized arrangement of contractile myofilaments actin and myosin
• Skeletal muscle contraction (overview)

  1. Neuromuscular junction stimulates sarcoplasmic reticulum

  2. Sarcoplasmic reticulum releases calcium ions

  3. Calcium ions bind to troponins, opening actin and myosin binding sites

  4. Actin and myosin form cross bridges

  5. Myosin pulls actin, shortening muscle and producing tension
• Action potentials


  1. Action potential arrives at presynaptic terminal

  2. Calcium ions enter presynaptic terminal, initiating release of acetylcholine

  3. Acetylcholine diffuses across synaptic cleft and binds to ligand-gated sodium channels

  4. Sodium channels open, depolarizing postsynaptic membrane

  5. Acetylcholinesterase removes acetylcholine from synaptic cleft

  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 sarcoplasmic reticulum to release calcium ions

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

  4. Myosin heads bind to actin, forming cross-bridges
• Skeletal muscle cells are multinucleated with nuclei located on the periphery
• Cardiac muscle fibers are extensively branched and connected by intercalated discs
• Endomysium surrounds muscle fibers and plays a role in transferring force to tendons
• Acetylcholine is the neurotransmitter released at the neuromuscular junction
• ATP provides the energy needed for the myosin heads to move and cause muscle contraction