Micro-ana of Sk. Musc. Tis, Sk Musc. Act, Types of Body Mov.

• Skeletal muscle fibers (cells)

  • Multinucleate
  • Many oval nuclei just beneath the plasma membrane (sarcolemma)
  • Long ribbonlike organelles (myofibrils) nearly fill the cytoplasm
• Myofibrils


  Chains of tiny contractile units called sarcomeres, which are the structural and functional units of skeletal muscle
• Sarcomeres


  • Aligned end to end like boxcars in a train along the length of the myofibrils
  • Precise arrangement of even smaller structures (myofilaments) within sarcomeres produces the striations in skeletal muscle fibers
• Thick filaments


  • Made mostly of bundled molecules of the protein myosin
  • Also contain ATPase enzymes, which split ATP to release the energy used for muscle contraction
  • Extend the entire length of the dark A band
  • Midpart is smooth, but ends are studded with small projections (myosin heads) that form cross bridges
• Thin filaments


  • Composed of the contractile protein actin, plus some regulatory proteins
  • Anchored to the Z disc
  • Do not extend into the middle of a relaxed sarcomere, so the central region (the H zone) looks a bit lighter
  • Slide toward each other during contraction, causing the H zones to disappear
• Sarcoplasmic reticulum (SR)


  • Specialized smooth endoplasmic reticulum that surrounds every myofibril
  • Stores calcium and releases it on demand when the muscle fiber is stimulated to contract
• Motor unit


  Composed of one neuron and all the skeletal muscle cells it stimulates
• Skeletal muscle activity

  1. Nerve impulse reaches axonal terminal

  2. Neurotransmitter (acetylcholine) is released

  3. Acetylcholine diffuses across synaptic cleft and attaches to receptors

  4. Sarcolemma becomes temporarily permeable to sodium ions which rush into the muscle cell

  5. Electrical current (action potential) is generated
• Skeletal muscles


  • Attached to bone, or to other connective tissue structures, at no fewer than two points
  • One point (origin) is attached to the immovable or less movable bone
  • Another point (insertion) is attached to the movable bone
  • When the muscle contracts, the insertion moves toward the origin
• Types of body movements

  • Flexion
  • Extension
  • Rotation
  • Abduction
  • Adduction
  • Circumduction
  • Pronation
  • Supination
  • Inversion
  • Eversion
  • Dorsiflexion
  • Plantar flexion