Muscular Tissue

• Muscular tissue


  Exhibits the property of contractility
• Types of muscle tissue

  • Skeletal muscle
  • Smooth muscle
  • Cardiac muscle
• Major functional characteristics of muscle tissue

  • Contractility - shorten and lengthen
  • Excitability - response to stimulus
  • Extensibility - can be stretched
  • Elasticity - retract and recoil
• Skeletal muscle


  • Striated muscle
  • Consists of muscle fibers, which are long, cylindrical multinucleated cells (myocyte)
• Membranes of skeletal muscle

  • Epimysium - surrounds the entire muscle (dense irregular connective tissue)
  • Perimysium - surrounds the entire bundle of muscle fibers known as fascicles
  • Endomysium - surrounds the external lamina of individual muscle fibers (thin reticular fibers)
• Myotendinous junction


  Where all three membranes overlie together to connect with the tendons
• Organization of skeletal muscle

  • Individual muscle fibers are covered by sarcolemma, where multiple nuclei are located within the periphery
  • Transverse tubules are large portions of the sarcoplasmic reticulum which plays in the secretion of Ca2+ which plays a major role in contraction
  • Sarcoplasm contains many bundles of myofilaments called myofibrils consisting of actin (thin) and myosin (thick) myofilaments
  • Striations show alternating dark (A-band) and light bands (I-band), contained in sarcoplasm
  • Z-disk separates the functional unit of muscle, sarcomere
• Components of thin filaments

  • Actin
  • Troponin - binding site for calcium
  • Tropomyosin - blocks off active sites of actin
• Thick filaments


  Entirely made up of myosin, including heavy chains and light chains
• Mechanism of contraction


  1. Nerve impulse triggers release of acetylcholine (ACh) from the synaptic knob into the synaptic cleft

  2. ACh binds to ACh receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber

  3. Muscle impulse spreads along transverse tubules, releasing calcium ions from terminal cisternae into the sarcoplasm

  4. Calcium ions bind to troponin, changing its shape and moving tropomyosin to expose active sites on actin

  5. Myosin heads attach to exposed active sites on actin, forming cross-bridges

  6. Myosin heads pivot, moving thin filaments toward the sarcomere center

  7. ATP binds myosin heads and is broken down, causing myosin heads to detach from thin filaments and return to their prepivot position

  8. Repeating cycle of attach-pivot-detach-return slides thick and thin filaments past one another, shortening the sarcomere and contracting the muscle

  9. When the impulse stops, calcium ions are actively transported back into the sarcoplasmic reticulum, tropomyosin re-covers active sites, and acetylcholine is no longer released at the neuromuscular junction
• Cardiac muscle


  • Each cardiac muscle cell usually has only one nucleus and is centrally located
  • Intercalated discs - transverse lines that cross the fibers at irregular intervals where the myocardial cells join
  • Mitochondria occupy up to 40% of the cell volume, higher than in slow oxidative skeletal muscle fibers
  • Muscle of the heart ventricles is much thicker than that of the atria, reflecting its role in pumping blood through the cardiovascular system
• Smooth muscle


  • Specialized for slow, steady contraction under the influence of autonomic nerves and various hormones
  • At each cell's central, broadest part, where its diameter, is a single elongated nucleus
  • Smooth muscle is not under voluntary motor control and its fibers typically lack well-defined neuromuscular junctions
  • Instead of troponin, calmodulin and myosin light-chain kinase (MLCK) regulates calcium binding to produce contraction