Muscle Histology

• Muscle tissue


  Contracts to cause movement

  • Muscle attached to bones (skeletal)
  • Muscles of heart (cardiac)
  • Muscles of walls of hollow organs (smooth)
• Muscle is comprised of cells called fibers. When cells are active, internal changes cause them to shorten. The result of shortening is movement.
• What are the 3 types of histological types of muscle in the body?

  1. Skeletal muscle tissue
  2. Cardiac muscle tissue
  3. Smooth muscle tissue
• Skeletal muscle tissue 


  Attached to bones of skeleton and sometimes skin

  • Cylindrical and long (some as long as whole muscle)
  • Multinucleated
  • Striated (striped internal appearance)
  • Voluntary
• Cardiac Muscle tissue


  Found in wall of the heart (myocardium)

  • Branched (Y-shaped) and shorter than skeletal fiber cells
  • Striated
  • Involuntary
  • Attached end-to-end by strong gap junctions at intercalated discs that allow rapid passage of electrical current from one cell to the next during each heartbeat
  • Contraction causes movement of blood
• Smooth muscle tissue


  Found in walls of most internal organs like the stomach, intestines, urinary bladder

  • Relatively short, wide in the middle, tapered at the ends (fusiform shape)
  • Uninucleate
  • Not striated
  • Involuntary
  • Contraction causes involuntary movement of food, blood
• Properties of muscle tissue

  • Excitability: ability to respond to stimulimembranelengthening
  • Extensibility: ability to be stretched beyond resting length
  • Elasticity: ability to return to resting length after shortening or
  • Contractility: ability to generate tension and shorten cell length
  • Conductivity: ability to transmit electrical events along the cell
• Functions of skeletal muscle tissue

  • Body movement
  • Protection and support
  • Regulating elimination of materials
  • Sphincters at orifices
  • Maintenance of posture
  • Heat production
• Structural organization of skeletal muscle

  • Bundles of muscle fibers is called a fascicle
  • Individual muscle fibers contain myofibrils (complex, cylindrical, organelles)
  • Myofibrils are composed of myofilaments

  
• Connective tissue components

  • Superficial fascia: Separates muscle from skin
  • Areolar and adipose connective tissue
  • Deep fascia: Large sheet external to epimysium. Surrounds each muscle, binds muscles with similar functions
  • Dense irregular connective tissue with vessels and nerves
  • Epimysium: Surrounds entire muscle
  • Dense irregular connective tissue
  • Perimysium: Surrounds fascicles
  • Dense irregular connective tissue
  • Endomysium: Surrounds and electrically insulates each muscle fiber
  • Areolar connective tissue with reticular fibers
• Microscopic anatomy of skeletal muscle

  Skeletal muscle fibers have many of the same components of a typical cell, but some are named differently.

  Ex.

  Sarcolemma: Plasma membrane

  Sarcoplasm: Cytoplasm

  Sarcoplasmic reticulum: Smooth ER
• Two main structures are unique to muscle fibers

  1. Transverse tubules (T-tubules): Deep invagination of sacrolemma that extend into the sarcoplasm
  2. Carry impulses from sarcolemma to help stimulate contraction
  3. Sarcoplasmic reticulum: Internal membrane complex
  4. Stores calcium (used to initiate contraction)
• Myofibrils

  • Cylindrical structures within muscle fibers that run the length of the cell
  • Make up 80% of fiber volume
  • Have the ability to shorten, resulting in contraction of the muscle fiber
  • Contain myofilaments: strands of proteins that allow for contraction
  • Two types of myofilaments: thick filaments and thin filaments
• Sarcomeres are structural and functional units within a myofibril. A sarcomere contains overlapping thick and thin filaments. One sarcomere spans from one Z disc to the next.

  
• Thick filaments 


  Composed of hundreds of bundled myosin molecules
• Thin filaments 


  Composed mainly of two strands of filamentous actin twisted around each other
• I band


  Contains thin filaments but not thick filaments

  • Shortness during contraction
• A band


  Contains thick filaments

  • Band is located in central part of sarcomere
  • H zone is center of A band: contains thick but not thin filaments
  • M line is a transverse protein structure in center of H zone that attaches and aligns thick filaments
• Motor Unit


  single motor neuron and the muscle fibers it controls

  
• When a motor unit is stimulated, all muscle fibers within it contract
• Types of skeletal muscle fibers

  • Slow oxidative fibers
  • Fast oxidative fibers
  • Fast glycolytic fibers
• Muscle attachments 


  Tendon: attaches a muscle to bone, skin or another muscle. Formed by merger of connective tissues at the end of muscle. Ropelike structure.

  Aponeurosis: thing flattened connective tissue attachment
• Muscle contraction usually causes one bone to move while the other bone remains fixed.

  Origin: Less moveable attachment

  Insertion: more moveable attachment