EM 122 Cardiac Muscle

Cardiac Muscle

Transmission of electron microscopy (TEM) of intercalated discs between two cardiac muscle cells (top cell, blue; bottom cell, green).

Cardiac muscle cells contain myofibrils arranged in a similar pattern to skeletal muscle, resulting in similar striations. The myofibrils are composed of longitudinal arrays of sarcomere that appear as alternating dark and light bands (i.e., striated).

• I bands (light blue, top cell; light green, bottom cell) - parallel arrays of thin (actin) filaments that point in opposite directions from Z lines into A bands
  • Narrow with contraction
• Z lines (black) - dark region in the center of I bands
  • Thin filaments anchor to Z lines
• A bands (dark blue, top cell; dark green, bottom cell) - parallel arrays of thick (myosin) filaments that point in opposite directions from M lines
• H bands (cyan, top cell; yellow; bottom cell) - light region in the center of A bands
  • Thin filaments do not extend into this region
• M lines - dark region in the center of H bands
  • Thick filaments anchor to M lines

A sarcomere usually extends from one Z line to the next Z line. However, the last sarcomere extends from a Z line to an intercalated disc.

Subcellular Structures:

• Mitochondria (red)
• Sarcoplasmic Reticulum (cyan)
• Cytoplasm (blue, top cell; green, bottom cell)

Intercalated discs are specialized attachment sites between adjacent cardiac muscle cells. They cross fibers in a linear or stepwise fashion.

An intercalated disc is composed of a transverse (black) and lateral components (purple).

• Transverse Component (black) - crosses at a right angle to the myofibrils.
  • Fascia Adherens - holds together the cells and is anchoring site for thin filaments of adjacent sarcomeres.
  • Desmosomes (maculae adherens) - reinforces the fascia adherens.
• Lateral Components (#1,#2) - parallel to the myofibrils.
  • Gap Junctions (purple) - electrically couple adjacent cells so muscle fibers function as a single functional unit or syncytium.

Subcellular Structures:

• Mitochondria (red)
• Sarcoplasmic Reticulum (cyan)
• Cytoplasm (green)