EM 138 Cardiac Muscle

Cardiac Muscle

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

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 sarcomeres that appear as alternating dark and light bands.

• I bands (light green, left cell; light blue, right 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 green, left cell; dark blue, right Cell) - parallel arrays of thick (myosin) filaments that point in opposite directions from M lines
• H bands (yellow, left cell; cyan, right 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 extends from one Z line to the next Z line.

Subcellular Structures:

• Sarcomeres (green, left cell; blue, right cell)
• Mitochondria (red)
• Cytoplasm (green, left cell; cyan, right cell)

Intercalated Disc

Cardiac muscle cells are joined together by intercalated discs (#1,#2; black), which coincide with Z lines. They cross fibers in a linear or stepwise function.

• Transverse Component - crosses perpendicularly to the muscle fiber
  • Finger-like interdigitations to maximize the surface area of contact
  • Fascia Adherens - holds together the cells and is anchoring site for thin filaments of adjacent sarcomeres.
  • Desmosomes (macula adherens) - reinforces the fascia adherens.
• Lateral Component - parallel to muscle fibers and connect transverse components
  • Gap Junctions (orange) - electrically couple adjacent cells so muscle fibers function as a single functional unit or syncytium.

These structures have two important roles:

• Attachment points for the characteristic branched pattern of cardiac muscle
• Allow cardiac muscle cells to function as a syncytium