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Cardiac Muscle Structure

(Sample Lesson)


There are two major types of cardiac muscle cells: myocardial contractile cells and myocardial conducting cells.

  • The myocardial contractile cells constitute the bulk (99%) of the cells in the atria and ventricles. They conduct impulses and are responsible for the contractions of the heart.
  • The myocardial conducting cells (1%) form the conduction system of the heart. They are generally much smaller than the contractile cells and have few contractile proteins. Their function is similar in many respects to neurons, although they are specialized muscle cells.

Contractile Cells

Compared to skeletal muscle, cardiac myocardial contractile cells,(= fibers; cardiomyocytes), are considerably shorter, smaller in diameter, and branched.

Mitochondria are plentiful, providing energy for the contractions of the heart. Typically, they have a single, central nucleus, but two or more nuclei may be found in some cells.

The myofilaments in myocardial contractile cells have the same composition and arrangement as found in skeletal muscle cells. Thick filament proteins are principally composed of myosin, and thin filament proteins consist mostly of actin.

The two myofilaments are organized into tubular bundles called myofibrils. Vertically oriented Z-line (disc) proteins anchor the thin filaments and divide the myofibrils into functional contractile units called sarcomeres. Repeated overlapping of the thick and thin filaments give cardiac muscle contractile cells a striated appearance. The dark bands are referred to as A bands and the light bands are called I bands.

General Structure of Contractile Cells

General structure of cardiac muscle cells (Illustration)

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Transverse (T) tubules penetrate from the surface plasma membrane, the sarcolemma, to the interior of the cell, allowing the electrical impulse to reach the interior. The T tubules are only found at the Z lines, whereas in skeletal muscle, they are found at the junction of the A and I bands. Therefore, there is one-half as many T tubules in cardiac muscle as in a skeletal muscle. Also, the sarcoplasmic reticulum stores few calcium ions, so most of the calcium ions must come from outside the cells, which slows the onset of contraction.

Ultrastructure of Contractile Cells

Ultrastructure structure of cardiac muscle cells (Illustration)

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Individual cardiac muscle cells are joined together at their ends by intercalated discs. Microscopically, the discs appear as folded, dark-staining lines that run perpendicular to the direction of cardiac muscle fibers.

Embedded in the intercalated discs are numerous intercellular junctions. They span the distance between adjacent cell membranes and anatomically and physiologically link the neighboring cells.

  • Anchoring junctions strongly bind the ends of adjacent muscle cells together and resist the forces exerted by contraction.
    • Adhering junctions bind to actin (thin) filaments.
    • Desmosomes bind to intermediate filaments.
  • Communicating junctions allow the passage of ions between the muscle cells and help synchronize contraction movements.
    • Gap junctions are small, intercellular pores consisting of adjacent membrane channel proteins.

Micrograph of Contractile Cells

Page Attributions

OpenStax, Anatomy and Physiology

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Reference: “Cardiac Muscle and Electrical Activity

Wikipedia, the free encyclopedia.

Reference 1: “Cardiac muscle

Reference 2: “Gap Junction