Cardiac Muscle Cell Types

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 heart’s contractions.

The myocardial conducting cells (1%) form the heart’s conduction system. They are generally much smaller than the contractile cells and have few contractile proteins. Although they are specialized muscle cells, their function is similar to that of neurons in many respects.

Contractile Cell Appearance and Components

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

Mitochondria are plentiful and provide energy for the heart’s contractions. 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 gives cardiac muscle contractile cells a striated appearance. The dark bands are called A bands, and the light bands are called I bands.

Contractile Cell Components

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Micrograph of Contractile Cells

Transverse (T) tubules extend from the surface plasma membrane, known as the sarcolemma, to the cell’s interior, enabling 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 are one-half as many T tubules in cardiac muscle cells as in skeletal muscle cells. Additionally, the sarcoplasmic reticulum stores a limited number of calcium ions, so most of the calcium ions must come from outside the cells, which slows the onset of contraction.

Ultrastructure of Contractile Cells

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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.

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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, resisting 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.

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References and Attributions

OpenStax, Anatomy and Physiology

Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Reference: “Cardiac Muscle and Electrical Activity“

Wikipedia, the free encyclopedia.

Reference 1: “Cardiac muscle“

Reference 2: “Gap Junction“