Striated Muscle Cells: Skeletal vs. Cardiac

Skeletal Striated Muscle Cells

Skeletal striated muscle cells are elongated and have hundreds of peripheral nuclei. The cytoplasmic membrane (sarcolemma) encloses the cytoplasm (sarcoplasm), which is packed with:

• Myofibrils
• Sarcoplasmic reticulum
• T tubules
• Mitochondria

Myofibrils are composed of sarcomeres arranged in series. Each myofibril consists of thin (actin) and thick (myosin) myofilaments.

Sarcomere Structure

The sarcomere (from Greek sarco meaning "flesh" or "meat" and meres meaning "part") features:

• A central, dark A band (anisotropic)
• Two peripheral, light I bands (isotropic)

The sarcomere contains globular proteins such as troponin and tropomyosin. The sliding of thin filaments between thick filaments causes muscle contraction. During contraction, the A bands remain the same length, while the I bands shorten, and the H zone may disappear.

Sarcoplasmic Reticulum (SR)

The sarcoplasmic reticulum (SR) is a modified endoplasmic reticulum responsible for storing, releasing, and retrieving calcium ions (Ca²⁺). It consists of interconnected tubules and terminal cisternae, located at the junctions of the A and I bands. The SR surrounds the myofibrils.

T Tubules

T tubules are transverse invaginations of the sarcolemma, situated between two adjacent terminal cisternae of the SR. A T tubule and two adjacent terminal cisternae form a triad.

Cardiac Striated Muscle

The heart is a hollow organ with walls composed of three layers:

• Endocardium: The innermost layer, consisting of endothelium and underlying connective tissue.
• Myocardium: The middle layer, primarily composed of cardiomyocytes and cardiac connective tissue.
• Epicardium: The outermost layer, containing blood vessels, nerves, and connective tissue covered by mesothelium.

Cardiomyocytes

Cardiomyocytes are smaller than skeletal muscle cells and have a cylindrical, branched shape. These branches connect to other cardiomyocytes, forming a complex three-dimensional network. These connections are called intercalated discs.

Structure

Cardiomyocytes have a single, central nucleus and an elongated morphology. Their myofibrils are similar to those in skeletal muscle cells. T tubules are narrower and located at the level of the Z discs (unlike skeletal muscle cells, where they are located between the A and I bands). The sarcoplasmic reticulum is similar to that of skeletal muscle cells but lacks terminal cisternae and triads.

Cardiac Conduction System

The heart generates its own electrical impulses and conducts them through specialized cells called cardiac conduction cells. This system, known as the heart's pacemaker, includes:

• Sinoatrial (SA) node (Keith and Flack's node)
• Atrioventricular (AV) node (Aschoff-Tawara node)
• Bundle of His and its branches
• Purkinje fibers

Histologically, the SA and AV nodes consist of nodal cells. The bundle of His and Purkinje fibers are composed of Purkinje cells. Purkinje cells are larger than nodal cells, fusiform in shape, and can be difficult to distinguish from fibroblasts. Purkinje cells are bulky and easily differentiated in connective tissue, with one or two central nuclei and irregular morphology.