Cardiac muscle tissue

Cardiomyocytes

Cardiomyocytes, also known as cardiac muscle cells, usually contain one elongated nucleus that lies in the centre, which is a distinguishing feature from skeletal muscle. By examining the ultrastructure, it becomes apparent that the myofibrils separate as they approach the nucleus, pass around it and re-assemble in their original pattern on the other side. You can visualize the arrangement by imagining two cones that are joined at their vertices, which represents the nucleus. In fact, cell organelles are also concentrated in this cytoplasmic region around the nucleus. These include mitochondria, Golgi apparatus, lipofuscin filled granules, and glycogen. Lipofuscin is a red-brown pigment, often called the wear and-tear-pigment, which gradually accumulates inside cardiac tissue with age. It is the remnant of lysosomal cell contents. The cytoplasm of cardiomyocytes, called sarcoplasm, is eosinophilic and appears as a 3D network. 

Due to the high energy requirements, cardiac muscle tissue contains additional large and elongated mitochondria located between the myofibrils. They can run the full length of the sarcomere and contain many internal cristae. In addition, extra glycogen granules are also located between the myofibrils to store the energy. Threads of collagenous tissue fibers together with capillaries are also present between the muscle fibers to provide the tissue with support and a blood supply.

Intercalated discs

Cardiac myocytes are joined together via intercalated discs, which coincide with Z lines. They appear as lines that transverse the muscle fibers perpendicularly when examined with a light microscope. However, if the ultrastructure is examined, the discs are far from linear because they have finger-like interdigitations to maximize the contact surface area. The discs also contain two compartments that are orientated transversely and laterally (parallel) in relation to the myofibrils, resembling a flight of stairs.

To accomplish their attachment roles, intercalated discs contain three types of cell junctions:

• Adherens junctions (fascia adherens) are a part of the transverse component and are the ones making the intercalated discs visible in hematoxylin and eosin (H&E) staining. They are responsible for actually connecting the ends of the myocytes together to form a fiber. In addition, they transmit the force of contractions from cell to cell because the actin filaments of terminal sarcomeres insert into these junctions.
• Desmosomes (maculae adherentes) are part of both components and they reinforce adherens junctions. They prevent the separation of myocytes during contractions by anchoring intermediate filaments.
• Communication (gap junctions) are part of the lateral component of intercalated discs. They allow cardiac tissue to function as a syncytium by providing pathways for various ions to pass between adjacent cells, resulting in the propagation of excitation and subsequent contraction.

Myofibrils and sarcomeres

Sarcomeres are the functional subunits of myofibrils and the contractile units of cardiac muscle tissue. They are arranged into a branched pattern, forming a 3D network in the cytoplasm. Sarcomeres are specific portions of myofibrils located between two Z lines and are responsible for the striated appearance of cardiac tissue. They are composed of thick and thin filaments. Thick filaments are composed of polymerised myosin type II protein and are attached to band called the M line that is situated in the middle of the sarcomere. Thin filaments consist of polymers of the protein alpha actin and are attached to the Z lines. These two lines, together with the A band that corresponds to the length of the myosin filaments, are electron rich and appear darker in electron microscopy. The I and H bands appear lighter and they represent regions which consist of only thin or thick filaments respectively, but not both.

The cytoplasmic regions between the sarcomere branches are filled with mitochondria and smooth endoplasmic reticulum (sER) called sarcoplasmic reticulum, which envelopes each myofibril. The membranous network of sarcoplasmic reticulum is transversed by structures called T tubules, which are extensions of the sarcolemma (plasma membrane of muscle cells). They form the T tubule system and their lumens are communicating directly with the extracellular space. Their course follows the Z lines of the sarcomeres, resulting in a single T tubule for every sarcomere. The region formed by the pair of flat terminal cisternae of the sarcoplasmic reticulum and a portion of a T tubule is called a diad.

Cardiac conducting cells

Contractions are initiated and propagated throughout the heart by specialised cardiac cells called cardiac conducting cells (they are not neurons). Collectively, they form the conducting system of the heart. These cells form specific structures like nodes, bundles, and conducting fibers. The initial, spontaneous stimulus starts from the sinuatrial node located in the wall of the right atrium at the level of the entry point of the superior vena cava. The impulses travel through the walls of the atria, resulting in contraction. They are then picked up by the atrioventricular (AV) node situated above the tricuspid valve in the medial wall of the right atrium. These two nodes are enveloped by collagenous tissue that is full of capillaries and autonomic nerves. After the AV node, the impulse passes through the bundle of His, the right and left bundle branches, and finally through the Purkinje system. The three bundles stain quite pale due to their high content of glycogen granules and mitochondria. The Purkinje fibers also contain a central area that stains pale. Cardiac conducting cells are connected strictly via desmosomes and gap junctions only. They also don’t have a T tubule system.

As you can see, the contraction of the heart is spontaneous. However, despite their autonomy, conducting cells are not isolated from the nervous system. The sympathetic branch increases the impulse frequency from the nodes to the conducting system, while the parasympathetic branch decreases it. As every impulse is followed by a contraction in normal situations, the rate of contraction is increased or decreased, respectively.