Corpus: Myocardium

The myocardium is the layer of cardiac muscle tissue located between the inner lining of the heart (endocardium) and the outer lining (epicardium). It includes structures within the heart chambers, such as the papillary muscles and trabeculae carneae.

2.1. ...by function

• Working myocardium: muscle cells that generate force to pump blood
• Excitatory and conductive muscle cells: muscle cells involved in the conduction system of the heart

2.2. ...by localization

• Atrial myocardium: muscle tissue of the atria
• Ventricular myocardium: muscle tissue of the ventricles

Myocardial cells are connected to form myocardial fibers, which are grouped into visible bundles by connective tissue (endomysium). These fibers are oriented in circular, oblique, and longitudinal directions. This arrangement enables both concentric contraction of the ventricles and longitudinal shortening between the valve plane and the apex of the heart.

3.1. Atria

The atrial muscle fibers form two layers:

• Outer layer: fibers run horizontally across the front and connect the two atria (horizontal interatrial fascicle). On the back, some fibers cross the interatrial sulcus, linking the right and left atria (transverse fascicles).
• Inner layer: Contains horseshoe-shaped muscle tracts (vertical fascicles) that remain within each atrium, especially visible as the pectinate muscles and crista terminalis inside the right atrium.

3.2. Ventricles

Ventricular fibers are arranged in a spiral pattern, forming two layers in the right ventricle and three layers in the left ventricle.

3.2.1. Subepicardial layer

In the right ventricle, circular fibers extend through the anterior interventricular sulcus into the left ventricle. The fibers run longitudinally on the diaphragmatic surface and descend obliquely on the lateral and anterior wall. At the apex of the heart, the subepicardial layer forms a twisting pattern known as the vortex cordis. Near the conus arteriosus, fibers attach to the conus tendon.

3.2.2. Middle layer

The middle layer is only present in the left ventricle and the interventricular septum, where fibers run in a circular direction.

3.2.3. Subendocardial layer

The subendocardial layer is the innermost muscle layer in both ventricles, with fibers running predominantly in a longitudinal orientation. This layer is especially thick around the papillary muscles and also covers both sides of the interventricular septum.

The myocardium is striated, resembling skeletal muscle but functionally intermediate between skeletal and smooth muscle. It consists of individual cardiac muscle cells, or cardiomyocytes, that may have one or two nuclei, located centrally within each cell.

Cardiomyocytes have a branched shape, creating a woven cell network. They are connected by intercalated discs, which contain gap junctions to propagate electrical signals, as well as desmosomes and adherens junctions to stabilize and transmit force across the network.

Similar to skeletal muscle, the myocardium has a structured arrangement of striated fibers. It also has a system for rapid calcium influx through terminal cisterns of the sarcoplasmic reticulum and T-tubules in the cell membrane. Unlike skeletal muscle, which has triads, the myocardium has dyads, each composed of a terminal cisterna and a T-tubule.

Specialized cardiac muscle cells in the sinoatrial node, atrioventricular node, and ventricular conduction system generate and conduct electrical impulses. Those pacemaker cells have a loose myofibrillar structure and lack T-tubules. Purkinje fibers are large cardiac cells with abundant glycogen and few myofibrils.

Some atrial cardiomyocytes produce and store secretory granules containing atrial natriuretic peptide (ANP), a hormone released when the atrial walls are stretched.

Myocardium that has been destroyed (e.g. as a result of a heart attack) is not replaced by functionally equivalent tissue, but by connective tissue (myocardial remodeling).

According to current knowledge, however, the myocardium is also capable of renewing itself to a limited extent. This has been demonstrated by ¹⁴C analyses of cardiomyocyte DNA. According to this, around 1 % of heart muscle cells are replaced per year in a 25-year-old. This rate drops to 0.45 % in 75-year-olds. During a lifetime, however, less than 50 % of cardiomyocytes are replaced.^[1]