The myocardium is the cardiac muscle, which forms the middle and the thickest layer of the heart wall. The rhythmic contraction of its fibers makes it possible the constant blood circulation throughout our bodies to supply every tissue and organ with oxygen and nutrients. Thus, the cardiac muscle is the key player and the essential part of the heart function, which is to pump blood and keep us alive. (The other two layers of the heart wall are the epicardium and the endocardium).
Like the skeletal muscle, the myocardium is striated muscle tissue. However, the heart fibers are joined together end to end and are cylinder-shaped. They are also arranged in a different fashion and their contraction is not voluntary but automatic and independent of our will. This functional autonomy is carried out by highly specialized cardiac fiber cells organized around the sinoatrial node, the atrioventricular node, the bundle of His, and the Purkinje fibers, with the first one initiating the chemical-electrical impulses that spread throughout the other three structures, contracting the myocardium. These impulses, in turn, are reinforced by sympathetic and parasympathetic nerve fibers, which arise from the autonomic nervous system and form the cardiac plexus.
The cardiac muscle consists of a dense aggregation of muscle cells, which are called cardiomyocytes. These elongated muscle fibers which are arranged longitudinally and transversally (because they branch out diagonally). They are attached to one another, end-to-end, by a specialized proteins called intercalated disks, with fascia adherens playing an important role in holding the muscle fibers together.
The myocardium surrounding the two atria is composed of two muscular layers; a superficial and a deep one. The thickest part of the cardiac muscle is the portion surrounding the two ventricles, especially the left one. This is called ventricular myocardium, which, along with the interventricular septum, is made up of three muscular layers. The deep layer is formed by bundles running upwards from the apex; the middle layer, in contrast, consists of circular muscle bundles that surround the ventricles; the outer layer, on the other hand, is relatively thin and consists of oblique rounded and flattened bundles.
The reason for the myocardium to be thicker on the ventricles is due to the fact that it makes a stronger exertion (with all the pumping power) at each ventricular contraction during systole to drive the blood to the lungs (right ventricle) and to the rest of the body (left ventricle).
Below, the myocardium of the heart, with the three layers exposed in the area around the ventricles.
Anterior aspect of human heart, which has been stripped of the pericardium to expose the myocardium.
