The epicardium is the outermost or superficial layer of the heart. It represents the visceral layer of the pericardium. As we get old, the epicardium is often the place when fat infiltrates. Within the close confinement of the bony thorax, the epicardium is the heart’s very own first line of defence. The epicardium is made up of tough, fibrous tissue that conceals the heart.
The layer next to the epicardium is called the myocardium. From the word myo which means muscles and the word cardi which pertains to the heart, this layer is composed mostly of heart or cardiac muscles and it forms the bulk of the heart. As the main pumping organ of the body, this is the part that contracts. With the body demanding an endless supply of blood, the heart needs to be a strong structure to withstand the enormous stress of pumping 24 hours a day, seven days a week. The cardiac muscle cells branch out and are connected to one another by strong connective tissue fibres that crisscross. These muscles are then arranged in either circular bundles or spiral. The interlacing and branching bundles are very effective in keeping all the parts of the heart together.
To further add to the stability of the heart, the fibrous skeleton of the heart arises from the dense network of the connective tissue fibres which reinforces internally the myocardium and serves to anchor the cardiac muscle fibres. The thickness of this collagen network depends on the location; it may be thicker in some areas. A good example of this is the great vessels of the heart and the heart valves. Rope-like rings of muscle fibres provide additional support to ensure that these vessels will withstand the enormous pressure of blood pulsing through them and they do not stretch out.
The connective tissues found in the myocardium are not electrically excitable. With this mechanism in place, it limits the direct spread of action potential across the heart to specific pathways.
The endocardium represents number three of the layers of the heart. The word endo means inside and cardi means heart so it literally translates to inside the heart. It has a characteristic white, glistening sheet of endothelium. This squamous epithelium finds refuge on a thin tissue layer.
The endocardium is situated in the inner layer of the heart and it covers fibrous skeletons of the heart. Furthermore, it also lines the heart chambers. The endocardium is continuous with the endothelial coating of the blood vessels that leaves and enters the heart. Furthermore, the endothelial cells found in the endocardium not only cover the walls of the chambers of the heart but also that of the valves.
Another thing to note in relation to the endocardium is that the Purkinje fibres are located in this layer of heart. Purkinje fibres are important because it participates in heart muscle contractions.