Which layer of the heart is primarily composed of involuntary muscle?

The myocardium is the layer of the heart that is primarily composed of involuntary muscle. This muscle tissue is responsible for the heart's ability to contract and pump blood throughout the body. Myocardial muscle fibers are specialized to withstand the continuous workload of pumping, and they work automatically without conscious control, which is characteristic of involuntary muscle.

The endocardium, while an essential layer that lines the heart's interior, is made up of endothelial cells, not muscle. The epicardium serves as a protective outer layer and is also formed of connective tissue and epithelial cells. The pericardium is a fibrous sac surrounding the heart that provides protection and support; it is not muscular in nature. Thus, the myocardium is distinct and crucial for heart function due to its structure and role in generating the cardiac contraction needed for circulation.