There are two types of cells within the heart: the myocardiocytes and the cardiac pacemaker cells. Myocardiocytes make up the atria (the chamber in which blood enters the heart) and ventricles (where blood is pumped out of the heart) of the heart. These cells must be able to shorten and lengthen their fibers and the fibers must be flexible enough to stretch. These functions are critical to the proper form during the beating of the heart.
Myocardiocytes can contain vimentin and desmin. Vimentin is a substance in mesenchymal cells that is responsible for holding the organelles in the cytosol. It is also responsible for adding flexibility to the cell and keeps them from being too delicate. The exact nature of desmin is not known. It is purported to have a similar function as vimentin although the levels of each vary at different points during cardiac muscle growth. Desmin is more readily available during early growth whereas vimentin is predominate in later stages.
Cardiac pacemaker cells carry the impulses that are responsible for the beating of the heart. They are distributed throughout the heart and are responsible for several functions. First, they are responsible for being able to spontaneously generate and send out electrical impulses. They also must be able to receive and respond to electrical impulses from the brain. Lastly, they must be able to transfer electrical impulses from cell to cell.
All of these cells are connected by cellular bridges. Porous junctions called intercalated discs form junctions between the cells. They permit sodium, potassium and calcium to easily diffuse from cell to cell. This makes it easier for depolarization and repolarization in the myocardium. Because of these junctions and bridges the heart muscle is able to act as a single coordinated unit.
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