These are cells that have the ability propagate and spread action potentials to surrounding cells.
These are the muscle cells of the heart, each one physically contracting to cause the heart muscle to shorten. They are relatively very large cells (30x30x150 micron), constitute up to 80% of the heart volume and have an elongated, brick-like appearance.
Smooth Muscle Cells
These cells are located inside the walls of large arteries and arterioles. Their primary purpose is to regulate vessel diameter by contracting and relaxing.
The pacemaker cells make up the two (S-A and A-V) nodes of the heart. These cells spontaneously depolarize (as a result of their high membrane permeability to sodium ions) and generate action potentials at regular intervals.
These are very large, elongated muscle cells which rapidly (speeds up to 4 m/s) spread impulses from the A-V node to the ventricles. This allows for an almost immediate transmission of the electrical impulse throughout the entire cardiac muscle.