Cardiac muscle also called as the heart muscle.It is one of the three types of vertebrate muscle,With the other two being skeletal and smooth muscles.It is an involantary striatel muscle that constitutes the main tissue of the walls of the heart.Cardiac muscle contracts with some important differences.An electrical stimulation in the form of an action potential delivered in a rhythmic pattern triggers the release of calcium from the cells internal calcium store,the sarcoplasmic reticulum.