What characteristic of cardiac muscle causes contraction of the heart as a unit?

The cardiac muscle has intercalated disk that allows rapid transmission of the impulse. This will allow the cardiac muscle to have a coordinated way. The heart muscle has pacemaker that will spread the signal in regular time. The transmission of impulse is also controlled by a few checkpoint like the bundle of his to make the signal to the Purkinje fiber of left and right ventricle at the same time.