The nodal cells of the heart, unlike cardiac contractile muscle fibers, have an intrinsic ability to depolarize

The nodal cells of the heart, unlike cardiac contractile muscle fibers, have an intrinsic ability to depolarize spontaneously.
This means that they are very unstable and no one can even predict when this is going to happen, as it all occurs within your body and the heart. These cells are responsible for the contraction of the heart - they produce electrical impulses that make the heart pump blood.