The atrial syncytium is a network of cardiac muscle cells connected by intercalated discs that lends to the coordinated contraction of the atria. Electrical resistance through intercalated discs is very low, thus allowing free diffusion of ions. The ease of ion movement along cardiac muscle fibers axes is such that action potentials are able to travel from one cardiac muscle cell to the next, facing only slight resistance. In sum, cardiac muscle is a syncytium, meaning when one cell contracts, they all contract. Ventricular syncytium refers to the same network in the ventricle.