The atrial syncytium

Syncytium

In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...

is a network of cardiac muscle cells connected by intercalated disc

Intercalated disc

When observing cardiac tissue through a microscope, intercalated discs are an identifying feature of cardiac muscle. Cardiac muscle consists of single heart muscle cells which have to be connected by intercalated discs to work as a functional organ. By contrast, skeletal muscle consists of...

s 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

Ventricular Syncytium

The ventricular syncytium is a network of cardiac muscle cells connected by gap junctions that lends to the coordinated contraction of the ventricles. Atrial syncytium refers to the same network in the atria....

 refers to the same network in the ventricle

Ventricle (heart)

In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

.