One unique anatomical feature of cardiac muscle, which isn't seen in skeletal or smooth muscle, is

The answer to this question is the Intercalated disks.

Intercalated discs are also known as the gap functions. Intercalated discs are microscopic details of the cardiac muscles. Intercalated discs functions as a connector in the two adjacent cardiac cells. This also contains membrane junctions which are the fascia adherens, gap junctions, and macual adherens.