The discs are seen here as extremely dense cross striations on the cells. They're somewhat irregular in appearance because, in fact, the ID is irregular in shape. Functionally the ID acts as a communication site and a point of adherence between two myocytes.
Gap junctions running between the two myocytes take care of the communication.
The ID is a site of lowered electrical resistance. The coordination of the heartbeat and the transmission of the contraction signal from one cell to the next is dependent on the integrity and proper functioning of the ID's gap junctions. Physically, ID's are located at the ends of the cells, where they butt up against other ones. There are normally no ID's on the lateral boundaries. In addition to their role in communication and adherence, they are the physical site of the first and last Z-lines in the cell.