Cardiac muscle, called the myocardium, is composed of a number of long, branching cells that are joined by intercalated discs. An intercalated disc is an area where cell membranes of adjacent cardiac muscle cells are joined. There are also small spaces in cardiac muscle cells that create a direct electrical connection between cells by allowing ions to move freely between cells. The interconnecting matrix joins cardiac muscle cells into a single, very large muscle cell called a syncytium (Latin for “joined cells”). Another difference between skeletal muscle and cardiac muscle is that cardiac muscle has pacemaker cells, which initiate contractions rhythmically rather than through neural stimulation. Cardiac muscle contraction lasts about 10 times longer than skeletal muscle contractions, and cardiac muscle cannot produce sustained contractions as skeletal muscles do.