The heart muscle is a special hardworking kind found nowhere else in the body, though it can get very tired when deprived of oxygen in a heart attack and can get stronger through exercise.
The other types of muscle are skeletal or striated muscle, the kind that lets the body move voluntarily, and smooth muscle, found in internal organs and the walls of blood vessels, which is arranged in sheets and is not under conscious control.
The heart muscle, or myocarclium, is called a synctial muscle or synctium, because its strands are so interconnected that they form a continuous network of cells that work in synchrony.
This lets internal electrical signals be coordinated so the whole muscle acts as a unit, contracting or relaxing together.
In fact, the heart was once thought not to be composed of individual cells.
The cells of the heart muscle have their nuclei buried deep within, rather than near the surface like those of skeletal muscle.
They also have an abundance of large mitochondria, the energy factories inside cells, presumably because of high energy demands.
The cells are arranged in parallel columns, as in skeletal muscle, but in still another difference from other muscle cells, heart cells are joined end to end in very long fibers that branch and interconnect.
The joining sites are marked by disks called intercalated disks.
In between the fibers are spaces richly supplied with capillaries to supply oxygenated blood. The cells also enjoy a good supply of glycogen and lipids, potential energy sources.
Inside the cells are myofibrils, a banded contractile substance.
The myofibrils have zones called sarcomeres, where thin filaments made of a substance called actin and thick filaments made of myosin contract and relax.
The filaments slide by each other as they act.