What makes heart muscle different from the muscles you move?

Heart muscle works with the same machinery as the muscles you move, but it is built for a different job: beating without rest, in step, for a lifetime. Cardiac muscle is one of three types of vertebrate muscle, alongside skeletal and smooth muscle, and it is an involuntary, striated muscle that forms the main tissue of the heart wall.

It contracts in a similar manner to skeletal muscle. An electrical signal triggers the release of calcium from the cell's internal store, and the rise in calcium makes the cell's myofilaments slide past each other, just as in your biceps. The important differences lie elsewhere.

First, the heart gives itself its orders. Its contractions are myogenic, initiated by the heart muscle cells themselves rather than by a nerve, though the autonomic nervous system can modulate them. Specialised pacemaker cells set the rhythm.

Second, its cells work as one. Individual cardiac muscle cells are joined by intercalated discs into a network, the cardiac syncytium, which lets electrical impulses pass quickly and the muscle contract in a coordinated way.

Third, it is built to keep going. Heart cells contain many mitochondria, which supply the energy and make them highly resistant to fatigue.

Heart muscle also has a surprising history of debate. It was commonly believed that heart muscle cells could not be regenerated, until a report published in 2009 contradicted this. Olaf Bergmann and colleagues at the Karolinska Institute dated heart cells with carbon-14 left in their DNA by Cold War bomb tests, and found that they do renew, but slowly: about 1 percent a year at 25, falling to 0.45 percent at 75.