Contraction of Cardiac Muscle

The walls of the heart are made of cardiac muscle, a type of muscle found only in the heart. It has special properties that allow it to contract rhythmically, around 70 times a minute at rest, for a whole lifetime.Myogenic. Cardiac muscle is myogenic: it contracts without needing to be stimulated by nerve impulses. The signal to contract starts within the heart itself. Nerves can speed up or slow down the heart rate, but they do not start each beat.Branched cells joined by intercalated discs. Cardiac muscle cells are branched and joined end to end by intercalated discs. These contain gap junctions that allow electrical impulses to pass very quickly from cell to cell. As a result, the cells in a chamber wall contract together, producing a strong, coordinated squeeze.Many mitochondria. Cardiac muscle cells contain large numbers of mitochondria, which provide a constant supply of ATP by aerobic respiration. This means cardiac muscle does not fatigue (tire) like skeletal muscle.How contraction pumps blood. When the muscle in the wall of a chamber contracts, the chamber gets smaller. Its volume decreases, so the pressure of the blood inside it increases. Blood is forced out of the chamber towards an area of lower pressure. When the muscle relaxes, the chamber gets larger again, its pressure falls, and it can refill with blood.After each contraction, cardiac muscle has a short period during which it cannot be stimulated again. This allows the chambers to relax and refill between beats.