Cardiac pacemaker heart. There are 3 main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase 1 or 2, just phases 0, 3, and 4. The key to the rhythmic firing of pacemaker cells is that, unlike other neurons in the body, these cells will slowly depolarize by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials. As in all other cells, the resting
Cardiac physiology contract, calcium is released from the sarcoplasmic reticulum of the cells as well as the T tubules. The calcium release triggers sliding of the actin and myosin fibrils leading to contraction. A plentiful supply of mitochondria provide the energy for the contractions. Typically, cardiomyocytes have a single, central nucleus, but can also have two or more. Cardiac muscle cells branch freely and are connected by junctions known as intercalated discs which help the synchronized contraction of the muscle. The sarcolemma (membrane) from adjacent cells bind together at the intercalated discs. They consist of desmosomes, specialized linking proteoglycans, tight junctions, and