<P> The cardiac action potential is a brief change in voltage (membrane potential) across the cell membrane of heart cells . This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins called ion channels . The cardiac action potential differs from action potentials found in other types of electrically excitable cells, such as nerves . Action potentials also vary within the heart; this is due to the presence of different ion channels in different cells (see below). </P> <P> Unlike the action potential in skeletal muscle cells, the cardiac action potential is not initiated by nervous activity . Instead, it arises from a group of specialized cells, that have automatic action potential generation . In healthy hearts, these cells are found in the right atrium and are called the sinoatrial node (SAN; see below for more details). They produce roughly 60 - 100 action potentials every minute . This action potential passes along the cell membrane causing the cell to contract, therefore the activity of the SAN results in a resting heart rate of roughly 60 - 100 beats per minute . All cardiac muscle cells are electrically linked to one another, by structures known as gap junctions (see below) which allow the action potential to pass from one cell to the next . This means that all atrial cells can contract together, and then all ventricular cells . </P>

Where is the normal cardiac action potential initiated