The contractions of the heart are controlled by chemical (electrolytes- potassium
, calcium, magnesium, sodium, chloride
) impulses, which fire at a rate which controls the beat of the heart. The cells that create the impulses are called pacemaker cells. IST effects the Sinoatrial Node.
The sinoatrial node (SA node) is responsible for the whole heart's beat and is made up of cardiac myocyte cells which is muscle fiber.