How is the heart beat controlled?

The heart is composed of myogenic muscle and generates its own rate in normal condition. This rate can be modified during increased or decreased respiratory demand. 
The heart beat is initiated by a group of specialized muscle cells in the right atrium called the sinoatrial node. The sinoatrial node acts as a pacemaker. It sends out an electrical signal that stimulates contraction as it is propagated through the walls of the atria and then the walls of the ventricles via the atrioventricular node and bundle of His. The heart rate can be modulated by impulses brought to the heart through two nerves from the medulla of the brain acting. The vagus nerve decreases heart rate (part of the parasympathetic nervous system). An accelerator nerve (part of sympathetic nervous system) increases heart rate. Epinephrine increases the heart rate to prepare for physical activity, while norepinephrine decelerates the heart rate.