How is heart rate controlled?

The heart muscle is myogenic, meaning that it is able to initiate its own contraction. The wall of the right atria has a region of specialised fibres known as the sinoatrial node (SAN). This node acts as the pacemaker of the heart. The SAN initiates a wave of electrical stimulation at regular intervals (60-80 times a minute at rest). This causes a coordinated contraction of the atria. When the wave reaches the base of the atria it is blocked by a band of insulating tissue, causing a delay in conduction. This delay allows time for blood to move from the atria into the ventricles. Eventually the wave of excitation is conducted through another node in the atrioventricular septum, called the atrioventricular node (AVN). The AVN passes the wave of excitation down the bundle of His towards the apex of the heart. The bundle of His then branches into the Purkinje fibres which carry the wave upwards. This causes the ventricles to contract.
The SAN is connected to two nerves originating from the medulla oblongata in the brain. The accelerator nerve is part of the sympathetic nervous system and increases the rate of impulse release from the SAN.This increases the heart rate. This can be in response to factors such as a high carbon dioxide concentration in the blood, which is detected by chemoreceptors in the carotid arteries, aorta and brain. The receptors send more frequent impulses to the medulla oblongata via a sympathetic pathway, which in turn sends more frequent impulses to the SAN via the accelerator nerve.
By contrast, the vagus nerve is parasympathetic and causes the heart rate to decrease. It acts in response to increased blood pressure, detected by baroreceptors in the carotid sinus. This results in an increased frequency of impulses from the medulla oblongata along the parasympathetic pathway, reducing the frequency of impulses from the SAN and slowing the heart rate.