Communication between the heart and brain

The rate at which the SAN fires (i.e heart rate) is unconsciously controlled by a part of the brain called the medulla.

Animals need to alter their heart rate to respond to internal stimuli, e.g to make sure the heart rate is high enough to supply the body with enough oxygen. Internal stimuli are detected by pressure receptors and chemical receptors:

•  There are pressure receptors called baroreceptors in the aorta and carotid arteries. They're stimulated by high and low blood pressure. 
• There are chemical receptors called chemoreceptors in the aorta, the carotid arteries and in the medulla. They monitor the oxygen level in the blood and also carbon dioxide and pH,

Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones.