Heart Cycle control
The pacemaker is the Sinoatrial node (SAN) which is responsible for the intrinsic heartbeat. Impulses originate from the SAN which causes atria to contract because the cardiac muscle transmits the impulse as it contracts. There is a slight delay of the impulse at the Atrioventricular node (AVN) to ensure that atrial contraction is complete before the ventricles contracts. The impulse passes to the ventricles via specially conductive fibres called Purkinje fibres which are grouped into the bundle of His in the septum and cause the rapid transmission of the impulse to the apex of the heart. From here the impulse spreads through the ventricles, causing them to contract from the apex upwards.
The cardiovascular centre in medulla modifies heartbeat. Chemoreceptors in the aortic arch, carotid body and medulla detect [CO2], [O2] and pH. Impulses are sent from these receptors to the cardiovascular centre in the medulla. Depending on the input from these receptors either:
Impulses are sent along sympathetic nerves to stimulate the SAN and increase cardiac output
Impulses are sent along parasympathetic nerves (vagus nerves) to inhibit the SAN and decrease cardiac output