The heart has specialized cells that generate electrical pulses to control heartbeats. There are three main parts to the heart's electrical system - the sinoatrial node, atrioventricular node, and His-Purkinje system. The sinoatrial node acts as the natural pacemaker to create electricity for heartbeats, which is then passed through the atrioventricular node to the ventricles via the His-Purkinje system. The brain and hormones can also influence heart rate through the sympathetic and parasympathetic nerves as well as hormones released by the thyroid gland and adrenal gland.

1. HEARTBEATS
Heartbeats are controlled by specialized cells located in the heart itself. These cells can
generate electrical pulses on their own for our heart to beat. These cells separate electrical
charges and released electrical charges will make our heart beats.
There are three parts of heart electrical system: SA node (sinoatrial node), AV node
(atrioventricular node) and His-Purkinje system.
The SA node is the natural pacemaker that creates electricity for heartbeats. The AV node
passes electrical signals from atria to ventricles. His-Purkinje system spreads the electricity
throughout the ventricles to make them contract.
Any electrical tissue in the heart has the potential to be the pacemaker; however, SA node
produces electricity faster so it is normally in control. If the SA node fails, other parts of our
heart can take over at a slower rate.
Pacemaker normally makes our heart beats at 60 to 100 times per minute.

2. Although heart already has its natural pacemaker, the brain and hormones sometimes have to
play its role.
The brain controls heart rate by sending nerves impulses: sympathetic nerves and
parasympathetic nerves. The sympathetic nerves increase heart rate and the force of
contraction while the latter do the opposite. These nerves are received by medulla oblongata
(a special part of the brain). Sympathetic nerves signals release hormone which causes SA
node to send electrical signals more often.
Sympathetic nerves are sent when our body tissue sends signals that it needs more oxygen.
This happens after exercising or doing strenuous activity. Excessive amount of carbon
dioxide in our blood also causes heart to beat faster. Parasympathetic nerves then are sent to
stop the action of sympathetic nerves once our body gets enough oxygen or at rest.
Hormones from thyroid gland and adrenal gland (during stress or in need for increased
cardiac output) also influence heart rate. Hormone from adrenal gland, norepinephrine,
triggers sympathetic nerves which circulate in bloodstream, causes heart to beat faster for
about several minutes while other nervous sympathetic system sends an instantaneous and
short-lived signal.