The pacemaker is the region of the heart for initiating each contraction of the heart. It is located in the wall of the right atrium. Every time the pacemaker sends out an electrical signal the heart contracts or beats.
The heart beats of its own accord. The pacemaker can cause contraction without stimulation of a nerve (as in quadriplegics). Normally nerves and hormones transmit messages to the pacemaker to modify the rate of heart beat.
Nerves:
a) One nerve carries messages from brain to pacemaker to stimulate an increase in heart contraction.
b) A second nerve carries messages from brain to pacemaker to tell it to slow the rate of contraction.
Hormones:
a) Adrenaline is a hormone carried to the pacemaker via the bloodstream that tells the pacemaker to increase the rate of contraction.