4. Outline the control of the heartbeat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and epinephrine (adrenaline)

http://www.sads.org.uk/heart_function.htm

- Myogenic muscle contraction means the heart muscle can contract by itself. 

- In the wall of the right atrium, the pacemaker (SA nodes) is located, region that initiates each heart contraction, and whenever the pacemaker sends a signal, the heart contracts and causes a heartbeat. 

- Nerves and hormones are responsible for transmitting messages to the pacemaker; the adrenaline, for example carries the massages from the medulla of the brain to the pacemaker telling it to speed up the beating.

- Another nerve sends a massage telling the pacemaker to slow down the beatings.