(a)[4]
Describe the functions of the sinoatrial node (SAN) and the atrioventricular node (AVN) in starting and regulating the cardiac cycle.
(b)[2]
Explain why it is necessary for the Purkyne fibres to reach the base of the heart.
(c)[4]
The nervous system controls SAN activity. Noradrenaline is released by nerve cells in the SAN. Fig. 4.1 illustrates how noradrenaline causes calcium ions ($\text{Ca}^{2+}$) to move into a cell in the SAN. With reference to Fig. 4.1, outline the process of cell signalling.