Which structure initiates the heartbeat?

In the heart, as the action potential reaches the $AV$ node from the $SA$ node, there is a delay of the action potential. This delay is important because:

$A$ specialised nodal tissue embedded in the lower corner of the right atrium,close to the atrioventricular septum,delays the spreading of impulses to the heart apex for about $0.1 \; sec$. This delay allows:

The rate of heart beat per minute is highest in case of

Define the following terms and give their location:
$(a)$ Purkinje fibres
$(b)$ Bundle of His

The wall of the human heart is thick due to: