Heart beats are controlled by a nodal tissue which is composed of

Since it is the sinu-auricular node which initiates the impulses in the heart of a mammal,it is called:

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:

The pace-setter in the heart is called

The histology of the heart superficially shows the following structures, $EXCEPT$:

Impulse of heart beat originates from