What is the membrane surrounding the heart called?

$SA$ node (Sinoatrial node) ke baare mein sahi kathan chuniye:
$I$. Located at the right upper corner of the right atrium.
$II$. Heart of heart.
$III$. It initiates the rhythmic contractile activity of the heart.
$IV$. It is called the pace-keeper of the heart.
$V$. It is called the pacemaker of the heart.
Which of the following options contains the correct statements?

Purkinje's fibres are special types of

During systole of the ventricle:

$A$ patch of nodal tissue responsible for initiating the rhythmic contractile activity of the heart is present in:

What are Purkinje fibers?