What's the key structure in cardiac muscle fibre that aids in the transmission of electrical impulse?

AV node

Function of the cardiac muscle: Spontaneous rhythmic contraction and is composed of contractile proteins: actin, myosin, tropomyosin, troponin and others; myoglobin, glycogen, calcium ions.
The cardiac contracting cells consists of cardiac conducting cells, Purkinje cells, pacemaker cells that helps in generation and conduction of impulses and these are located in sinoatrial node (pacemaker), atrioventricular node, atrioventricular (His) bundle, Purkinje fibers.
The molecular details of working of these cells involves a type of protein that we call as channel proteins, through which movements of calcium ions generates an electric impulse leading to the generation of electric impulse.

The impulse originates in the SA nodes in the left atrium but it travels down the bundle of HIS (main route) to the ventricles where it is spread over a larger area by the perkunjie ( not sure its spelled right) fibers

I think sodium and potasium level as one thing,
these muscles are non streated or involuntary .

therefore they keep working constantly

SA node transmits signals to heart and in case if SA node is not working then other parts of heart such as purkinjee fibres can generate there own signals at a rate slower than normal