P-wave of ECG indicates:
(1) Depolarisation of atrial muscles
(2) Activation of the SA node
(3) Spread of excitation from the SA node to the V node
(4) Repolarisation of atria and depolarisation of ventricle
1 and 2 are correct.
The P wave is a small upward wave which represents electrical excitation or atrial depolarisation which leads to atrial contraction. The impulses of contraction start from the SA node as the P wave activates the SA node.