Given below is the diagrammatic representation of a standard $ECG$. Label its different peaks.

(N/A) The standard $ECG$ (Electrocardiogram) represents the electrical activity of the heart. The peaks are labeled as follows:
$1$. $P$-wave: Represents the electrical excitation (or depolarization) of the atria,which leads to the contraction of both the atria.
$2$. $QRS$-complex: Represents the depolarization of the ventricles,which initiates the ventricular contraction. The contraction starts shortly after $Q$ and marks the beginning of the systole.
$3$. $T$-wave: Represents the return of the ventricles from an excited to a normal state (repolarization). The end of the $T$-wave marks the end of the systole.