The SA Node is the heart's normal pacemaker. When the SA Node depolarizes, electrical signal spreads through the atrial muscle causing contraction.
Step-by-step explanation:
Inherent rhythmicity / automaticity - The property of the pacemaker cells of the heart to start electrical sequence of depolarization and repolarization.
SA Node or Sino-atrial Node generates the electrical signal.
The electrical signal passes through internodal pathways, atrial fibres, atrioventricular node (AV Node), the bundle of His, Purkinje fibres, right bundle branch, left bundle branch.
Systole - Contraction of cells when the electrical signal of depolarization reaches the contractile cells.
Diastole - Relaxation of cells when the electrical signal of repolarization reaches the myocardial cells.