After a short delay that gives the ventricles time to fill with blood, the electrical signal diverges and is conducted through the left and right bundle branches of His to the respective Purkinje fibers for each side of the heart, as well as to the endocardium at the apex of the heart, then finally to the ventricular epicardium; causing the ventricles to contract.

On the microscopic level, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disc.

These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction (heart attack).

Embryologic evidence of generation of the cardiac conduction system illuminates the respective roles of this specialized set of cells.

It is then followed by rapid growth of a second order sympathetic adrenergic system arising from the formation of the thoracic spinal ganglia.

After a delay, potassium channels reopen, and the resulting flow of K+ out of the cell causes repolarization to the resting state.

The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage.

The left posterior fascicle transmits impulses to the papillary muscles, leading to mitral valve closure.

This allows pre-tensioning of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction.

The two bundle branches taper out to produce numerous Purkinje fibers, which stimulate individual groups of myocardial cells to contract.

The T (and occasionally U) waves are chiefly influenced by the parasympathetic nervous system guided by integrated brainstem control from the vagus nerve and the thoracic spinal accessory ganglia.

An impulse (action potential) that originates from the SA node at a relative rate of 60–100 bpm is known as a normal sinus rhythm.

When an arrhythmia cannot be treated by medication (or other standard cardioversion measures), an artificial pacemaker may be implanted to control the conduction system.