Editing Cardiac conduction system (section)

===Ventricular repolarization===
The last event of the cycle is the repolarization of the [[ventricle (heart)|ventricles]]. It is the restoring of the resting state. In the ECG, repolarization includes the J point, ST segment, and T and U waves.<ref name="pmid12906963">{{cite journal | vauthors = Yan GX, Lankipalli RS, Burke JF, Musco S, Kowey PR | title = Ventricular repolarization components on the electrocardiogram: cellular basis and clinical significance | journal = J Am Coll Cardiol | volume = 42 | issue = 3 | pages = 401–409 | date = August 2003 | pmid = 12906963 | doi = 10.1016/s0735-1097(03)00713-7 | doi-access = free }}</ref>
The transthoracically measured PQRS portion of an electrocardiogram is chiefly influenced by the [[sympathetic nervous system]]. 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&nbsp;bpm is known as a normal [[sinus rhythm]]. If SA nodal impulses occur at a rate less than 60&nbsp;bpm, the heart rhythm is known as [[sinus bradycardia]]. If SA nodal impulses occur at a rate exceeding 100 bpm, the consequent rapid heart rate is [[sinus tachycardia]]. These conditions are not necessarily bad symptoms, however. Trained athletes, for example, usually show heart rates slower than 60 bpm when not exercising. If the SA node fails to initialize, the AV junction can take over as the main pacemaker of the heart. The AV junction consists of the AV node, the bundle of His, and the surrounding area; it has a regular rate of 40 to 60 bpm. These "junctional" rhythms are characterized by a missing or inverted P wave. If both the SA node and the AV junction fail to initialize the electrical impulse, the ventricles can fire the electrical impulses themselves at a rate of 20 to 40&nbsp;bpm and will have a QRS complex of greater than 120&nbsp;ms. This is necessary for the heart to be in good function.