<P> The last event of the cycle is the repolarization of the ventricles . It is the restoring of the resting state . In the ECG, repolarization includes the J point, ST - segment, and T - and U-waves . </P> <P> 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 . </P> <P> An impulse (action potential) that originates from the SA node at a relative rate of 60 - 100bpm is known as normal sinus rhythm . If SA nodal impulses occur at a rate less than 60bpm, the heart rhythm is known as sinus bradycardia . If SA nodal impulses occur at a rate exceeding 100bpm, 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 60bpm 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 60bpm . 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 40bpm and will have a QRS complex of greater than 120 ms . This is necessary for the heart to be in good function . </P> <P> An' arrhythmia' refers to an abnormal rhythm or speed of rhythm of the heartbeat . An abnormal rhythm or speed is defined as one which is not physiological . </P>

Trace the pathway of the electrical impulse in the heart