J point is a critical point in the ECG when the ventricles hand over the baton in the electrical relay race from depolarization to repolarization .This the time the sodium channels extinguish itself and the potassium current begins its activity from Phase 0 to 1 .
If the potassium channels activate little early and snatch the baton prematurely from sodium , we get early repolarization pattern .When this happens , the J point of ECG show a conspicuous wave called J wave , originally denoting Junctional wave between QRS/ST segment (Now perceived as Jitter waves ?) The other implication of premature K+ activity is , lifting up of ST segment , making it the most common cause of non ischemic ST elevation.
* J wave in hypothermia is referred to as Osborne wave and may not be not related to ERS(Ref.4)
Image source.www.cardiology.org
The Ito current is responsible for the phase 1 of action potential (AP), where a rapid outward k + ion flux take place and draws the dome of AP . The dynamics of Ito is complex .It depends upon the density of epicardial K + channels , which are clustered in a heterogeneous manner .There seems to be a concentration gradient along the epicardium and endocardium , making the wave appear prominent in some. This is especially true in healthy, athletic male population where we have some evidence for androgen to play a role on how these channels will behave.Here comes the overlap between Brugada syndrome and ERS as well.
Image source : http://www.research.chop.edu
The subset of patients with J wave pattern were recently shown to have increased risk of primary VF due to phase 2 reentry , when they develop ACS. (Rather J wave pattern was more common in patients who had primary VF following STEMI(Ref 1).This resulted in a spate of worrying articles .Now we know , the fear is largely unfounded ,the risk is far less.
Current thinking is, persons who have asymptomatic ERS pattern with prominent J waves should not be investigated electro-physiologically . (Please remember , every human heart can be induced to VF in EP lab if appropriately stimulated ! )
In fact , I used to tell the young men who harbor prominent J wave , as a marker of healthy heart rather. Let us not fear them with a remote risk that could be as negligible as risk of intercontinental flight crashing into the ocean !
References
Dr.S.Venkatesan MD 
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