The stress response increases the propensity for ectopic beats, as adrenaline encourages the electrical signal of the heart to bypass the sino atrial node (the 'pacemaker') and initiate an ectopic beat.
I think not quite. Most of the heart cells have the potential to initiate the electrical action stimulating the heart muscle contraction and pumping. The sinus node positioned in the right atrium will generate pulses slightly quicker so will generally initiate the heart beat. The electrical activity will then spread through the atrial heart muscle causing the muscle contraction. It will then be momentarily delayed through the AV node to then stimulate the ventricle muscle similarly spreading through the muscle causing the contraction and pumping. You need the delay through the AV node or the atria and ventricle would beat together - a pretty useless arrangement that would be.
While adrenalin does stimulate the sinus node so does a whole range of nerve stimuli - both the vagal and sympathetic systems, its their job to speed it up or slow it down. For athletes it is frequently raised vagal tone which encourages atrial miss beats. However worry, anger, stress, alcohol, caffeine, relaxation, waking, shock, large meals, ejaculation, breath holding - I could go on - can all initiate atrial miss beats.
With ectopics that you feel this is an issue with the ventricle beat, not the atria. The premature ventricle beat will have been stimulated by an electrical impulse from within the ventricle muscle. As everyone has said these are benign and unimportant - except to the person having them. If you have lots they can be literally a real pain. They will effect blood flow for each beet effectively missed. Only if you have a history of prior heart attack or you really have lots is there normally a need for further investigation but you should still talk to your Dr. Alcohol and coffee can encourage them as of course does hard exercise, especially endurance sports like cycling. Physical activity can increase electrical disturbance in both atria and ventricle.
When the atria produces premature beats the situation becomes more "interesting". This can cause atrial flutter or atrial fibrillation which I have. Either of which mean a much less effective atrial pump and a confused signal for the AV node to pass to the ventricles. Well worth a Dr visit. These are generally much more of an embug_gerance but in the absence of other heart conditions are also benign.
Studies now show that from 5 to 10% of seriously trained endurance athletes like cyclists will develop an atrial arrhythmia compared to less than 1% of the less fit population.
No I am not a Dr but I have done a little home work.