The electrical signal which drives the contraction of the heart takes a very specific pathway in the normal heart.
It is the re routing of this pathway, through various pathologies, that causes the ECG to change.
So in this post lets look at the structures involved.
Sino-Atrial Node (SA Node)
Located in the wall of the right atrium. It is surrounded by connective tissue which prevents electrical activity in the atria affecting it.
It initiates the action potentials. (Cardiac pacemaker cells action potential)
Innervated by the sympathetic nervous system which will release noradrenaline.
This will increase the flow of sodium and potassium into the cells which will then increase the heart rate.
The parasympathetic system will release acetylcholine which will then slow the heart rate.
Atrio-Ventricular Node (AV Node)
Lies at the lower back section of the intra-atrial septum.
It receives signals from the SA Node and it delays that signal for 0.09 secs allowing the ventricles time to fill before contracting.
It is capable of decremental conduction- the faster it is stimulated the slower it conducts. This prevents fast atrial rhythms from being conducted to the ventricles.
Normal conduction rate is 40-60 bpm. If there is loss of conduction for the SA node the slower AV node will take over.
Bundle of His
Continues down the intraventricular septum.
Divides into the left and right bundles.
Approximately 1.8 cm long in the adult heart.
Located deep within the connective tissue
Purkinje Fibres
Located in the inner ventricular walls just beneath the endocardium.
They carry the impulse form the left and right main bundles to the myocardium.
They also have automaticity- they can generate their own impulse- if the other nodes fail they can generate a rate of 20-30 bpm.