Secondary (AV Junction & Bundle of His)
If the SA node does not function, a group of cells further down the heart will become the heart's pacemaker, this is known as an ectopic pacemaker. These cells form the atrioventricular node (AV node), which is an area between the left atrium and the right ventricle, within the atrial septum.
The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.
Further down the electrical conducting system of the heart is the Bundle of His. The left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential at a rate of 30-40 beats per minute, if the SA and AV node both do not function. The reason the SA node controls the whole heart is that its action potentials are released most often to the heart's muscle cells; this will produce contraction. The action potential generated by the SA node passes down the cardiac conduction system, and arrives before the other cells have had a chance to generate their own spontaneous action potential. This is the normal conduction of electrical activity in the heart.
Read more about this topic: Cardiac Pacemaker
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