Primary SA node creates 70 beats per minute and secondary AV node 40-60 beats per minute. Is the AV node inactive (and other subsequent nodes) while the SA node is functional? If the primary, secondary and tertiary nodes are working simultaneously, wouldn't the difference in the beats per minute cause a mess?
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The AV node and any other nodes are suppressed by the SA node because it is the fastest. If the SA node is sick or slow or there is a conduction problem one of these other nodes can take over calling the beat.
http://www.cvphysiology.com/Arrhythmias/A018. bold emphasis mine.
The higher frequency of SA nodal firing suppresses other pacemaker sites by a mechanism called overdrive suppression. If a latent pacemaker is being depolarized at a higher frequency than its intrinsic rate of automaticity by an adjacent cell that is driven by the primary pacemaker, then the increased frequency of depolarizations leads to an increase in intracellular sodium ions because more sodium ions enter the cell per unit time. This increased sodium stimulates the Na+-K+-ATPase (increases its activity) to expel more sodium from the cell in exchange for potassium (see figure). Because this pump is electrogenic, increased pump activity increases the amount of hyperpolarizing currents generated by the pump. This drives the membrane potential more negative, thereby offsetting the depolarizing pacemaker currents (If) being carried into the cell. This effectively prevents the pacemaker currents from depolarizing the cell to its threshold potential, and thereby prevents the spontaneous generation of action potentials. If the cell ceases to be driven by the SA node (e.g., because of AV block), then the additional hyperpolarizing currents will be lost and spontaneous depolarization and action potential generation can occur.
