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Cardiovascular Physiology Concepts

Richard E. Klabunde, PhD

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Sinoatrial Node Action Potentials

Cells within the sinoatrial (SA) node are the primary pacemaker site within the heart. These cells are characterized as having no true resting potential, but instead generate regular, spontaneous action potentials. Unlike non-pacemaker action potentials in the heart, and most other cells that elicit action potentials (e.g., nerve cells, muscle cells), the depolarizing current is carried into the cell primarily by relatively slow Ca++ currents instead of by fast Na+ currents. There are, in fact, no fast Na+ channels and currents operating in SA nodal cells. This results in slower action potentials in terms of how rapidly they depolarize. Therefore, these pacemaker action potentials are sometimes referred to as "slow response" action potentials.

cardiac pacemaker action potential
SA nodal action potentials are divided into three phases. Phase 4 is the spontaneous depolarization (pacemaker potential) that triggers the action potential once the membrane potential reaches threshold between -40 and -30 mV). Phase 0 is the depolarization phase of the action potential. This is followed by phase 3 repolarization. Once the cell is completely repolarized at about -60 mV, the cycle is spontaneously repeated.

The changes in membrane potential during the different phases are brought about by changes in the movement of ions (principally Ca++ and K+, and to a lesser extent Na+) across the membrane through ion channels that open and close at different times during the action potential. When a channel is opened, there is increased electrical conductance (g) of specific ions through that ion channel. Closure of ion channels causes ion conductance to decrease. As ions flow through open channels, they generate electrical currents (i or I) that change the membrane potential.

In the SA node, three ions are particularly important in generating the pacemaker action potential. The role of these ions in the different action potential phases are illustrated in the above figure and described below:

During depolarization, the membrane potential (Em) moves toward the equilibrium potential for Ca++, which is about +134 mV. During repolarization, g’Ca++ (relative Ca++ conductance) decreases and g’K+ (relative K+ conductance) increases, which brings Em closer toward the equilibrium potential for K+, which is about -96 mV). Therefore, the action potential in SA nodal cells is primarily dependent upon changes in Ca++ and K+ conductances as summarized below:

Em = g'K+ (-96 mV) + g'Ca++ (+134 mV)

Although pacemaker activity is spontaneously generated by SA nodal cells, the rate of this activity can be modified significantly by external factors such as by autonomic nerves, hormones, drugs, ions, and ischemia/hypoxia.

It is important to note that action potentials described for SA nodal cells are very similar to those found in the atrioventrcular (AV) node. Therefore, action potentials in the AV node, like the SA node, are determined primarily by changes in slow inward Ca++ and K+ currents, and do not involve fast Na+ currents. AV nodal action potentials also have intrinsic pacemaker activity produced by the same ion currents as described above for SA nodal cells.

RK Revised 12/13/08



DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.