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. Show 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 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. Revised 12/03/2022 DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice. What happens if sodium leak channels are blocked?Blocking voltage-gated sodium channels (NaV) will prevent action potential initiation and conduction and therefore prevent sensory communication between the airways and brainstem. In so doing, they would be expected to inhibit evoked cough independently of the nature of the stimulus and underlying pathology.
What would happen to the resting potential if you blocked the leak channels?Neurons and muscle are just unique in using RMP in generating AP. If potassium leak channels are blocked, what will happen to the membrane potential? It will reduce the resting membrane potential, making the cell less negative (or more positive).
What happens to the resting membrane potential of MV when sodium channels open?During the application of action potential, sodium channels open and sodium rushes into the cell. The large sodium current takes the membrane potential from its negative resting state toward ENa.
Is leakage ion channels are responsible for resting membrane potential?Resting membrane potentials are maintained by two different types of ion channels: the sodium-potassium pump and the sodium and potassium leak channels. Firstly, there is a higher concentration of thepotassium ions inside the cell in comparison to the outside of the cell.
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