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ILLUMINATIONS

SIMPLIFIED INTERPRETATION OF THE PACEMAKER POTENTIAL AS A TOOL FOR TEACHING MEMBRANE POTENTIALS

Osijek Medical Faculty, Dept. of Physiology, 31000 Osijek, Croatia E-mail: sven{at}jware.hr

	Introduction

TOP Introduction Text for Study by... Activities for Students References

 
Most courses of physiology start by teaching about membrane potentials in different cells. Many of our students find these ideas difficult to understand. Often they try to memorize facts rather than understand mechanisms. The most difficult task may be interpretation of the pacemaker potential generation in sinoatrial (SA) cells. This illumination attempts to improve students’ understanding of membrane potentials by giving them a simplified interpretation of pacemaker potential generation before they have group discussions.

This simplified model interprets the pacemaker potential as a hyperpolarization wave that is not able to reach the hypothetical resting membrane potential (calculated to be about -14 mV), because the triggering potential of calcium/sodium channels (-40 mV) initiates the next cycle. The continuous cycle of electrical activity in SA cells would then consist of four stages: depolarization (opening of calcium/sodium channels), repolarization with hyperpolarization (opening of several types of potassium channels), hyperpolarization recovery (cessation of potassium permeability), and triggering of depolarization without ever reaching the resting membrane potential. Although this is an oversimplified interpretation of SA potential generation, students who can understand and discuss it are better able to interpret resting and action membrane potentials.

The following text is given to medical students as reading material for discussion that is usually scheduled for the following week. The students can use their textbooks (1, 3) or other references (2, 4). In Simplified characteristics of SA cells below, students are asked to calculate the hypothetical resting potential for the SA cells based on a Na/K permeability ratio of 0.58:1.00 and the following ion concentrations: extracellular Na⁺ = 142 meq/l and K⁺ = 4; intracellular Na⁺ = 14 meq/l and K⁺ = 142 meq/l (4).

Answer

where EMF is electromotive force.

They are also expected to draw a graph similar to Fig. 1.

View larger version (20K): [in this window] [in a new window]   FIG. 1 Hypothetical resting potential for sinoatrial (SA) cells. Area of hyperpolarization is marked. A continuous cycle of electric activity in SA cells consists of depolarization (opening of calcium/sodium channels), repolarization with hyperpolarization (opening of several types of potassium channels), hyperpolarization recovery (cessation of potassium permeability), and triggering of depolarization without ever reaching the resting membrane potentials.

	Text for Study by Students.

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Simple facts about membrane potentials.
1) More permeable membranes have better defined membrane potentials that are less variable than potentials of less permeable membranes. The high permeability seems to anchor the membrane potential near the Nernst potential of that ion. The cost of stabilization is the high ion flux that must be compensated by more work of ion pumps.

2) Hyperpolarization can be described as a more pronounced negativity of the membrane potential after repolarization. The occurrence, quantity, and duration of the hyperpolarization wave necessarily reflects momentary membrane permeability for certain ions. Hyperpolarization is caused by the temporary relative increase in potassium permeability compared with the resting phase. If this increase in potassium permeability is small, the hyperpolarization wave will also be small. For example, if a certain membrane is almost exclusively permeable to K⁺ ions, it will have a membrane potential near the potassium Nernst potential. During repolarization, this membrane will not hyperpolarize, because additional opening of the potassium channels cannot move the potential any closer to the Nernst potential.

Simplified characteristics of SA cells.
In SA cells, permeability for Na⁺ and K⁺ is similar, so the membrane potentials are between the Nernst potentials for potassium and for sodium. Because the overall permeability is small, the membrane potential is also less stable. Calculate the resting potential if the Na/K permeability ratio is 0.58:1.00 and extracellular Na⁺ = 142 meq/l, K⁺ = 4; intracellular Na⁺ = 14 meq/l and K⁺ = 142 meq/l (4).

Proposed hyperpolarization-based interpretation of SA potentials.
1) During repolarization from the peak depolarization, slow K⁺ channels that are gradually activated during depolarization make the membrane much more permeable to potassium. So the membrane potential progressively becomes more negative, moving toward the potassium Nernst potential. This hyperpolarization wave is shaped by the opening and closing of different sets of K⁺ channels.

2) When all the K⁺ channels that were opened during depolarization are closed, the membrane potential will reach the resting potential that reflects the Na/K permeability ratio. In SA cells, the resting membrane potential is never reached, because it lies above the triggering potential of calcium/sodium channels.

3) A continuous cycle of electrical activity in SA cells consists of the following four stages: depolarization (opening of calcium/sodium channels), repolarization with hyperpolarization (opening of several types of potassium channels), hyperpolarization recovery (cessation of potassium permeability increase), and triggering of depolarization without ever reaching the resting membrane potentials.

	Activities for Students

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1) Use figures of SA potentials from your textbook and mark the calculated level of the hypothetical resting membrane potential of SA-nodal cells.

2) Does this make sense to you?

3) Draw a graph of two SA action potentials. Label the vertical axis in mV (+ and -) and the horizontal axis in seconds.

4) List pros and cons for the proposed model.

5) Discuss the model with your classmates.

	Acknowledgments

 
I thank Drs. D. C. Randall and D. F. Speck from the University of Kentucky Department of Physiology for their support.

DISCLOSURES This work was supported by the Croatian Ministry of Science (Grant 127051).

	References

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1. Guyton AC and Hall JE. Medical Physiology. Philadelphia, PA: WB Saunders, 2000, p. 52–62.
2. Irisawa H, Brown HF, and Giles W. Cardiac pacemaking in the sinoatrial node. Physiol Rev 73: 197–227, 1993.[Free Full Text]
3. Richardson DR, Randall DC, and Dexter DF. Cardiopulmonary System. Madison, CT: Fence Creek, 1998, p. 74–98.
4. Seyama I. Which ions are important for the maintenance of the resting membrane potential of the cells of the sinoatrial node of the rabbit? In: The Sinus Node, edited by Bonke FIM. The Hague: Nijhoff, 1978, p. 339–347.

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