Question 1

Describe and compare the action potentials from cardiac ventricular muscle cells and the sino-atrial node.

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College Answer

This question details an aspect of cardiac physiology which is well described in multiple texts. Comprehensive answers included both a detailed description of each action potential and a comparison highlighting and explaining any pertinent differences. The question lends itself to well-drawn, appropriately labelled diagrams and further explanations expressed in a tabular form. Better answers included a comparison table with points of comparison such as the relevant RMP, threshold value, overshoot value, duration, conduction velocity, automaticity, ion movements for each phase (including the direction of movement) providing a useful structure to the table. Incorrect numbering of the phases (0 – 4) and incorrect values for essential information (such as resting membrane potential) detracted from some responses.

Discussion

Appropriately labelled diagrams? Tabular form? Who knew that these might be the keys to answering a CICM exam question.

Phases of the myocyte and pacemaker action potential

A Comparison of the Ionic Events
During the Cardiac Action Potentials
in the Myocyte and the Pacemaker Cell
Cell	Ventricular Myocyte	Pacemaker cell
Resting potential	-90 mV	-60 mV
Threshold Potential	-70 mV	- 40 mV
Phase 4	Resting potential Stable plateau Maintained by the I_k1 inward rectifying potassium current	Slow depolarisation Maintained by the If "funny" sodium curent
Phase 0	Rapid depolarisation Mediated by fast voltage-gated sodium channels	Slow depolarisation Mediated by L-type calcium channels
Phase 1	Rapid repolarisation Mediated by transient outward potassium currents (Ito) and the	No Phase 1 in the pacemaker action potential
Phase 2	Prolonged plateau at ~ 0mV Lasts ~ 100-200 msec Mediated by L-type calcium channels	No real Phase 2 in the pacemaker action potential
Phase 3	Rapid repolarisation Mediated by the Ikr, Iks and Ik1 potassium currents	More gradual repolarisation Mediated by the Ikr, Iks and Ik1 potassium currents

References

Antoni, H. "Electrophysiology of the heart at the single cell level and cardiac rhythmogenesis." Comprehensive Human Physiology. Springer, Berlin, Heidelberg, 1996. 1825-1842.

Pinnell, Jeremy, Simon Turner, and Simon Howell. "Cardiac muscle physiology." Continuing Education in Anaesthesia, Critical Care and Pain 7.3 (2007): 85-88.

Bers, Donald M. "Cardiac excitation–contraction coupling." Nature 415.6868 (2002): 198-205.

Draper, Morrell Henry, and Silvio Weidmann. "Cardiac resting and action potentials recorded with an intracellular electrode." The Journal of physiology 115.1 (1951): 74-94.

Bers, Donald M. "Cardiac excitation–contraction coupling." Nature 415.6868 (2002): 198-205.

DiFrancesco, Dario. "Pacemaker mechanisms in cardiac tissue." Annual review of physiology 55.1 (1993): 455-472.

Noma, Akinori. "Ionic mechanisms of the cardiac pacemaker potential." Japanese heart journal 37.5 (1996): 673-682.

Grunnet, Morten. "Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti‐arrhythmic principle?." Acta physiologica 198 (2010): 1-48.

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