Question 11

Compare the action potentials of a sino-atrial node cell and a myocardial cell.

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College Answer

A good answer included a well labelled sketch with a description of ion channels and relative
directional flow. When using sketches in an answer they should be correctly labelled, and
when used as a comparison with another sketch, the differences should be clear e.g. shape,
duration and voltage difference.

Discussion

A "well labelled sketch with a description of ion channels and relative directional flow" would probably be intolerably messy because there would be a considerable amount of text present on the diagram, which in many ways defeats the objectives of having a diagram. However, to humor the examiners, such a sketch is presented below:

Phases of the myocyte and pacemaker action potential with ionic events labelled

A more efficient use of art would be to label the phases of the cardiac action potential in a diagram and then explain their ionic correlations in a tabulated legend, as follows:

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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