Question 19

Describe and compare the action potentials from cardiac ventricular muscle and the sinoatrial node.

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College Answer

A fundamental aspect of cardiac physiology, that overall was well answered. The majority of
candidates used figures to good effect. Candidates are reminded that all figures must be correctly labelled (e.g. X and Y axis, phases of action potential, etc.). Common omissions were those that reflected an adequate depth of knowledge (e.g. some of the current flows).

Discussion

One could surely produce a well-labelled figure to describe action potentials, which might look something like this (just ignore the missing horizontal axis label):

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

However, it may be easier to compare these things in a table:

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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