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.


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 Ik1  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 


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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.