G2i / 25B03 / 20B01 / 17B21 / 16B11: Compare the action potential of SAN and myocardial cell

25B03: Exam Report

  1. Describe the action potential of the following:
    1. the sinoatrial node (35% of marks)
      the cardiac ventricular muscle cells (35% of marks)
    2. Include in your answer the phases and ionic events involved with each action potential.
  2. Explain the implications of the differences between these action potentials (30% of marks).

83% of candidates passed this question.

  1. Descriptions of the action potentials of the SA node and cardiac myocyte are well described in most physiology textbooks and were relatively well answered. Shape, duration and ionic events were covered in good detail in most answers.
  2. An overview of the differences between the SA node and cardiac myocyte action potentials was required and then the implications of these differences. The differences expected to form a good answer included the absence of a true resting membrane potential in the SA node and differences in the duration of the action potentials, conduction velocities, action potential phases and refractory period.

20B01: Exam Report

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

72% of candidates passed this question.

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.

17B21: Exam Report

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

95% of candidates passed this question.

This topic was well understood and answered by most candidates. Some candidates had a good knowledge base but missed out on potential marks by failing to compare and contrast. A diagram outlining the various phases was a useful way to approach the question.

16B11: Exam Report

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

75% of candidates passed this question.

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.

G2i / 25B03 / 20B01 / 17B21 / 16B11: Describe and compare the action potential from a cardiac ventricular muscle and the sinoatrial node

Fast

Slow

Site

Normal atrial, ventricular & Purkinje myocardial fibres

SA Node, AV Node, ventricular conduction system

Duration

250 msec

150 msec

RMP

-90mV

-60mV

Threshold

-65mV

-40mV

Slow cell has less negative RMP because:

  1. Ca2+/Na+ leakiness
  2. Fewer RECTIFYING K CHANNELS
  3. Fast Na+ channels are permanently inactivated (at -60mV their h-gates shut)

Phases

Myocardial Cell

SA Node

Phases

4

SA Node

TRUE RMP

  • RMP -90mV
  • Restored by INWARD RECTIFYING K+ CHANNELS

Myocardial Cell

SPONT SLOW DECAY

  • Multiple ionic currents involved
  • ↓K+ perm
  • Slow inward Na+ movement KA ‘funny currents’
  • ↑Ca2+ perm through T-type Ca2+ channels
  • Approaches threshold

Phases

0

SA Node

FAST

  • Nearby AP causes RMP to rise
  • Fast Na+ channel start opening
  • THRESHOLD -65mV → all open
  • Mass Na+ influx
  • RAPID DEPOLARISATION
  • At 55mV h-gates close
  • At -40mV slow L-type Ca2+ channels open
  • Small but steady Ca2+ influx

Myocardial Cell

SLOW

  • At -40mV L-type Ca2+ channel open
  • Depolarisation
  • Transient ↓K+ perm contributes to depol.

Phases

1

SA Node

  • Some K+ channels open briefly
  • Outward K movement
  • Returns MP to 0mV

Myocardial Cell

Absent

Phases

2

SA Node

  • Ca2+ channel still open
  • Small constant Ca2+ influx
  • K+ leaks out through DELAYED RECTIFIER K CHANNEL
  • Electrical balance ∴plateau maintained

Myocardial Cell

Brief, no plateau

∴shorter AP

Phases

3

SA Node

  • Ca2+ ch is inactive
  • Persistant K+ outflow via RECTIFYING K+ CHANNELS
  • Brings MP back to 90mV
  • Ion gradients restored by Na/Ca exchange, Ca/ATPase, Na/K/ATPase

Myocardial Cell

  • Inward RECTIFIER K+ channel open → repol.
  • Ca2+ close & inactive

Ph 3 ends when RMP = -65mV