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

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 / 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