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
Normal atrial, ventricular & Purkinje myocardial fibres
SA Node, AV Node, ventricular conduction system
Slow cell has less negative RMP because:
- Ca2+/Na+ leakiness
- Fewer RECTIFYING K CHANNELS
- Fast Na+ channels are permanently inactivated (at -60mV their h-gates shut)
- RMP -90mV
- Restored by INWARD RECTIFYING K+ CHANNELS
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
- 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
- At -40mV L-type Ca2+ channel open
- Transient ↓K+ perm contributes to depol.
- Some K+ channels open briefly
- Outward K movement
- Returns MP to 0mV
- Ca2+ channel still open
- Small constant Ca2+ influx
- K+ leaks out through DELAYED RECTIFIER K CHANNEL
- Electrical balance ∴plateau maintained
Brief, no plateau
- 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
- Inward RECTIFIER K+ channel open → repol.
- Ca2+ close & inactive
Ph 3 ends when RMP = -65mV