2020A Question 09
Compare and contrast the action potentials from the sino-atrial node and a ventricular myocyte.
Examiner Report
The domains for this question were:
- Diagram of SAN action potential
- Concept of automaticity
- Phase 0
- Phase 3
- Phase 4
- Factors affecting
This question was answered well with some excellent responses. Although a diagram wasn't specifically asked for, the vast majority of candidates provided one. To get a 3, candidates needed to correctly draw the SAN action potential and describe the ion currents contributing to each phase of the potential as well as describe an understanding of the concept of automaticity. Most candidates did this successfully. Further marks were given for explanation of the factors that affect the potential with a focus on the effects of sympathetic and parasympathetic input. The better answers included a detailed description of the interplay of sodium, calcium and potassium currents and how these contribute to the pacemaker potential.
Model Answer
Structure:
- Introduction
- Graph and conductance
- Pacemaker action potential
Introduction
| Term | Detail |
|---|---|
| Action potential | - Allows rapid, co-ordinated signaling - Rapid depolarisation followed by repolarization - Ion channels cycle through resting → Activated → Inactivated states |
| Determinants of ion flux | - Channel permeability: Voltage dependent - Electrochemical gradients - Na+ and Ca2+ into cell, K+ out of cell - Mainly due to Na+K+ATPase |
Graphs and Conductance
Pacemaker Action Potential
| Factor | Detail |
|---|---|
| Features | - Automaticity - i.e. Intrinsic, rhythmic, frequent depolaristion and contraction - HCN channel produces ‘funny current’ – i.e. mixed Na+ and K+ influx - [cAMP] ∝ If ∝ slope of phase 4 ∝ heart rate - Note depth of repolarisation ∝ IK ∝ 1/(heart rate) - Post-repolarisation refractoriness - RRP extends into phase 4 |
| Refractory periods | - ERP: Insufficient HCN and Ca2+ channels resting; action potential (AP) impossible - RRP: Enough HCN and Ca2+ channels resting but fewer than normal; AP possible with higher than normal stimulus |
| Factors increasing rate | - ↑ SNS → ↑ If = ↑ iCa2+ > iK+ → ↑ Steepness all slopes, especially phase 4 - ↑ Temp → ↑ Cell metabolic rate (enzymes, pumps) - ↓ K+ → Less hyperpolarisation - Neonate: ↑ Resting HR (fixed stroke volume) |
| Factors decreasing rate | - ↑ PSNS: → ↓ If, ↑ IK → ↓ Steepness phase 4, increased hyperpolarisation - ↓ Temp → ↓ Cell metabolic rate (enzymes, pumps) - Hypoxia → Failure of Na+K+ATPase, unable to repolarize cell, HCN not reactivated - Elderly: Fibrous/fatty change in conduction system, ↓ β1 adrenoceptors |