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Cardiac Conduction and Contraction Quiz
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Which ion channel is primarily responsible for the slow, spontaneous depolarisation that initiates each action potential in sinoatrial node cells?
- A HCN (funny current) channels You picked ✓
- B Fast voltage-gated sodium channels You picked ✕
- C Ligand-gated chloride channels You picked ✕
- D Voltage-gated potassium channels You picked ✕
- E L-type calcium channels You picked ✕
Correct!
Incorrect
The answer is A: HCN (funny current) channels
Explanation
Why A is correct
See Sino-atrial Node Action Potential. Option A is correct. Cells of the sinoatrial node, the heart's natural pacemaker in the right atrium, have no steady resting membrane potential. Instead, once an action potential finishes, the voltage drifts upwards until it reaches threshold and the next beat fires. This drift is the pacemaker potential (phase 4). It is produced by HCN channels, short for hyperpolarisation-activated cyclic nucleotide-gated channels. They are unusual because they open when the membrane becomes more negative, not less, and they let sodium flow in; the resulting inward current is called the funny current (If) because of this odd behaviour. When the voltage has risen to around -40 mV, voltage-gated calcium channels open and produce the actual upstroke, and potassium channels then repolarise the cell. The steepness of the pacemaker potential sets the heart rate, which is why the autonomic nervous system controls the rate by altering it. Memory aid: The funny current is the slow tide that carries the cell to its firing point.
- Pacemaker potential: The slow phase 4 drift to threshold in sinoatrial node cells.
- HCN channels: They carry the inward funny current (If).
- Upstroke: Calcium channels open at about -40 mV to fire the action potential.
Why the other options are incorrect
- B. Fast voltage-gated sodium channels: Fast sodium channels produce the sharp upstroke of the action potential in working atrial and ventricular muscle and in Purkinje fibres. They do not generate the slow pacemaker depolarisation of sinoatrial node cells. Your answer
- C. Ligand-gated chloride channels: Ligand-gated chloride channels are not part of cardiac pacemaker activity. Chloride entry would tend to hold a cell more negative, so they would not drive a rise towards threshold. Your answer
- D. Voltage-gated potassium channels: Voltage-gated potassium channels open at the peak of the action potential and let potassium leave the cell, which repolarises it. They bring the voltage down, whereas the pacemaker potential is a slow rise. Your answer
- E. L-type calcium channels: L-type calcium channels open only once the pacemaker potential has reached about -40 mV, and then they produce the upstroke. They respond to the slow depolarisation rather than creating it. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which ionic movement is primarily responsible for sustaining the plateau phase of the ventricular action potential?
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2
Which mechanism directly links calcium entry through the cardiac cell membrane to the large rise in intracellular calcium that initiates contraction?
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