Outline the structure of fast cardiac sodium channels and describe in detail how they work.
The expectation for this question was an appropriate description of the fast sodium channel structure included outlining its single alpha and 2 beta sub-units, activation (m) and inactivation (h) gates and sodium selectivity. It was expected that candidates would comment on cycling between the 3 states (resting, open and inactive), corresponding conformational changes to the fast sodium channel, triggers for these changes (voltage or time) and ionic events, including reference to and description of absolute and relative refractory periods. Better candidates were able to relate events to a diagram of a fast action potential, simply drawing a diagram without referencing events at the sodium channel scored few marks. No marks were awarded for description of drug effects on the fast sodium channel. The question specifically relates to fast Na channels, so descriptions of other channels (e.g. ligand gated sodium
channels) did not score marks. Details of the subsequent contraction of cardiac muscle following an action potential was also not part of the question and did not score marks
Structure of the voltage-gated sodium channel
Three states of activity:
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