Cardiac Conduction and Contraction Quiz

See Sino-atrial Node Action Potential. HCN cation channels (A) carry the funny current, If, in sinoatrial node cells. Their name means hyperpolarisation-activated cyclic nucleotide-gated: they open when the membrane voltage becomes more negative and can be modulated by cyclic AMP, an intracellular signalling molecule. They conduct both sodium and potassium, but the combined current is inward at pacemaker voltages, helping the voltage rise towards threshold. This gradual rise is diastolic depolarisation, the electrical preparation for the next heartbeat. HCN current is one contributor to pacemaking, alongside calcium currents and changing potassium currents, so it should not be taught as the only route to spontaneous firing.

Fast sodium channels (B) generate the rapid upstroke in working ventricular muscle, whereas the upstroke in central SA nodal cells mainly uses calcium. L-type calcium channels (E) contribute late in diastolic depolarisation and to that nodal upstroke, but do not carry If. D mainly allows outward potassium current during repolarisation; decreasing this outward current can help the next depolarisation, but it is not a hyperpolarisation-activated HCN channel. C identifies a different type of ion channel and does not account for the mixed sodium-potassium funny current.