Describe the gating and ion selectivity properties of HCN channels. How do HCN channels function?
a) HCN channels are voltage-gated and selectively allow the passage of sodium ions.
b) HCN channels are ligand-gated and selectively allow the passage of potassium ions.
c) HCN channels are voltage-gated and selectively allow the passage of potassium ions.
d) HCN channels are ligand-gated and selectively allow the passage of sodium ions.