Editing Cardiac action potential (section)

===HCN channels===
{{Main |HCN channel}}
[[HCN channel|Hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels)]] are located mainly in pacemaker cells, these channels become active at very negative membrane potentials and allow for the passage of both Na<sup>+</sup> and K<sup>+</sup> into the cell (which is a movement known as a funny current, I<sub>f</sub>). These poorly selective, cation (positively charged ions) channels conduct more current as the membrane potential becomes more negative (hyperpolarised). The activity of these channels in the SAN cells causes the membrane potential to depolarise slowly and so they are thought to be responsible for the pacemaker potential. Sympathetic nerves directly affect these channels, resulting in an increased heart rate (see below).{{sfn|Sherwood|2012|pp=310-1}}<ref name="DiFrancesco funny">{{Cite journal |last=DiFrancesco |first=Dario |date=2010-02-19 |title=The role of the funny current in pacemaker activity |journal=Circulation Research |volume=106 |issue=3 |pages=434–446 |doi=10.1161/CIRCRESAHA.109.208041 |issn=1524-4571 |pmid=20167941 |doi-access=free}}</ref>