The "funny" channel plays an important role in regulating the heart rate. The "funny" channel is an ion channel that allows sodium ions to enter pacemaker cells in the heart. The "funny" channel is a Hyperpolarization-activated Cyclic Nucleotide-gated channel. The major protein component of the "funny" channel in humans is the protein of the HCN4 gene.
The first ion channels studied by electrophysiologists open and allow ion current to cross cell membranes when the membrane potential becomes less negative inside (depolarization). In contrast, the "funny" channels opens when the membrane potential becomes more negative inside (membrane potential changes in the hyperpolarizing direction).
The sympathetic division of the autonomic nervous system can speed the heart rate by increasing sodium movement through "funny" channels. This involves increased levels of cyclic AMP in pacemaker cells. cAMP binds to "funny" channels and activates them.
"Funny" channels can be blocked by ivabradine, a drug used to treat some heart disease patients.