The funny current or pacemaker current is predominantly a feature of the sinoatrial node. It is also seen in atrioventricular node and the Purkinje fibres. It is a mixed sodium-potassium current which is inward and gets activated on hyperpolarization. The funny current is responsible for the spontaneous diastolic depolarization which ultimately leads to the automaticity of the sinus node. Since it controls the rate of the sinus node activity, it determines the heart rate. In addition to the diastolic voltage, the funny current activation is also dependent on cyclic AMP and hence can be modulated by the autonomic nervous system. HCN channel (hyperpolarization activated cyclic nucleotide) mediates the funny current (If). Four types of HCN channels (HCN 1-4) are known at present. HCN4 mutations have been implicated in sinus node dysfunction.
Though the funny current has been described over a quarter of a century back, it has come into attention recently due to the availability of selective If current blockers like ivabradine, a pure sinus node inhibitor without any other hemodynamic effect. Zatebradine and cilobradine are two analogues. A similar Ih current has been described in different types of neurons. If current has also been targeted in the development of a potential biological pacemaker.
If pacemaker current was discovered by Professor D. DiFrancesco in 1979.