Pacemaker current on hyperpolarization, and very slow kinetics. The funny current is highly expressed in spontaneously active cardiac regions, such as the sinoatrial node (SAN, the natural pacemaker region), the atrioventricular node (AVN) and the Purkinje fibres of conduction tissue. The funny current is a mixed sodium–potassium current that activates upon hyperpolarization at voltages in the diastolic range (normally from −60/−70 mV to −40 mV). When, at the end of a sinoatrial action potential, the membrane repolarizes below the I threshold (about −40/−50 mV), the funny current is activated and supplies inward current, which is responsible for starting the diastolic depolarization phase
Sinoatrial node hyperpolarisation-activated cyclic nucleotide–gated (HCN) channels, are activated. Activation of these channels at very negative membrane potential is an unusual property for ion channels, therefore the flow of sodium (Na) and some potassium (K) through the activated HCN channel is referred to as a "funny current" (I). This funny current causes the membrane potential of the cell to gradually increase, as the positive charge (Na and K) is flowing into the cell. Another mechanism involved in pacemaker potential is known as the calcium clock. Here, calcium is released spontaneously from the sarcoplasmic reticulum (a calcium store) into the cell, this is