current

The normal function of neurons and muscle cells depends on their ability to produce action potentials.

To produce rapid changes in a cell's membrane potential, ions such as Na+ must rapidly cross the cell membrane. Such movements of ions is called "current" and in electrophysiology, currents are often represented by either "I" or "g".

Examples. In heart pacemaker cells, action potentials are spontaneously initiated because of the "funny" current (I_f). This is an inward sodium ion current that moves through "funny channels" that are composed of proteins such as the HCN4 gene product.

All of the current flow in the beating heart can be detected as an electrocardiogram.

Panel A, below, shows a neuronal action potential. The sodium (gNa⁺) and potassium (gK⁺) currents that produce the action potential are shown in panel B. Sodium current is high during the rising phase. Potassium current is high during the falling (repolarization) phase.