An ion channel specifically for the passage of potassium ions (K⁺) through cell membranes. There are several types. Ungated or ‘leak’ potassium channels are largely responsible for the resting potential of cells. These are permanently open and allow K⁺ ions to move in and out of the cell. Outward migration down the K⁺ ion concentration gradient creates a net negative charge inside the cell. Voltage-gated K⁺ channels play a key role in transmitting electrical signals in excitable cells, particularly neurons and muscle cells. During a nerve impulse, the sudden reversal of polarity (i.e. the action potential) causes the voltage-gated K⁺ channels to open, allowing excess positive charge, in the form of K⁺ ions, to flood out of the cell thereby restoring the negative resting potential. Ligand-gated, or ‘chemical’ K⁺ channels open in response to a chemical stimulus, and cause an excess negative charge inside the cell (i.e. hyperpolarization) by allowing more K⁺ ions to leave than normal. They operate e.g. to reduce excitability of cells in parts of the brain during non-REM sleep. See ligand-gated ion channel; sodium ion channel; voltage-gated ion channel. See also sodium-potassium pump.