text: Gating also includes activation and inactivation by second messengers from the inside of the cell membrane – rather than from outside the cell, as in the case for ligands.
  *Some potassium channels:
  **Inward-rectifier potassium channels: These channels allow potassium ions to flow into the cell in an "inwardly rectifying" manner: potassium flows more efficiently into than out of the cell. This family is composed of 15 official and 1 unofficial member and is further subdivided into 7 subfamilies based on homology. These channels are affected by intracellular ATP, PIP, and G-protein βγ subunits. They are involved in important physiological processes such as pacemaker activity in the heart, insulin release, and potassium uptake in glial cells. They contain only two transmembrane segments, corresponding to the core pore-forming segments of the K and K channels. Their α subunits form tetramers.
  **Calcium-activated potassium channels: This family of channels is activated by intracellular Ca and contains 8 members.
  **Tandem pore domain potassium channel: This family of 15 members form what are known as leak channels, and they display Goldman-Hodgkin-Katz (open) rectification. Contrary to their common name of Two-pore-domain potassium channels, these channels have only one pore but two pore domains per subunit.
  *Two-pore channels include ligand-gated and voltage-gated cation channels, so-named because they contain two pore-forming subunits. As their name suggests, they have two pores.
  *Light-gated channels like channelrhodopsin are directly opened by photons.
  *Mechanosensitive ion channels open under the influence of stretch, pressure, shear, and displacement.
  *Cyclic nucleotide-gated channels: This superfamily of channels contains two families: the cyclic nucleotide-gated (CNG) channels and the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. This grouping is functional rather than evolutionary.
  **Cyclic nucleotide-gated channels: This family of channels is characterized by activation by either intracellular cAMP or cGMP. These channels are primarily permeable to monovalent cations such as K and Na. They are also permeable to Ca, though it acts to close them. There are 6 members of this family, which is divided into 2 subfamilies.
  **Hyperpolarization-activated cyclic nucleotide-gated channels
  *Temperature-gated channels: Members of the transient receptor potential ion channel superfamily, such as TRPV1 or TRPM8, are opened either by hot or cold temperatures.