<P> 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 . </P> <Ul> <Li> Some potassium channels: <Ul> <Li> 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 members 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 . </Li> <Li> Calcium - activated potassium channels: This family of channels is activated by intracellular Ca and contains 8 members . </Li> <Li> Two - pore - domain potassium channels: This family of 15 members form what are known as leak channels, and they display Goldman - Hodgkin - Katz (open) rectification . </Li> </Ul> </Li> <Li> Light - gated channels like channelrhodopsin are directly opened by photons . </Li> <Li> Mechanosensitive ion channels open under the influence of stretch, pressure, shear, and displacement . </Li> <Li> 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 . <Ul> <Li> 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 . </Li> <Li> Hyperpolarization - activated cyclic nucleotide - gated channels </Li> </Ul> </Li> <Li> 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 . </Li> </Ul> <Li> Some potassium channels: <Ul> <Li> 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 members 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 . </Li> <Li> Calcium - activated potassium channels: This family of channels is activated by intracellular Ca and contains 8 members . </Li> <Li> Two - pore - domain potassium channels: This family of 15 members form what are known as leak channels, and they display Goldman - Hodgkin - Katz (open) rectification . </Li> </Ul> </Li> <Ul> <Li> 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 members 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 . </Li> <Li> Calcium - activated potassium channels: This family of channels is activated by intracellular Ca and contains 8 members . </Li> <Li> Two - pore - domain potassium channels: This family of 15 members form what are known as leak channels, and they display Goldman - Hodgkin - Katz (open) rectification . </Li> </Ul>

What kinds of gated channels are typically found on axons