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| Gap junctions, also referred to as cell-to-cell channels, are present in most animal tissues and form aqueous channels between adjoining cells. Inorganic molecules as well as numerous [[Metabolites|metabolites]], such as [[Amino acids|amino acids and]] sugars, can pass through these channels. However, [[Macromolecules|macromolecules]], such as [[Proteins|proteins]] and [[Nucleic acids|nucleic acids]], cannot move across. Cell-to-cell channels allow communication between cells, including excitable cells. Gap junctions allow communication between cells in both directions. They also allow the spread of an extracellular signal; this is achieved by intracellular intermediaries, such as [[CAMP|cyclic AMP and]] [[Calcium|calcium ions]], moving between channels. <ref>Bruce Alberts et al, 2007. Molecular Biology of the Cell. 5th edition. USA: Garland Publishing Inc.</ref> One example of communication occurs in heart muscle cells, whereby a rapid flow of ions through these cells enables a speedy and coordinated response to a [[Stimulus|stimulus]], ultimately leading to contraction of the muscle. <ref>Berg J., Tymoczko J and Stryer L., 2007. Biochemistry. 7th edition. New York: WH Freeman</ref><br>
| | See [[Gap Junction]] |
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| Gap junctions are also present in an electrical synapse, particularly in hormone-secretory neurons, as they allow action potientials to be triggered in multiple cells at once promoting a burst of hormone to be secreted into the circulatory system. <ref>National Center for Biotechnology Information</ref>
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| Specialised forms of [[Transmembrane proteins|transmembrane proteins]], called [[Connexins|connexins]], form gap junctions. Twelve connexin molecules assemble to form a gap junction. A single connexin molecule has four membrane spanning proteins. [[Connexons|Connexons]], also called hemichannels, are formed when six connexin molecules are arranged in a hexagonal structure; a cell-to-cell channel is formed when two connexons are aligned and joined together, 1 from each adjacent cell.<br>
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| Gap junctions differ from other membrane channels in a number of different ways. This includes cell-to-cell channels spanning across two membranes, instead of one, and channels forming between the cytoplasm of two cells, rather than to the extracellular matrix. <br>
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| An important point to note is that cell-to-cell channels are closed by high calcium and hydrogen ion concentrations; this mechanism protects healthy cells from damaged ones. <ref>Berg J., Tymoczko J and Stryer L., 2007. Biochemistry. 7th edition. New York: WH Freeman</ref> This mechanism is vital, for example if one cell is damaged and the plasma membrane becomes 'leaky' there will be an influx of Ca<sup>2+ </sup>ions into the cell, due to it's concentration gradient. This triggers the gap junction to close, effectively sealing it from the other cell, preventing further damage. (Alberts et al., 2002, pg 1074-1077) <ref>Alberts, B. et al., 2002. Molecular Biology of the Cell, Fourth Edition, New York: Garland Science.</ref><br>
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| = Refererences =
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| <references />
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