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Gap junction
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<p>Gap junctions are <a href="/facts/Membrane_channel/x34araRI">membrane channels</a> between adjacent cells that allow the direct exchange of cytoplasmic substances, such small molecules, substrates, and metabolites. </p><p>Gap junctions were first described as <i>close appositions</i> alongside tight junctions, however, electron microscopy studies in 1967 led to gap junctions being named as such to be distinguished from tight junctions. They bridge a 2-4 nm gap between cell membranes. </p><p>Gap junctions use protein complexes known as <a href="/facts/Connexon/Nn1FYQJq">connexons</a>, composed of <a href="/facts/Connexin/RK87nd90">connexin</a> proteins to connect one cell to another. <a href="/facts/Gap_junction_protein/oIa3xdWw">Gap junction proteins</a> include the more than 26 types of connexin, as well as at least 12 non-connexin components that make up the gap junction complex or <i>nexus,</i> including the <a href="/facts/Tight_junction_protein_ZO-1/ocSWYsQG">tight junction protein ZO-1</a>—a protein that holds membrane content together and adds structural clarity to a cell, <a href="/facts/Sodium_channel/pXb4n2Vz">sodium channels</a>,[6] and <a href="/facts/Aquaporin/d9pI3N4C">aquaporin</a>. </p><p>More gap junction proteins have become known due to the development of <a href="/facts/Next-generation_sequencing/FjLl8GGa">next-generation sequencing</a>. Connexins were found to be structurally homologous between vertebrates and invertebrates but different in sequence. As a result, the term <a href="/facts/Innexin/6kdyAJT0">innexin</a> is used to differentiate invertebrate connexins. There are more than 20 known innexins, along with unnexins in parasites and <a href="/facts/Vinnexin/oNlH1GEc">vinnexins</a> in viruses. </p><p>An <a href="/facts/Electrical_synapse/3IuCLs3v">electrical synapse</a> is a gap junction that can transmit <a href="/facts/Action_potential/Sz8yCRqx">action potentials</a> between <a href="/facts/Neuron/G7CyTVdH">neurons</a>. Such synapses create bidirectional continuous-time electrical coupling between neurons. Connexon pairs act as generalized regulated gates for ions and smaller molecules between cells. Hemichannel connexons form channels to the extracellular environment. </p><p>A gap junction or <i>macula communicans</i> is different from an <a href="/facts/Ephaptic_coupling/DSzLzvPn">ephaptic coupling</a> that involves electrical signals external to the cells. </p>