Gap junctions, electric synapses

Signal transmission between neurons is mediated in multiple distinct ways. In the adult brain, transmission via neurotransmitters (chemical synapsis) constitutes the most common form and has been comprehensively considered in Chapter XXX. An alternative way for interneuronal signal transmission, which lacks the machinery required for neurotransmitter storage and release including the appropriate postsynaptic receptor inventory, happens through a direct mediation of signal transfer between cells. This is made possible by specific channel-forming proteins (the connexins in chordates and the innexins in invertebrates), which bridge the intercellular space and allow direct communication between the intracytoplasmic compartments of adjoining cells. The term gap junctions was coined for these unique cell-to-cell contacts by Revel and Karnovsky in 1967 because of a 2.5-nm space in the contact zone between the outer leaflets of the apposed plasma membranes that is evident in electron micrographs. Gap junctions evolved early in evolution and can be found in primitive multicellular organisms such as sponges. They serve for intercellular exchange of ions and small metabolites up to a molecular mass of about 1 kDa and are considered to be responsible for coordination and synchronization of individual cells in a social complex, thereby forming tissues and organs. It is therefore not surprising that almost all cells in an organism are coupled through gap junctions, except freely circulating sperm and erythrocytes and mature skeletal muscle. In brain tissue, gap junctions are present between neurons; glial cells, including astrocytes, oligodendrocytes, and microglia; ependymal cells; and leptomeningeal cells. They provide the direct conduits connecting the cerebral endothelium and can even be found in neuronal precursor cells. In a narrow sense, gap junctions are the structure of electrical synapses between neurons, although gap junctions with the same general architecture are found between non-neuronal cells of the brain and other tissues.