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Gap junctions occur in virtually all tissues of the body, with the exception of adult fully developed skeletal muscle and mobile cell types such as sperm or erythrocytes. In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. When two identical connexons come together to form a Gap junction channel, it is called a homotypic GJ channel. Several gap junction channels (hundreds) assemble within a macromolecular complex called a gap junction plaque. Allows for direct electrical communication between cells, although different connexin subunits can impart different single channel conductances, from about 30 pS to 500 pS. 3) and calcium (Ca2+),[7] although different connexin subunits can impart different selectivity for particular small molecules. In general, allows transmembrane movement of molecules smaller than 485 Daltons[20] (1,100 Daltons through invertebrate gap junctions[21]), although different connexin subunits may impart different pore sizes and different charge selectivity. Ensures that molecules and current passing through the gap junction do not leak into the intercellular space. Gap Junctions have been observed in various animal organs and tissues where cells contact each other.
Gap junctions may be seen to function at the simplest level as a direct cell to cell pathway for electrical currents, small molecules and ions. In the 1980s, more subtle but no less important roles of gap junction communication have been investigated. The "bystander effect" with its connotations of the innocent bystander being killed is also mediated by gap junctions.
While there has been a tendency to focus on the bystander effect in disease due to the possibility of therapeutic avenues there is evidence that there is a more central role in normal development of tissues. Gap junctions electrically and chemically couple cells throughout the body of most animals. Gap junctions are particularly important in cardiac muscle: the signal to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in unison. There has been some observation of weak neuron to glial cell coupling in the locus coeruleus, and in the cerebellum between Purkinje neurons and Bergmann glial cells. Neurons within the retina show extensive coupling, both within populations of one cell type, and between different cell types. Gap junctions were so named because of the "gap" shown to be present at these special junctions between two cells.[87] With the increased resolution of the transmission electron microscope (TEM) gap junction structures were first able to be seen and described in around 1953. The term "gap junction" appeared to be coined about 16 years later circa 1969.[88][89][90] A similar narrow regular gap was not demonstrated in other intercellular junctions photographed using the TEM at the time.
Well before the demonstration of the "gap" in gap junctions they were seen at the junction of neighboring nerve cells. Implicit or explicit in most of the early studies is that the area of the gap junction was different in structure to the surrounding membranes in a way that made it look different.
Because of the widespread occurrence of gap junctions in cell types other than nerve cells the term gap junction became more generally used than terms such as electrical synapse or nexus.
The purification[101][102] of the intercellular gap junction plaques enriched in the channel forming protein (connexin) showed a protein forming hexagonal arrays in x-ray diffraction. Ultrastructure and biochemistry of isolated gap junctions already referenced had indicated the connexins preferentially group in gap junction plaques or domains and connexins were the best characterized constituent. Early descriptions of "gap junctions" and "connexons" did not refer to them as such and many other terms were used.

Studies allowing views inside the plane of the membrane of gap junctions during formation indicated that a "formation plaque" formed between two cells prior to the connexins moving in. The formation plaque and non-connexin part of the classical gap junction plaque have been difficult for early researchers to analyse. This relatively large value of S32 indicates there is little isolation between the two output ports.
Gap Junctions, however, are not found in simpler organisms such as sponges and slime molds. In turn, hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. When one homomeric connexon and one heteromeric connexon come together, it is called a heterotypic gap junction channel.
Large biomolecules, for example, nucleic acid and protein, are precluded from cytoplasmic transfer between cells through gap junction connexin channels.
From the 1950s to 1970s they were detected in crayfish nerves,[22] rat pancreas, liver, adrenal cortex, epididymis, duodenum, muscle,[23] Daphnia hepatic caecum,[24] Hydra muscle,[25] monkey retina,[26] rabbit cornea,[27] fish blastoderm,[28] frog embryos,[29] rabbit ovary,[30] re-aggregating cells,[31][32] cockroach hemocyte capsules,[33] rabbit skin,[34] chick embryos,[35] human islet of Langerhans,[36] goldfish and hamster pressure sensing acoustico-vestibular receptors,[37] lamprey and tunicate heart,[38][39] rat seminiferous tubules,[40] myometrium,[41] eye lens[42] and cephalopod digestive epithelium.[43] Since the 1970s gap junctions have continued to be found in nearly all animal cells that touch each other. The control of this communication allows complex downstream effects on multicellular organisms as described below. It was discovered that gap junction communication could be disrupted by adding anti-connexin antibodies into embryonic cells.[50][51] Embryos with areas of blocked gap junctions failed to develop normally. When cells are compromised due to disease or injury and start to die messages are transmitted to neighboring cells connected to the dying cell by gap junctions.
Gap junctions are expressed in virtually all tissues of the body, with the exception of adult fully developed skeletal muscle and mobile cell types such as sperm or erythrocytes.
The electrical synapse was discovered using electrical measurements before the gap junction structure was described. Those mRNAs appear to be down-regulated or destroyed by micro interfering RNAs ( miRNAs ) that are cell-type and cell-lineage specific. The gap junction had been shown to create a micro-environment between the two cells in the extra-cellular space or "gap". The observation was largely without explanation until vesicles were shown by Peracchia using TEM thin sections to be systematically associated with gap junction plaques.[98] Peracchia's study was probably also the first study to describe paired connexon structures, which he called somewhat simply a "globule". Another dimension in the relationship between nerve cells and gap junctions was revealed by studying chemical synapse formation and gap junction presence.
Now systematic study and identification of the predominant gap junction protein[103] became possible. It has been noted that the organisation of proteins into arrays with a gap junction plaque may be significant.[29][110] It is likely this early work was already reflecting the presence of more than just connexins in gap junctions.
It is likely that "synaptic disks"[114] were an accurate reference to gap junction plaques. They were particle free areas when observed by TEM FF indicating very small or no transmembrane proteins were likely present.
It appears in TEM FF and thin section to be a lipid membrane domain that can somehow form a comparatively rigid barrier to other lipids and proteins.
The input time average power Pin should be divided between ports 2 and 3 in a desired ratio.
While an ephapse has some similarities to a gap junction, by modern definition the two are different.
By the 1990s new technology such as confocal microscopy allowed more rapid survey of large areas of tissue.
Tissues in this section have well known functions observed to be coordinated by gap junctions with inter-cellular signaling happening in time frames of micro-seconds or less.
It is thought that annular gap junctions result from engulfment by one of the two cells of the membrane plaque to form a vesicle within the cell.

This portion of extra-cellular space was somewhat isolated from the surrounding space and also bridged by what we now call connexon pairs which form even more tightly sealed bridges that cross the gap junction gap between two cells.
Studies showing vesicles associated with gap junctions and proposing the vesicle contents may move across the junction plaques between two cells were rare, as most studies focused on the connexons rather than vesicles. By tracing nerve development in leeches with gap junction expression suppressed it was shown that the bidirectional gap junction (electrical nerve synapse) needs to form between two cells before they can grow to form a unidirectional "chemical nerve synapse".[100] The chemical nerve synapse is the synapse most often truncated to the more ambiguous term "nerve synapse".
Refined ultrastructural studies by TEM[104][105] showed protein occurred in a complementary fashion in both cells participating in a gap junction plaque.
Combining the emerging fields of freeze-fracture to see inside membranes and immunocytochemistry to label cell components (Freeze-fracture replica immunolabelling or FRIL and thin section immunolabelling) showed gap junction plaques in vivo contained the connexin protein.[111][112] Later studies using immunofluorescence microscopy of larger areas of tissue clarified diversity in earlier results. While the detailed structure and function of the connexon was described in a limited way at the time the gross "disk" structure was relatively large and easily seen by various TEM techniques. Little is known about what structures make up the formation plaque or how the formation plaque's structure changes when connexins and other components move in or out. There has been indirect evidence for certain lipids being preferentially involved with the formation plaque but this cannot be considered definitive.[121][122] It is difficult to envisage breaking up the membrane to analyse membrane plaques without affecting their composition. Since the 1970s even tissues that were traditionally considered to possibly have isolated cells such as bone showed that the cells were still connected with gap junctions, however tenuously.[44] Gap junctions appear to be in all animal organs and tissues and it will be interesting to find exceptions to this other than cells not normally in contact with neighboring cells. Many of those affect the skin because this tissue is heavily dependent upon gap junction communication for the regulation of differentiation and proliferation.
A later study using a combination of microscopy techniques confirmed the early evidence of a probable function for gap junctions in intercellular vesicle transfer.
The gap junction plaque is a relatively large area of membrane observed in TEM thin section and freeze fracture (FF) seen filled with trans-membrane proteins in both tissues and more gently treated gap junction preparations. Gap junction plaques were confirmed to have variable composition being home to connexon and non-connexin proteins as well making the modern usage of the terms "gap junction" and "gap junction plaque" non-interchangeable.[113] In other words, the commonly used term "gap junction" always refers to a structure that contains connexins while a gap junction plaque may also contain other structural features that will define it. Disks allowed researchers using TEM to easily locate the connexons contained within the disk like patches in vivo and in vitro.
One of the earlier studies of the formation of small gap junctions describes rows of particles and particle free halos.[118] With larger gap junctions they were described as formation plaques with connexins moving into them. By study of connexins still in membranes lipids associated with the connexins have been studied.[123] It was found that specific connexins tended to associate preferentially with specific phospholipids.
The membrane from each cell is the dark line with the whiter narrow gap between the two darkly stained membranes. With the apparent ability for one protein alone to enable intercellular communication seen in gap junctions[106] the term gap junction tended to become synonymous with a group of assembled connexins though this was not shown in vivo.
As formation plaques precede connexins these results still give no certainty as to what is unique about the composition of plaques themselves. The network is also lossless and because it’s a three-port, we know that not all ports will be simultaneously matched.
It may be argued that if present in skeletal muscle, gap junctions might propagate contractions in an arbitrary way among cells making up the muscle.
Other findings show connexins associate with protein scaffolds used in another junction, the zonula occludens ZO1.[124] While this helps us understand how connexins may be moved into a gap junction formation plaque the composition of the plaque itself is still somewhat sketchy.
That is, port 2 (port 3) will not be matched when ports 1 and 3 (ports 1 and 2) are terminated with matched loads.

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