Electrical Synapse

A type of electric synapse that is comprised of intercellular channels that direct connect the cytoplasm of two cells and facilitate the cell-to-cell passage of ions and pocket-size molecules.

From: Encyclopedia of the Eye , 2010

Nervous Arrangement, Arrangement of

Jay B. AngevineJr., in Encyclopedia of the Human Brain, 2002

VIII.A.1. Electric Synapses

Electrical synapses are gap junctions. When present betwixt neurons, they are very different from chemical synapses where the separateness of the cells is not in question. They allow the direct spread of electric current from ane prison cell to another, without filibuster or need for receptor and decoding systems. But the individuality of the coupled cells is partly lost, and hence their utility is diminished for large nervous systems with labeled lines like those of mammals. Electrical synapses are mutual in invertebrate and nonmammalian nervous systems but infrequent in mammals except between neuroglial cells, where they offer the chief manner of advice. Nevertheless they take been found between mammalian neurons and shown to transmit in a few cases. In the embryonic CNS, they are seen in many places, even in the cerebral cortex, but decline in number as chemical synapses develop. In the developed, they are unremarkably found in cell clusters that burn down action potentials synchronously, as in the lateral vestibular nucleus, which furnishings a rapid increase in ipsilateral extensor tone for postural maintenance, or clusters that spread influences widely, like the horizontal cells of the retina. Studies testify that electrical synapses can be modulated, that they may have mechanisms favoring unidirectional conduction, and that electrical and chemic synapses have important reciprocal influences.

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Distribution and Function of Gap Junction Coupling in Cortical GABAergic Neurons

Daniele F. Condorelli , ... Natale Belluardo , in Gap Junctions in the Brain, 2013

Properties of Neuronal Gap Junctions (Electrical Synapses) Integrated in Neuronal Jail cell Physiology

Electric synapses between neurons are bidirectional and act as a low-pass filter (for reviews see Bennett and Zukin, 2004; Hestrin and Galarreta, 2005). The and so-chosen coupling ratio, estimated as the ratio of the aamplitude of the voltage change in the non-injected cell to that in the injected cell in paired recording in connected cells, is higher for low-frequency signals and ranges from a fraction of 1% upwards to 10%. Yet, electrical synapses can transmit, although with a lower efficiency, fast action potentials to the continued cells, originating a "spikelet", with an amplitude ranging from hundreds of microvolts to several millivolts. Nonetheless, it has been repeatedly shown that gap junction-mediated advice can promote the synchrony of both depression-frequency signals, such every bit subthreshold oscillations of membrane potentials, and fast phenomena, such equally trains of action potentials. Indeed, the availability of Cx36 KO animals allowed researchers to show that neuronal gap junctions in the junior olive are not necessary for the generation of spontaneous subthreshold oscillations in single neurons, simply that electrototonic coupling can serve to synchronize rhythmic activeness among inferior olivary neurons (Long et al., 2002).

Information technology should be pointed out that the neuronal gap junction is not but a ionic aqueduct, but as well a conduit for organic molecules up to 1   kDa, such every bit messengers and metabolites, with an of import bear upon on neuronal physiology.

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Beefcake and Physiology, Systems

A. Rollenhagen , J.H.R. Lübke , in Encephalon Mapping, 2015

Full general Structural Features of Synapses

Two fundamentally dissimilar types of synapses are found in the neocortex.

Electrical synapses : Here, the pre- and postsynaptic opposing cell membranes are connected by special channels then-chosen gap junctions. In vertebrates, gap-junctional hemichannels are primarily human- or heterohexamers of connexin proteins belonging to different families. The most mutual types of connexins in the neocortex are connexin 36 expressed in neurons and connexin 43 expressed in astrocytes (Hormuzdi, Filippov, Mitropoulou, Monyer, & Bruzzone, 2004). Electrical synapses can serve unlike functions: (1) electric and metabolic exchange through hemichannels, (2) electric and metabolic coupling between neurons, and (3) adhesive office independent of conductive gap-junctional channels. Electrical synapses were long thought to be expressed only in prenatal and early postnatal life and are involved in neuronal migration in the neocortex. Information technology is now established that gap junctions connect unlike types of GABAergic interneurons that form independent networks responsible for the induction of unlike rhythmic and oscillatory activities, thereby controlling and modulating the behavior of the entire cortical network. Electric synapses are characterized by fast bespeak transduction between neurons but lack synaptic plasticity (Gibson, Beierlein, & Connors, 2005).

Chemic synapses: The 2nd and almost abundant blazon of synapses in the neocortex is the chemical synapse ( Figures 1 and ii(b, b1) ). Here, the arriving activeness potential in the presynaptic neuron elicits (via the activation of voltage-gated presynaptic Ca2   + channels) a chemic point by the specific release of either an excitatory or inhibitory neurotransmitter. Quanta of the released neurotransmitter specifically bind to appropriate receptors located at a highly organized membrane complex of the postsynaptic neuron, the postsynaptic density (PSD) ( Figures i, 2(b) , and three ). The neurotransmitter may initiate an electrical response or actuate a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. Because of the complexity of receptor point transduction, chemical synapses have a much broader and complex issue on the postsynaptic neuron than electric synapses, namely, on synaptic transmission and plasticity (Cowan, Südhof, & Stevens, 2003).

Effigy 2. Comparison of ii cortical synapses embedded in different microcircuits. (a) 2 hippocampal mossy fiber boutons (MFB1 and MFB2, transparent xanthous) terminating on a big proximal dendrite (transparent blue) of a CA3 pyramidal neuron. Here, synaptic contacts are established exclusively with the spiny excrescences (se, transparent blue), whereas puncta adherentia that are regarded as adhesion complexes (red asterisks) are just found at the apposition zone between the two boutons and the target dendrite. Scale bar is 0.5   μm. (a1) iii-D volume reconstruction of an en passant MFB (yellow) and its postsynaptic target dendrite. (b) Electron micrograph of a neocortical axospinous synapse with a perforated postsynaptic density (reddish arrowheads). Calibration bar is 0.v   μm. (b1) 3-D volume reconstruction of the synaptic complex shown in (b). Notation that the mossy fiber terminal is ~   20-fold larger than the neocortical bouton as indicated by the white scale bar (5   μm).

Figure 3. Distribution pattern of AMPA and NMDA receptors at neocortical synapses. (a) Intrasynaptic distribution of the AMPA receptor (subunits GluR1–4, black gold grains) at a postsynaptic density (PSD) every bit revealed by the dense accumulation of then-chosen intramembranous particles on a dendritic spine using freeze-fracture replica preparations combined with postimmunogold labeling. Here, the synaptic cleft (as marked past the arrowheads) separating the pre- (on the left) and PSD (on the right) is clearly visible. (b) Intrasynaptic distribution of the NR1 subunit of the NMDA receptor (black gold grains) at a PSD at a dendritic shaft. Annotation the clustered distribution of the NR1 subunit within the PSD. (c) Co-localization of the AMPA receptor (labeled with five   nm aureate) and the NR1 subunit of the NMDA receptor (labeled with 10   nm gold) at a PSD located at the somatic region of a neuron. Scale bars in (a)–(c) are 100   nm.

The so-chosen synaptic active zone, that is, the shut apposition betwixt the pre- and postsynaptic elements, is the site of neurotransmitter release. In full general, chemical synapses are equanimous of 3 subelements: a pre- and postsynaptic compartment and a cleft between the ii elements ( Figures 1(e) inset and 3(a) ). The presynaptic chemical element contains a highly variable pool of synaptic vesicles and several mitochondria associated with the pool of vesicles ( Figure 1 (a–c) and 1(e) ). At the contact zone with the postsynaptic neuron, a dense accumulation is formed, the presynaptic density ( Figure 1(a), 1(c) , and 1(e) inset). This density is constituted by a cocktail of diverse synaptic proteins including the SNARE and SNAP circuitous, various MUNC and RIM proteins, and many others all involved in the send, binding, and fusion of synaptic vesicles to the presynaptic density (Südhof, 2012). Also synaptic proteins, different high and low voltage-gated Ca2   + channels of the North, P/Q, R, and T type exist, which are thought to be specifically organized in clustered domains forth the presynaptic density (Stanley, 1997).

The second structural chemical element is the synaptic cleft ( Figures 1(e) inset and 3(a) ), not only a gap but also a bridge connecting the pre- and postsynaptic density (PSD), containing a matrix of fuzzy material of still unknown function and origin. Neurotransmitter molecules released from fused synaptic vesicles diffuse via the synaptic cleft ( Figure 1(east) inset) to the PSD and bind to appropriate neurotransmitter receptors. In the neocortex, PSDs often are perforated past periodic interruptions of the protein matrix ( Figure 2(b) ). Synapses with perforated PSDs are regarded to be highly efficient in synaptic strength and plasticity (Geinisman, 1993; Geinisman, Detoledo-Morrell, & Morrell, 1991; Nicoletta, Fenghua, Gregers, Maurizio, & Randal, 2013). Proteins at the PSD are involved in anchoring and trafficking neurotransmitter receptors, thereby modulating the activity of these receptors, in particular their temporal and spatial sensitization and desensitization. The different composition and content of neurotransmitter binding sites and synaptic proteins at the PSD is critically involved in synaptic transmission and in the modulation of long- and short-term synaptic plasticity (Südhof, 2012).

The so-called puncta adherentia, a second membrane specialization that is thought to function as adhesion complexes at large key nervous system (CNS) synapses (due east.thou., hippocampal mossy cobweb bouton; Rollenhagen et al., 2007), are not establish at neocortical synapses (compare Figure 2(a) with two(b) ).

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Synaptic transmission

Paul Johns BSc BM MSc FRCPath , in Clinical Neuroscience, 2014

Electrical synapses

Electrical synapses are as well known as gap junctions and are straight points of contact betwixt the cytoplasm of adjacent neurons (Greek: sunapsis, bespeak of contact). This allows very rapid two-mode communication and synchronization of electric discharges.

A gap junction is composed of effectually 100 intercellular channels called connexons that are inserted into the plasma membranes of adjacent cells (Fig. 7.i). Each connexon is composed of a hexagonal assortment of proteins chosen connexins, surrounding an aqueous channel that is 2 nm wide. The pores in adjacent cell membranes are aligned to grade a 'tunnel' between the two cells. These tin can be opened or closed by a conformational change in the constituent proteins, regulated by phosphorylation state.

Gap junctions correspond a low-resistance pathway that allows charged particles and small-scale molecules to menstruation freely in either direction and couples the electrical activity of adjoining cells. Groups of cells linked past gap junctions form an electrical syncytium which can generate big, synchronized discharges. This happens in sure brain stem nuclei that control animate and may contribute to the generation of abnormally synchronized discharges in some forms of epilepsy (Ch. eleven).

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Gap Junctions in the Nervous Organization

D.C. Spray , ... R. Dermietzel , in Encyclopedia of the Neurological Sciences (2nd Edition), 2014

Gap junctions between neurons mediate electric coupling

Examples of electrical synapses in mammalial encephalon include the cortical GABAergic interneuronal system, the junior olive with its projections into the cerebellar cortex, and the rhythmic activeness of the thalamic reticular nucleus and its project into the cognitive cortex. Electrotonic coupling occurs betwixt chief cells such as pyramidal cells (PC) of the neocortex and in the hippocampal germination. This remains somewhat controversial. The beginning bear witness of coupling of hippocampal PCs tin can be traced back to spikelets or fast prepotentials recorded in the 1980s, but electrical coupling from cells identified as PCs required new recording and imaging techniques. Such junctions are present only between small numbers of PCs merely coupling strength is high, so that action potential transmission may occur. PC electrical coupling thus appears to play a role in synchronization of neuronal assemblies.

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Structure, Part, and Development of the Nervous Organization

Mish Shoykhet , Robert S.B. Clark , in Pediatric Critical Care (4th Edition), 2011

Electric Synapses

At the electrical synapse, jail cell membranes of the adjoining neurons are tightly bound together into a gap-junction plaque. 5 Each plaque contains numerous channels fabricated of connexin proteins. In that location are 21 known connexin genes in humans. Each channel consists of two hemichannels, with 1 on each jail cell membrane. 2 hemichannels join together to form a functional gap junction between two neurons, assuasive intercellular diffusion of ions and modest molecules such as glucose, circadian AMP, and ATP. Gap junctions thus allow neurons to share data virtually their metabolic and excitable states, providing a machinery for large-scale regulation of energy demands and neuronal network dynamics. Additionally, gap junction channels close in response to lowered intracellular pH or elevated Catwo+ levels; since both events occur in damaged cells, paired hemichannels at the gap junction may office to isolate healthy neurons from those damaged during ischemia or trauma. Contempo testify suggests that unpaired hemichannels outside of the gap junction plaques may also contribute to ischemic neuronal prison cell death. six

Glia, like neurons, are too connected by gap junctions. For example, encephalon astrocytes form an interconnected cellular network, which allows long-altitude propagation of calcium signals across many cells. Additionally, layers of myelin generated past oligodendrocytes in the CNS and by Schwann cells in the PNS are linked by gap junctions. Myelin gap junctions provide structural stability to the myelin sheath and allow for rapid diffusion of nutrients and other substances beyond the sheath towards the underlying axon. In humans, mutations in gap junction poly peptide connexin 32 event in X-linked Charcot-Marie-Molar affliction, a demyelinating neuropathy. 7

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Biology of Gap Junctions

Richard D. Veenstra , in Cell Physiology Source Volume (4th Edition), 2012

Two Advantages of Electrical Synapses in Excitable Cells

The utility of electrical synapses in excitable cells is apparent. They may pass action potentials or subthreshold electrical activity more than rapidly from one neuronal element to another than can their chemical counterparts. This reward is especially obvious in neuronal pathways that serve as escape mechanisms, such as those in the tail muscles of crayfish and lobsters and those in the pectoral fins of fishes. In these cases, the rapidity of the animal'due south response to imminent danger has selective value. On the other paw, such permeable junctions connecting smooth muscle cells of the uterus or cardiac muscle cells of the heart wall, provide a mechanism for the systematic cell-to-cell spread of depolarization that is required for coordinated and effective contractile activity.

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Gap Junctions

David C. Spray , ... Eliana Scemes , in Encyclopedia of the Neurological Sciences, 2003

Gap Junctions between Neurons

The concept that electric synapses might play a major role in neural manual has been supported past recent identification of frequent, although tiny, gap junctions between neurons in the central nervous system and by the discovery that at least one connexin [connexin36 (Cx36)] is nearly exclusive in its neuronal expression and leads to loss of coupling amid neurons when its cistron is silenced (Cx36 knockout mice). Electrotonic synapses between GABAergic interneurons formed of Cx36 are believed to facilitate neuronal synchrony and contribute to large-range oscillatory rhythms in cerebral cortex, striatum, hippocampus, and cerebellum. Gap junctions between neurons too announced to play an important role in development, analogous activity of afferents and so as to optimize the formation of chemical synaptic inputs onto appropriate targets, as in the example of tectal projection of retinal ganglion neurons.

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Rod Photoreceptor Cells: Soma and Synapse☆

D. Križaj , R.One thousand. Smith , in Reference Module in Neuroscience and Biobehavioral Psychology, 2017

Electric Coupling

Near all vertebrates accept electrical synapses that interconnect photoreceptors to remove uncorrelated point noise and permit the mixing of rod and cone signals. Increased coupling between rods reduces photoreceptor racket (especially for depression frequency signals), increases the point-to-noise ratio and the reliability of rod output signals, allowing the retina to detect large dim objects in the visual field. Gap junctions likewise allow diffusion of ions, 2nd messengers (campsite, IP3) and metabolically significant molecules between rods and rods-cones. This aids in survival merely can hasten demise in instance of proapoptotic cistron diffusion ("the standby mechanism" of photoreceptor death).

Electrical coupling between rods reduces their uncorrelated voltage noise by averaging their signals, reduces the amplitude of the single-photon indicate and too affects the low-cal response of each rod. The amplitude of the single-photon signal is reduced in proportion to the number of coupled rods yet the amplitude of photon noise is reduced by the square root of the equivalent number of rods, which results in net reduction in signal-to-dissonance ratio. If rod bipolars were to sum rod signals linearly, the noise would swamp unmarried-photon signals, necessitating postsynaptic nonlinear filtering. Alternately, the distribution of each single-photon signal by electrical coupling to nearby rod terminals may permit vesicle fluctuation racket from rod ribbons to be averaged downstream. At mesopic backgrounds, rods receive ∼i–1000 photons per integration time, so they temporally integrate photon signals and function more similar cones except that their response gain is 30-to100-fold higher than the cone indicate.

As the rod synapse is specialized for single-photon signals and the rod bipolar pathway is specialized for high-gain spatial summation, the robust signals in rods at twilight need an alternate pathway. The electrical coupling between rods and cones allows twilight signals to laissez passer from rods into neighboring cones and thence into cone pathways. Thus, rather than behaving every bit contained and parallel units functioning over nonoverlapping ranges of environmental brightness, rods and cones communicate extensively, thereby allowing the animal'south vision to transition seamlessly beyond the mesopic range. Mixing of rod and cone signals has been demonstrated in fish, rabbit, mouse and primate retinas, and was shown to be regulated by an ingenious biological pathway based on the circadian clock and light-dependent release of the neurotransmitter dopamine. At dawn, weak low-cal stimulates rods, which through the rod bipolar pathway stimulate the release of dopamine from dopaminergic amacrine cells. Dopamine diffuses into the outer retina through paracellular book transport and activates D4 dopamine receptors on photoreceptors and D1 receptors on postsynaptic bipolar/horizontal cells, which has the event of opening the rod pathway to cone signals and facilitating cone inputs vis a vis rod inputs postsynaptically. The presynaptic switch from scotopic to mesopic vision occurs through the classical D4-Gi-camp-PKA signaling cascade within rods through which dopamine opens Cx36-based gap junctions betwixt rods and cones. This issue of dopamine is antagonized past adenosine 2a receptors, somatostatin SST2 and melatonin receptors which are activated in the absence of light. The mixing of rod and cone signals in vertebrate retinas therefore involves an elegant neuromodulatory push–pull betwixt dopamine-adenosine/peptidergic pathways that, in turn, are regulated by calorie-free/circadian rhythms.

In most mammals, rod–cone coupling is not selective for cone type, so dissimilar cone spectral types take a like rod contribution. This is convenient because opponent color signals computed by subtraction in retinal circuits can then remove the contribution from rods. The mixing of the cone'south signal with the higher-gain rod indicate represents a form of adaptation because it extends by two–iii log units the background range over which the cone ribbon synapse can transmit a signal. Rods of some species (frog, turtle, and fish) are much larger in bore than mammalian rods and therefore, when warm, generate more dark thermal events and are more probable to receive multiple photon events at scotopic backgrounds; in this example rod–rod coupling imitates mesopic cone–cone coupling and is advantageous. However, at low temperatures these poikilotherms (cold-blooded species) accept a depression nighttime thermal effect charge per unit and can respond to stimuli that evoke single-photon signals.

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How nerves work

In Auricular Acupuncture & Habit, 2009

The synapse

There are 2 broad types of synapse: electrical and chemic. An electrical synapse, also known as a gap junction, is a mechanical link betwixt 2 neurons that allows for the conduction of electricity. Electrical synapses incorporate channels that allow charges (ions) to menses from one cell to another ( Fig. 2.2).

In humans, virtually synaptic transmission is of the chemical type and information technology is this type that is afflicted by ingested chemicals. For these reasons, this text focuses on the chemical synapse (Fig. 2.3).

In chemic synapses the flow of information tends to exist unidirectional. The first neuron in a communicating pair is referred to as presynaptic and the second postsynaptic. Information technology should be noted that the communicating neurons are not in concrete contact. Due to this lack of concrete communication there exists the need for a means to span the synaptic gap. Communication across the synapse is brought nigh by neurotransmitters. The office of a neurotransmitter is to convert the electrical signal of the presynaptic neuron into a chemical bespeak that tin can cantankerous the synaptic gap and reach the postsynaptic neuron, where the chemical bespeak is converted dorsum to an electric one. In this way, neurons can exchange information, in a sense accept a conversation with each other and – via circuitous chain reactions of interconnected neurons – antipodal with millions of others. From out of this neuronal chit-chat springs human behaviour.

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