Electrical synapses

Communication between neurons involves neurotransmitters. Up until the beginning of the last century, synaptic transmission was regarded as probably electrical. It was suggested that the close apposition of two neurons allowed the current to jump the synaptic cleft, rather like an electrical spark between two closely positioned wires. There is indeed evidence for electrical synapses in animal species where the synaptic cleft is particularly narrow (2 nm, or nanometres), as well as in the myocardium where the close coupling of cells allows electrical current to flow from one cell to the next,... 

Finally, with regard to synaptic type, there is the well-characterized electrical synapse [20], where current can pass from cell to cell across regions of membrane... 

The taste bud is a polarized structure with a narrow apical opening, termed the taste pore, and basolateral synapses with afferent nerve fibers. Solutes in the oral cavity make contact with the apical membranes of the TRCs via the taste pore. There is a significant amount of lateral connectedness between taste cells within a bud both electrical synapses between TRCs and chemical synapses between TRCs and Merkel-like basal cells have been demonstrated to occur [39]. Furthermore, there are symmetrical synapses between TRCs and Merkel-like basal cells [39]. In addition, these basal cells synapse with the afferent nerve fiber, suggesting that they may function in effect as interneurons [39]. The extensive lateral interconnections... 

The communication between neurons occurs at either gap junctions (electrical synapses) or chemical synapses with release of neurotransmitters from a presynaptic neuron and their detection by a postsynaptic nerve cell (Fig. 17.1). Neurotransmitters not used in the synaptic cleft are removed promptly by either uptake into adjacent cells, reuptake in the presynaptic neuron, or are degraded by enzymatic systems. 

In an electrical synapse the electrical signal is transferred directly because the membranes of the two cells are in close vicinity. This is the fastest possible intercellular propagation system of information. Electrical synapses are present and have been studied mainly in lower animals. 

In electrical communication, changes in membrane potential are used to conduct a stimulus within a nerve cell. Changes in membrane potential can also be used for intercellular commimication. In this case, communication between the cells takes place via electrical synapses at which the potential change can be directly passed on to neighboring cell. Central components of electrical communication are voltage-dependent ion channels with open states regulated by changes in the membrane potential. 

The communication between neurons in the CNS occurs through chemical synapses in the majority of cases. (A few instances of electrical coupling between neurons have been documented, and such coupling may play a role in synchronizing neuronal discharge. However, it is unlikely that these electrical synapses are an important site of drug action.) The events involved in synaptic transmission can be summarized as follows. 

De Vries SH, Schwartz EA Modulation of an electrical synapse between solitary pairs of catfish horizontal cells by dopamine and second messengers. J Physiol 1989 414 351-375. 

Most but not all neuron connections rely on the passage of neurotransmitters across a synaptic deft. The neurons that connect to musdes controlling eye movement, however, are different in that they have what are called electrical synapses, which are direct connections between the neurons and the musdes (there s no gap). The high speeds of these direct connections provide for eye motion that is quick and jerky—a useful trait. Some fish have electrical synapses in their tails, an arrangement that provides for rapid escape from predators. 

Gap junctions in synapses. Not all neurons communicate via chemical synapses. Gap junctions, which are found in both neurons, astrocytes, and other cells, serve as electrical synapses. Thus, heart cells are all electrically coupled together by gap junctions.606-608 Gap junctions are formed with the aid of hexameric connexons, which are present in each of the opposed membranes and are aligned one with the other (Fig. 1-15F,G).607 609 610 There may be thousands of connexons in a single gap junction, which resemble ion channels in appearance but contain pores 1.5 nm in diameter. They are formed from 26- to 43- kDa... 

Bone cells are electrically active [10, 11, 24, 121, 170]. In addition to permitting the intercellular transmission of ions and small molecules, gap junctions exhibit both electrical and fluorescent dye transmission [93, 183, 187, 133], Gap junctions are electrical synapses, in contradistinction to intemeur-onal, chemical synapses and, significantly, they permit bi-directional signal traffic (e.g., biochemical, ionic, electrical etc.). In a physical sense, the CCN represents the hard wiring [30, 140, 141, 150] of bone tissue. 

As noted above, gap junctions as electrical synapses permit bi-directional flow of information. This is the cytological basis for the oscillatory behavior of a CCN. The presence of sharp discontinuities between groups of pheno-typically different osteoblasts is related also to an associated property of gap junctions, i.e., their ability to close and so prevent the flow of information [98, 106], Significantly, informational networks can also transmit inhibitory signals, a matter beyond our present scope [116],... 

SYNAPTIC FEATURES DENDRITIC RELEASE OF DOPAMINE AND ELECTRICAL SYNAPSES... 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Gap junctions provide in the nervous system the structural correlate of one class of electrical synapses, characterized by very close apposition between the presynaptic and postsynaptic membranes. It should be noted, in this respect, that different junctional specializations can mediate different forms of electrical transmission between neurons (Bennett, 1997). Electrical synapses transmit preferentially, but not exclusively, low-frequency stimuli, that allow the rapid transfer of a presynaptic impulse into an electrical excitatory potential in the postjunctional cells. Electrical transmission, via the intercellular channels, can be bidirectional. The widely held opinion that electrical transmission is characteristic of lower vertebrates probably derives from the large cell systems in which electrical synapses were identified in the initial period of intracellular recording (reviewed by Bennett, 1997). Contradicting this view, electrotonic coupling between neurons has now been demonstrated in many areas of the mammalian central nervous system and has been implicated in neuronal synchronization. Gap junctional intercellular communication can occur between glial cells, glia and neurons, as well as between neurons. 

Horzmuzdi SG, Filippov MA, Mitropoulou G, Montyer H, Bruzzone R (2004) Electrical synapses a dynamic signaling system that shapes the activity of neuronal networks. Biochim Biophys Acta 1662 113-137. 

Electrical synapses, which are tubular structures (called connexions) and form gap junctions the membranes of the two cells are separated by a distance of 2 nm. They may allow the two-way transmission of impulses ... 

Electrical synapses, in which two cells actually touch, allow the action potential to pass directly from one membrane to the next. They are very fast, but are quite rare, found only in the heart and the eye. 

There is also evidence that retinoic acid directly modulates transmission at electrical synapses of retinal cells. This is independent of G-proteins and second messengers, and involves a nonnuclear RAR-like binding site associated with ion channels (Zhang and McMahon, 2000). 

Zhang DQ and McMahon DG (2000) Direct gating by retinoic acid of retinal electrical synapses. Proceedings of the National Academy of Sciences of the USA 97, 14754-9. 

Synapses are electrical or chemical communicative contacts between neurons. Electrical synapses (neuronal gap junctions) function by the propagation of electrical impulses from one cell to another (and vice versa) via direct, physical contact. As a consequence, these synapses are characterized by a relatively simple organization of membrane structure and associated organelles (Zoidl et al. 2002). Electrical synapses are also less mutable, in terms of their function and molecular characteristics, and thus exhibit little of the plasticity that typifies the chemical synapse. 

In nervous tissue, some neurons are connected by gap junctions through which Ions pass rapidly, thereby allowing very rapid transmission of electrical signals. Impulse transmission through these connections, called electrical synapses. Is almost a thousandfold as rapid as at chemical synapses (Chapter 7). Gap junctions are also present in many non-neuronal tissues where they help to Integrate the... 

---