Synapse electrotonic

Auerbach AA, Bennett MVL A rectifying electrotonic synapse in the central nervous system of a vertebrate. J Gen Physiol 1969 53 211-237. 

Pathways in the central nervous system. A shows two relay neurons and two types of inhibitory pathways, recurrent and feed-forward. The inhibitory neurons are shown in black. B shows the pathway responsible for presynaptic inhibition in which the axon of an inhibitory neuron synapses on the axon terminal of an excitatory fiber. C Diagram illustrating that dendrites may be both pre-and postsynaptic to each other, forming reciprocal synapses, two of which are shown between the same dendrite pair. In triads, an axon synapses on two dendrites, and one of these dendrites synapses on the second. In serial synapses, a dendrite may be postsynaptic to one dendrite and presynaptic to another, thus connecting a series of dendrites. Dendrites also interact through low-resistance electrotonic ("gap") junctions (two of which are shown). Except for one axon, all... 

Pineyro G, de Montigny C, Weiss M, Bher P. Autoiegulatory properties of dorsal raphe 5-HT neurons possible role of electrotonic coupling and 5-HT1D receptors in the rat brain. Synapse 1996 22 54-62. 

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. 

Reticulospinal neurons form mixed electrotonic and chemical output synapses with motoneurons and different classes of intemeurons along the spinal cord (Rovainen, 1974, 1979 Buchanan and Grillner, 1987 Grillner et al., 1995). The chemical synapses release glutamate. 

At the ultrastructural level the majority of the release sites (65-70%) appear as a single cluster of synaptic vesicles accumulated at a single active zone (Fig. ID), which may contain specializations of pure chemical or mixed synapses (Pfenninger and Rovainen, 1974 Ringham, 1975 Christensen, 1976 Rovainen, 1979). The latter type thus contains gap junctions as well as active zones (Fig. ID). Most of the synapses are established on dendritic shafts. A distinct class of intemeuron, however, receives axo-somatic synapses (OS and LB unpublished). The majority of the postsynaptic cells receive multiple chemical or mixed synaptic contacts from a single axon. Therefore most of the EPSPs recorded in postsynaptic neurons are composed of two components, an electrotonic and a chemical component. 

Postsynaptic responses to neurotransmitters are invariably initiated by the binding of the transmitter to a specific recognition site, or receptor. This finding is true both for interneuronal communications and the transmission of signals from neurons to effector cells. Perhaps the only known exception to this observation is the presumed communication in the central nervous system between electrotonic synapses, a topic beyond the scope of this chapter [see Weight (1971) and Schmitt et al. (1976) for further details]. 

A brief comparison between electrical and chemical synapses, however, is instructive. Electrical transmission, by definition, requires no chemical substance for the transmission of an electrical signal from one neuron to another rather, electrotonic impulses are thought to move from one cell to another via syncytoid connections. The rate of transmission is orders of magnitude faster in electrical, as versus chemical, synapses. Although inhibition of chemical transmission can occur in a variety of ways (i.e., inhibition of... 

This suggest that some of the synapses for the "late IPSP" are remote from the cell body. In such a case, E IPSP> since the polarizing potential in the cell body required to reverse the "late IPSP" at the remote synapses located along the axon would decay electrotonically. Such differences have been similarly accounted for by others (Rail, 1962 Burke and Ginsborg, 1956 Calvin, 1969). However, it is also possible that either a transmitter other than ACh may be involved in synaptic action or that E is different for synaptic and extrasynaptic sites as has been recently shown in frog neuromuscular junction by Mallart and Feltz (1969). The fact that E means that the cell body also has ACh recep7... 

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