Synapse

A specialized junction between neurons, or between neurons and other types of excitable cells, capable of transmitting, processing, and retaining neural information.

The term 'synapse' (Greek for syn-haptein, 'to make contact') is commonly attributed to Sherrington (Foster and Sherrington 1897). It was actually proposed by Verrall, an expert on "classical drama, to replace syndesm (Greek for 'chained together'), which was Sherrington's first choice (Shepherd and Erulkar 1997). 'Synapse' was preferred because it was judged to yield a better adjectival form. It is left for the reader to judge whether 'synaptic "plasticity' indeed rhymes better than 'syndesic plasticity', but, in any case, it is too late for a change. In the background of the introduction of 'synapse' was one of the most important and heated debates in the history of the neurosciences (e.g. Brazier 1988; Finger 1994). Two 'theories' ("model) concerning the cellular organization of the nervous system coexisted toward the end of the nineteenth century. One, 'the reticular theory of nervous organization', promoted among others by Golgi, held that nerve cells are physically interconnected to form an uninterrupted web

(reticulum). This theory held the contacts between the extensions that branch of neuronal cell bodies are only specializations in a fused continuum of tissue. The opposing theory, dubbed 'the neuron theory' or 'the neuron doctrine', promoted among others by Cajal, held that nerve cells are discrete entities.1 According to this doctrine, the junctions between neurons are specialized miniature devices that engage the juxtaposed individual units in the net. The neuron doctrine triumphed, although not without some skirmishes waged by the retreating 'reticularists' well into the twentieth century (Szentagothai 1975). Sherrington was a neuro-physiologist who trusted that nerves terminate in free endings and that the transfer of information from these endings to their targets differs markedly from the propagation of information along neuronal branches. When requested to revise his contribution to an authoritative textbook of physiology (Foster and Sherrington 1897), he reasoned that as research on the functional junction between nerve cells had already matured to become an important topic in physiology, this type of junction deserved a special term. Hence the 'synapse' was born.

Synapses come in many flavours. They can be classified by their morphology, location, function (e.g. inhibitory vs. facilitatory), types of "neurotransmitters and their "receptors, etc. A major "taxonomy distinguishes 'chemical' from 'electrical' synapses. In chemical synapses, information is transmitted from one cell to another by chemical messages (neurotransmitters) over an intercellular gap. In electrical synapses, there is direct electrical communication between the juxtaposed cells by way of a specialized contact, called 'gap junction', which contains "channel complexes called 'connexons' (Goodenough et al. 1996). Two connexons, each contributed by one of the juxtaposed cells, interact, and align to form an intercellular channel, which subserves electrical coupling and exchange of small molecules. Cell-cell 'on-line interneting' via connexons occurs in many types of tissues. Electrical synapses are hence specializations in the nervous system of a ubiquitous type of cellular device that allows direct communication and synchronization of activity between adjacent cells. Communication via electrical synapses is fast but the direct coupling imposes some constraints, such as the inability to reverse the sign of the signal or amplify it on location. Electrical synapses do, however, display use-dependent plasticity, which may involve "intracellular signalling cascades triggered by chemical transmitters (Goodenough et al. 1996; Pereda and Faber 1996). Synaptic complexes that share elements of both electrical and chemical transmission are termed 'mixed synapses'. An example for the use of electrical synapses in the mammalian brain is provided by widespread networks of inhibitory neurons in the "cerebral cortex (Gibson et al. 1999). So far, electrical synapses have received less attention than chemical synapses, but recent years have witnessed a growing interest in their function in the brain.

As noted above, in chemical synapses, in contrast to electrical synapses, the two opposing neurons are separated by an intercellular gap, called the 'synaptic cleft'. The use of 'gap' or 'cleft' does not imply void; there is a rich microcosm in between the pre- and the postsynap-tic membranes, and some of the molecules physically bridge the two sides. It is also noteworthy that, although we now take it for granted that chemical synapses exchange chemical messages, the nature of the information transmitted over the synaptic cleft in such synapses had been debated over many years. Two types of possibilities were considered. One, that the information is mediated by electric currents. The other, that it is transmitted by chemical substances. The existence of chemical transmission in the neuromuscular junction was proposed by Du Bois-Reymond in 1877 (cited in Dale 1938). A series of investigations conducted independently by Lewandowsky, Langley, Elliot and later Dale, Loewi, and others have provided convincing evidence for chemical ('neurohumoral') transmission (Elliot 1904; Dale 1938,1954; also "acetylcholine, "noradrenaline). The evidence had been provided first for synapses in the peripheral nervous system and only later for the central nervous system. Actually, for a while, some leading investigators considered chemical messages too sluggish to be useful for fast communication in the central nervous system. Notable among them was Eccles (1982). But even the lingering opposition finally succumbed to the data: 'Eccles and his team concluded that... (transmission) could only be due to the release of a chemical agent from the endings of the afferent fibre... A remarkable conversion indeed! One is reminded, almost inevitably, of Saul on his way to Damascus, when the sudden light shone and the scales fell from his eyes' (Dale 1954).

Since their discovery, it has been realized that synapses are faced with an inherently tough job. Basically, they have to transmit information ("stimulus) with acceptable fidelity. But they had also been evolved into miniature transducers that filter, encode, process, modulate, associate and register chunks of that information for their own use and for the sake of the circuit. Accordingly, in the discipline of memory research, synaptic components have been assigned roles of elements in

Fig. 62 (a) Sherrington, who is accredited with the introduction of the term 'synapse', and (b) a highly schematic representation of a chemical synapse. Only a few elements of the synapse are shown.The postsynaptic terminal and an adjacent glia cell are depicted in grey. Glia cells (from the Greek 'glue') are traditionally considered to provide neurons with physical and metabolic support; although it is now clear that they do much more (Araque etal. 1999; Ullian etal. 2001), their exact role in synaptic communication and in synaptic plasticity is still mostly an *enigma. For more on synaptic components and their function, see *calcium, *ion channel, 'neurotransmitter, *receptor.

Fig. 62 (a) Sherrington, who is accredited with the introduction of the term 'synapse', and (b) a highly schematic representation of a chemical synapse. Only a few elements of the synapse are shown.The postsynaptic terminal and an adjacent glia cell are depicted in grey. Glia cells (from the Greek 'glue') are traditionally considered to provide neurons with physical and metabolic support; although it is now clear that they do much more (Araque etal. 1999; Ullian etal. 2001), their exact role in synaptic communication and in synaptic plasticity is still mostly an *enigma. For more on synaptic components and their function, see *calcium, *ion channel, 'neurotransmitter, *receptor.

miniature learning machines, such as "acquisition devices, "coincidence detectors and associators, storage devices, etc. (Dudai 1993; "Aplysia, "Drosophila, "long-term potentiation).2 The synaptic job is carried out by webs of macromolecules in the presynaptic and the postsynaptic terminals. Parts of this web extend into the synaptic cleft as well. The macromolecular web includes "ion channels, "receptors, cytoskeleton, adhesion molecules, and more (Kuno 1995; Hagler and Goda 1998; Kim and Huganir 1999). The presynaptic terminal contains a highly specialized apparatus for transmitter release (Kuno 1995; Schiavo et al. 1995; Geppert and Sudhof 1998), regulated, inter alia, by presynaptic receptors for the same or other transmitters (Langer 1997; Miller 1998). The structure and function of the synaptic machinery is a dynamic function of the stage in "development, the individual experience of the synapse, and the ambient extracellular and intracellular input (e.g. "calcium; "glutamate; Okabe et al. 1999; Thomson 2000). Plastic changes could manifest themselves in anything from gradual alterations in synaptic efficacy, subserved by pre-, post-, or pre + postsynaptic mechanisms (Jessel and Kandel 1993; Markram et al. 1998; Thomson 2000), to a transformation from a silent synapse to one that starts talking (Atwood and Wojtowicz 1999). Some manifestations of synaptic "plasticity last for a fraction of a second to a few seconds only, but other forms of synaptic plasticity could last much longer ("long-term potentiation). To reshape itself in the long term, the synapse apparently takes advantage of local as well as cell-wide "protein synthesis facilities (Steward 1997; Casadio et al. 1999; Gardiol et al. 1999).

The notion that synaptic plasticity subserves developmental and behavioural plasticity is a tenet of neuroscience. In the neurobiology of memory as well as in "modelling, the twentieth century was in many respects the century of the synapse. It has become the major and sometimes the sole focus of attention at the molecular, cellular, and circuit "levels of analysis. The synapse was also identified as the prime target for multiple types of neuroactive drugs, including some that affect memory ("lotus, "nootropics). The popularity of the synapse stems from a smart choice of a domineering "reductive research programme, and from the availability of certain advanced research "methodologies. It is tempting to predict that in the twenty-first century, analysis of the synapse will become even further integrated into the analysis of "internal representations and their experience-dependent modification at the circuit and system level. The data so far do tell us that synapses are critical in making memories. But synapses may not be sufficient and even more so exclusive agents in establishing a memory ("criteria). Because they are so interesting and captivating, and because we find them more or less amenable to analysis, becoming immersed in the fascinating world of the synapse could easily make one forget what the ultimate goal of memory research is. As far as memory is concerned, what counts for the behaving brain is not the individual synapse but rather the integrated contribution of many synapses to the coherent activity of the networks in which they operate.

Selected associations: Algorithm, Ion channel, Long-term potentiation, Plasticity, Reduction

1When the term 'synapse' was introduced, the term 'neuron' had itself been a newcomer to the scientific jargon. It was coined by Waldeyer (1891), an ardent supporter of Cajal's neuron doctrine, only a few years earlier.

2Beware, however, of the '*homunculus fallacy', which equates the behaviour of the synapse with the behaviour of the brain or even the organism.

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