Trends in Neurosciences
Volume 21, Issue 2, 1 February 1998, Pages 81-88
Journal home page for Trends in Neurosciences

Diversity of structure and function at mammalian central synapses

https://doi.org/10.1016/S0166-2236(97)01170-3Get rights and content

Abstract

Our appreciation of the relationship between synaptic structure and function, and in particular our understanding of quantal synaptic transmission, is derived from classical studies on the neuromuscular junction. However, physiological studies of quantal transmission at mammalian CNS synapses have produced a variety of results, and thus no consensus of opinion has emerged.  This variability could be due, in part, to experimental and analytical limitations or to differences in the structural and functional features of central synapses, or both. Some of the experimental limitations have recently been overcome by the use of novel preparations that permit direct measurement of quantal synaptic events in the CNS.  Although these studies reveal similarities between the synaptic mechanisms of the neuromuscular junction and CNS synapses, important differences and specializations are also evident. The purpose of this review is to highlight the structural and functional diversity of synapses in the mammalian CNS, and to discuss the potential relevance of structural features to synaptic function.

Section snippets

Structure of mammalian CNS synapses

Mammalian central synapses exist in a variety of shapes and sizes, and several examples are illustrated in Fig. 1. Some small boutons contact dendritic spines, such as those arising on pyramidal neurons in the hippocampus[22], whereas others end on the soma or smooth dendritic shafts, such as the contacts between muscle spindle afferents and motoneurons or dorsal spinocerebellar tract (DSCT) cells in the spinal cord6, 23, 24. At the other extreme, some of the largest presynaptic terminals in

The time course and probability of quantal transmitter release

The functional consequences of variations in the size and shape of synaptic specializations are potentially reflected in several parameters of synaptic transmission, in particular the probability of presynaptic transmitter-release and the amplitude and time course of the postsynaptic current. Following the arrival of an action potential at the presynaptic terminal, the secretion of a quantum of neurotransmitter from a release site occurs in an intermittent or probabilistic manner, and with a

The postsynaptic current

Spontaneous synaptic currents, thought to be generated by vesicular release of transmitter at many release sites over the surface of a neuron, and often exhibit a wide range of amplitudes and time courses within the same cell (Fig. 5). This variability at individual synaptic specializations could be caused by: the concentration and time course of neurotransmitter in the synaptic cleft; and the location and properties of the postsynaptic receptor/channels. Both of these changes could be

Concluding remarks

Experimental studies have demonstrated a large diversity of synaptic structure and function in the mammalian CNS. Although central synapses can share some basic mechanisms, the structural and functional diversity reviewed here raises the possibility that different synapses might be dominated by distinct properties linked to their particular physiological role. Further direct evidence concerning the correlation between structural and functional properties of central synapses will be extremely

Acknowledgements

We are very grateful to Alicia Fritchle for drawing Fig. 1, and to Drs Mark Bellingham, Steve Redman, Christian Stricker and Dirk van Helden for helpful comments on the manuscript. BW is supported by the National Health and Medical Research Council (Australia). FJA and REWF are supported by the National Institutes of Health (NINDS).

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