The synapses of the nervous system

The spatial relationship between the perineuronal proteoglycan network and the synaptic boutons as visualized by double staining with cationic colloidal iron method and anti-calbindin-D-28K immunohistochemistry in rat cerebellar nuclei

The present study demonstrated the precise spatial relationship between meshes in the perineuronal proteoglycan network and the terminal boutons of synaptically associated axons. Sections from the rat cerebellum were stained with cationic colloidal iron (pH 1.0-1.5), and successively immunostained with anti-calbindin-D-28K monoclonal antibody. Cationic iron stained sulfated proteoglycans around the nerve cell of the medial cerebellar nucleus, whereas the anti-calbindin antibody labeled the Purkinje cells including their axons terminating on large neurons in the cerebellar nucleus. It was found that each synaptic bouton fits into a mesh of the perineuronal network. The individual meshes appeared to be divided by partitions faintly stained with the colloidal iron. Electron microscopy of cationic colloidal iron-stained ultrathin sections revealed that the synaptic boutons were separated from each other by the proteoglycan matrix and that each of them was further divided into two or more contact areas of presynaptic membrane by the same matrix. This suggests that individual synapses are protected against the effects of adjacent synaptic transmission, and that each of them may be subdivided by this manner of partitioning, like pads of a cat's paw.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Contributions of dendritic spines and perforated synapses to synaptic plasticity

The dynamic nature of synaptic connections has presented morphologists with considerable problems which, from a structural perspective, have frustrated the development of ideas on synaptic plasticity. Gradually, however, progress has been made on concepts such as the structural remodelling and turnover of synapses. This has been considerably helped by the recent elaboration of unbiased stereological procedures. The major emphasis of this review is on naturally occurring synaptic plasticity, which is regarded as an ongoing process in the postdevelopmental CNS. The focus of attention are PSs, with their characteristically discontinuous synaptic active zone, since there is mounting evidence that this synaptic type is indicative of synaptic remodelling and turnover in the mature CNS. Since the majority of CNS synapses can only be considered in terms of their relationship to dendritic spines, the contribution of these spines to synaptic plasticity is discussed initially. Changes in the configuration of these spines appears to be crucial for the plasticity, and these can be viewed in terms of the significance of the cytoskeleton, of various dendritic organelles, and also of the biophysical properties of spines. Of the synaptic characteristics that may play a role in synaptic plasticity, the PSD, synaptic curvature, the spinule, coated vesicles, polyribosomes, and the spine apparatus have all been implicated. Each of these is assessed. Special emphasis is placed on PSs because of their ever-increasing significance in discussions of synaptic plasticity. The possibility of their being artefacts is dismissed on a number of grounds, including consideration of the results of serial section studies. Various roles, other than one in synaptic plasticity have been put forward in discussing PSs. Although relevant to synaptic plasticity, these include a role in increasing synaptic efficacy, as a more permanent type of synaptic connection, or as a route for the intercellular exchange of metabolites or membrane components. The consideration of many estimates of synaptic density, and of PS frequency, have proved misleading, since studies have reported diverse and sometimes low figures. A recent reassessment of PS frequency, using unbiased stereological procedures, has provided evidence that in some brain regions PSs may account for up to 40% of all synapses. All ideas that have been put forward to date regarding the role of PSs are examined, with particular attention being devoted to the major models of Nieto-Sampedro and co-workers.(ABSTRACT TRUNCATED AT 400 WORDS)

The postsynaptic density: a possible role in long-lasting effects in the central nervous system

A theory is proposed that biochemical changes at the synapse that occur as a result of stimulation of specific neuronal circuits can lead to long-term changes only if alterations occur in synaptic structures in these circuits. The main synaptic structure that is thought to undergo this alteration is the postsynaptic density (PSD). There are many reports in the literature of overall structural changes at the synapse, including the PSD, resulting from various neuronal stimuli. These structural changes are here envisaged to include those of concentration and conformation of PSD proteins, changes that could alter the neural physiology of dendritic spines and even that of the presynaptic terminal.

Three-dimensional analysis of dendritic spines. II. Spine apparatus and other cytoplasmic components

A total of 342 dendritic spines (193 from the visual and 149 from the cerebellar cortex of the mouse) were analyzed in serial and several hundred of thousands of them in single sections, with respect to the presence and organization of the spine apparatus and other cytoplasmic components. The continuity of the spine apparatus with the smooth endoplasmic reticulum of the dendritic trunk was shown in three-dimensional reconstructions. The dense material of the spine apparatus was divided into "inner dense plate" and "outer dense plate". The close relationship between the outer dense plate and the postsynaptic density suggests that the spine apparatus functions as a postsynaptic protein synthesizing centre. The material from the outer dense plate could be used for a dynamic extension of the synaptic active zone. An extraspinous spine apparatus of the axon initial segment was partially reconstructed. Polyribosomes were found in all large spines of the visual cortex but were not so frequent in small spines and in Purkinje cell dendritic spines. Microfilamentous network and intermediate filaments occurred in the spines. The smooth endoplasmic reticulum of Purkinje cell dendritic spines was reconstructed. No spine apparatus and dense material were present in these spines.