Synaptic Vesicle Clusters Research Articles

Synaptic organization of the substantia gelatinosa glomeruli in the spinal trigeminal nucleus of the adult cat.

In the spinal trigeminal nucleus of the adult cat, the neuropil of the substantia gelatinosa (SG) layer is made up of axonal endings of V nerve axons, dendrites and axons of SG neurons and dendrites of marginal and magnocellular neurons. A typical SG glomerulus consists of a centrally located V nerve ending which contains large synaptic vesicles and forms asymmetrical axodendritic synapses on two kinds of dendritic spines and on dendritic shafts. The dendritic processes of the SG glomerulus are linked by two kinds of dendrodendritic synapses: type 2 dendritic spines, containing large synaptic vesicles, form slightly asymmetrical (intermediate) synapses on type I dendritic spines. Dendritic shafts, containing clusters of small synaptic vesicles, form symmetrical dendrodendritic synapses amongst themselves and on type I dendritic spines. Small axonal endings, containing small synaptic vesicles form intermediate axodendritic synapses on type I dendritic spines and on dendritic shafts. These small axons also form symmetrical axoaxonic synapses on the V nerve ending. The V nerve endings in the SG layer are dark and differ morphologically from the pale V nerve endings in other parts of the trigeminal nuclei.

Neural organization of the median ocellus of the dragonfly. II. Synaptic structure.

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.

Ultrastructural features of synaptic regions in walking leg muscles of the horseshoe crab, Limulus polyphemus (L.)

Synaptic regions in muscles of the walking legs of Limulus polyphemus were examined with the electron microscope. These regions consist principally of a large evagination of sarcoplasm from the surface of the muscle fiber and numerous axon branches and their accompanying envelope of glial elements. The axon branches penetrate the sarcoplasm and ultimately form multiple neuromuscular synapses within the sarcoplasmic evagination. These observations provide the first morphological evidence for the polyneuronal and multi-terminal innervation of neuromuscular systems in the horseshoe crabs. The neuromuscular synapses themselves closely resemble those of other arthropods by the close apposition of pre- and postsynaptic membranes, the absence of glial elements in the synaptic cleft, and the presynaptic clustering of synaptic vesicles. Both large, granular and small, agranular synaptic vesicles are present in the presynaptic terminals. An extensive postsynaptic complex of vesicles and tubules also occurs in the sarcoplasm of the synaptic region.

Freeze-fracturing of presynaptic membranes in the central nervous system

That synaptic vesicles with a diameter of about 50 nm could be demonstrated by the freeze-fracturing technique was an important contribution to the knowledge of synaptic fine structure (Moor, Pfenninger & Akert 1969; Akert, Moor, Pfenninger & Sandri 1969). The finding u spheric profiles (figures 1, 3 a, c) in rapidly frozen tissue renders an artefactual origin of vesicles very unlikely. However, the chief advantage of freeze-fracturing lies in the visualization of membrane surfaces which are of special interest in presynaptic nerve terminals. The present micrographs were obtained from freeze-fractured monkey, cat, and rat spinal cord, cerebral cortex, and subfornical organ as well as from pigeon optic tectum. Special attention was paid to areas in which large pieces of membranes remained attached to the surfaces of nerve cells (figures 1, 3 a ). This situation suggests a special type of affinity between the two elements, and the association of one of the membranes with clusters of synaptic vesicles indicates that presynaptic membranes may be involved.

A quantitative ultrastructural study of the inner plexiform layer of the rat retina.

AbstractThe inner plexiform layer (IPL) and adjacent regions of the rat retina were studied by electron microscopy. The IPL contains neural processes of amacrine, bipolar, and ganglion cells, and glial (Müller) processes. Type 1 amacrine processes (maximum diameter 3.1 μ) traced from their somata structurally resemble amacrine perikarya. These processes rarely form synapses. Type 2 amacrine processes (maximum diameter 0. 7 μ) are radially oriented, forming synaptic expansions en passant, giving origin to collaterals, also with expansions. The type 2 amacrine processes form conventional synapses onto bipolar, ganglionic and other amacrin processes, onto amacrine and ganglionic somata, and onto dendritic spines. Amacrine perikarya in rare cases contain synaptic vesicles and form somatic synapses onto surrounding processes.Bipolar processes traverse the IPL, forming terminal expansions up to 6 μ in diameter near the ganglion somata, which they contact without synaptic vesicle clustering. Bipolar processes form synapses onto dyads. In 73% of dyads, both processes are vesiculated. One process of the dyad pair could usually be identified as an amacrine by its reciprocal synapse back onto the presynaptic bipolar process. The other dyad process, when vesiculated, contains vesicles which are significantly larger and less concentrated than in the adjacent amacrine, and is presumably a ganglion cell dendrite.The ratio of the incidence of amacrine synapses: bipolar synapses is about 8:1. A subdivision of the IPL into thirds shows that the highest incidence of amacrine synapses is in the middle third. Amacrine somata are significantly larger than bipolar somata. Synaptic vesicles in receptor terminals of the outer plexiform layer are significantly larger and more concentrated than those in axonal processes of the IPL. The functional aspects of these findings are discussed in relation to physiological investigations and interspecies differences described in the literature.

Ultrastructure of synaptic terminals in nucleus infundibularis and nucleus supraopticus of Passer domesticus.

Axo-dendritic and axo-somatic synapses and complex axo-dendritic junctions from the nucleus infundibularis and nucleus supraopticus of Passer domesticus were studied in the electron microscope. A morphological asymmetry, represented by clustering of synaptic vesicles (of about 500 A diameter) at the presynaptic membrane and a thickening of and subsynaptic web formation at the postsynaptic membrane, was observed in the axo-dendritic and axo-somatic synapses, and to a smaller extent in the complex axo-dendritic junctions. In the hypothalamic synaptic axon terminals, as in the perikarya of nucleus infundibularis, dense-core vesicles of 800–1,000 A mean diameter were observed in addition to the synaptic vesicles, but the 800–1,000 A vesicles were not concentrated at the presynaptic membrane. Granules of 2,000 A mean diameter, observed in the neurons of nucleus supraopticus, were not seen in any hypothalamic synaptic terminals. Intersynaptic filaments and an intrasynaptic band lying in the intersynaptic cleft were frequently observed. The significance of the synaptic structures was discussed in relation to neurotransmission and to possible mechanisms by which perikarya of the nucleus infundibularis and nucleus supraopticus receive nervous information.

Development of fine structure of spinal cord in monkey fetuses. II. Pre‐reflex period to period of long intersegmental reflexes

AbstractA series of 13 macaque fetuses of estimated ovulation ages 42 to 51 days (17 to 41 mm) was subjected to behavioral analysis with umbilical cord intact (in and out of the amnion), and to light and electron microscopic analysis of the cervical spinal cord. This period is a critical one in terms of onset and early development of spontaneous activity and of cutaneous reflexes. Behavioral criteria, reinforced by well‐defined microscopic characteristics, suggested a grouping of specimens into three major stages, namely, a pre‐reflex group (Stage 1: 17–22 mm); a group in the period of onset of primitive spontaneous activity and of precocious local cutaneous reflexes in the trigeminal‐cervical region (Stage 2: 24 to 28 mm); and a group characterized by the development of more vigorous activity, spontaneous and otherwise, and of long intersegmental and crossed reflexes (Stage 3: 32 to 41 mm). The evidence suggests that junctional differentiation and occurrance of a cluster of agranular spheroid synaptic vesicles characterize the primitive synaptic knobs which appear on motoneuron dendrites coincident with earliest reflexes. Onset of active long intersegmental reflexes in Stage 3 is coincident with development of axosomatic synaptic knobs, and of F type (flattened) synaptic vesicles in a rapidly increasing fraction of all synaptic bulbs in the motoneuron neuropil. Total volume of all synaptic bulbs increases gradually as a fraction of the volume of the motoneuron neuropil until birth, but a steep increase in proportionate number of F type synaptic knobs occurs in Stage 3.

The electron microscopic localization of cholinesterase activity in the central nervous system of an insect, Periplaneta americana 1.

The distribution of esterase activity in the last abdominal ganglion, the connectives and the cereal nerves of the cockroach Periplaneta americana has been investigated cytochemically. Activity of an unspecific eserine-insensitive esterase (or esterases) has been found in glial elements in these regions of the nerve cord. In addition, sites of cholinesterase (eserine-sensitive) activity have been found in association with (a) the glial sheaths of the axons in the cereal nerves and connectives, (b) the glial folds encapsulating the neuron perikarya in the ganglion, and (c) in localized areas along the membranes of axon branches within the neuropile, often flanked by focal clusters of synaptic vesicles. These results are discussed with particular reference to the previously reported insensitivity of the insect nerve cord to applied acetylcholine, and to the probable existence of a cholinergic synaptic mechanism in the central nervous system of this insect.

The Fine Structure of Synapses in the Ciliary Ganglion of the Chick

Ciliary ganglia of chick embryos and newly hatched chicks were examined in the light and electron microscopes. Particular attention was given to the fine structure of calyciform synapses, which are characteristically found in ciliary ganglia of birds. The calyciform endings are characterized by large expansions of the presynaptic axons upon ganglion cells, and the terminal processes extend over a considerable area of the cell surface. Often, indeed they appear to envelop the cell. In the electron microscope image, the appositional membranes are separated by a space about 300 to 400 A wide; i.e., the synaptic cleft. At irregularly spaced regions, the appositional membranes show areas of increased density. The presynaptic processes contain clusters of synaptic vesicles, localized at these dense regions. Thus the fine structure complex typical of other synapses is evident. The unique structural features of this synapse are as follows: (a) The calyx or presynaptic terminal derives from a single axon, does not arborize, and terminates upon a single ganglion cell. Thus, unlike the classical bouton terminal, this represents an anatomical device for firing single cells by single axons. (b) The surface area in contiguity, i.e., the area of appositional membranes, is far more extensive than the bouton terminal. The fine structure of this synapse is compared with others, for example, the classical boutons terminaux and purely electrical synapses, in an attempt to correlate fine structure with function.