Morphological bases of interaction between motoneurons in spinal cord isolated from young rats using HRP techniques

The structure of connections between lumbar motoneurons was investigated in preparations of spinal cord isolated from young rats. This involved applying horseradish peroxidase to the ventral root and intracellular injection of the same enzyme into motoneurons. The possibility of dendro-dendritic, dendro-somatic, and somato-somatic contacts between motoneurons was shown up in light mocroscopy studies. Recurrent collaterals of motor axons were revealed and they are though to form contacts with dendrites and perikarya of the motoneurons. The findings obtained from morphological experiments are discussed in the light of data from electrophysiological analysis of motoneuronal postsynaptic potentials produced by ventral root stimulation.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 340–350, May–June, 1988.     

Various types of non-synaptic intercellular contacts in the developing brain of the rat

Peculiarities of ultrastructural organization and localization of early forms of avascular nonsynaptic types of junctions formed in 14-18-day-old rat embryos have been studied; cerebral structures different in their phylogenic relations (the sensomotor cortex and nucleus caudatus) are taken as an example. Five main types of nonsynaptic intercellular junctions have been revealed: desmosome-like, gap, symmetric, asymmetric and mixed junctions. They differ by their ultrastructural organization. These types of junctions make the main types of contacts: soma-somatic, dendro-somatic, dendro-dendritic, axo-somatic, axo-dendritic. Desmosomes form the greatest number of the contacts. The earliest and the most primitive are gap junctions; they, evidently, reflect functional activity of desmosome-like junctions. The mixed junctions, perhaps, reflect the developmental stages of the intercellular contacts of transition from one type of junctions into another. Localization peculiarities of the nonsynaptic intercellular contacts are demonstrated: glomerule-like formations, establishment of numerous contacts looking like a successive chain, and so on. For some other indices a longer period of intercellular contact formation in the nucleus caudatus is noted, comparing the sensomotor cortex, though the latter is a newer structural cerebral formation from the phylogenic point of view.     

Ultrastructure of interneuronal connections in the superior cervical sympathetic ganglion in cats

The structure of interneuronal synapses in the superior cervical sympathetic ganglion was studied in cats under normal conditions and after division of the cervical sympathetic nerves and removal of spinal ganglia T₁₂–L₂. A definite number of dendro-dendritic and dendro-somatic junctions is observed in the ganglion and most of them remained intact after operations of both types; they are probably synapses formed by dendrites of neurons located in the ganglion. Synapses of this sort participate in the formation of nest-like complexes, consisting of consecutive junctions of one neuron with several dendrites. The formation of such complexes may provide the anatomical basis for synchronization of rhythmic neuronal activity in the cellular glomeruli of the ganglion. The results of an ultrastructural study of dendro-dendritic junctions suggests that they are synaptic in nature. Some dendro-dendritic junctions underwent degeneration after both types of operation and are probably endings of neurons in spinal ganglia. Wide club-like structures, probably receptor endings, formed by dendrites of afferent neurons of spinal ganglia, also are found in the ganglion. These structures lie freely in the stoma of the ganglion or form contacts with axon terminals and dendrites of neurons located in the ganglion; some of them degenerate after removal of spinal ganglia T₁₂–L₂.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 299–306, May–June, 1981.     

Changing patterns of ganglion cell coupling and connexin expression during chick retinal development

We have used dye injection and immunolabeling to investigate the relationship between connexin (Cx) expression and dye coupling between ganglion cells (GCs) and other cells of the embryonic chick retina between embryonic days 5 and 14 (E5-14). At E5, GCs were usually coupled, via soma-somatic or dendro-somatic contacts, to only one or two other cells. Coupling increased with time until E11 when GCs were often coupled to more than a dozen other cells with somata in the ganglion cell layer (GCL) or inner nuclear layer (INL). These coupled clusters occupied large areas of the retina and coupling was via dendro-dendritic contacts. By E14, after the onset of synaptogenesis and at a time of marked cell death, dye coupling was markedly decreased with GCs coupled to three or four partners. At this time, coupling was usually to cells of the same morphology, whereas earlier coupling was heterogeneous. Between E5 and E11, GCs were sometimes coupled to cells of neuroepithelial morphology that spanned the thickness of the retina. The expression of Cx 26, 32, and 43 differed and their distribution changed during the period studied, showing correlation with events such as proliferation, migration, and synaptogenesis. These results suggest specific roles for gap junctions and Cx's during retinal development.     

Cell-type specific dendritic contacts between retinal ganglion cells during development.

The extent of a neuron's dendritic field defines the region within which information is processed. The dendritic fields of functionally distinct ON and OFF center retinal ganglion cells (RGCs) form separate mosaics across the retina. Within each mosaic, neighboring dendritic fields overlap by a constant amount, sampling the visual field with the appropriate coverage. Contact-mediated lateral inhibition between neighboring RGCs has long been thought to regulate both the extent and overlap of dendritic fields during development. Here we show that dendro-dendritic contact exists between developing RGCs and occurs in a manner that would regulate the formation of ON and OFF mosaics separately. Dye-filled neighboring ON and OFF ferret alpha RGCs were reconstructed using multiphoton microscopy. At all neonatal ages examined, we observed dendro-dendritic contacts between RGCs of the same sign (ON/ON; OFF/OFF), but never between cells of opposite signs (ON/OFF). Terminal dendrites of one cell often touched a dendrite of its neighbor as they intersected. In some instances, the distal dendrite of one cell formed a fascicle with the proximal process of its neighbor. Alpha cells did not form contacts with neighboring beta cells of the same sign. Together, these observations suggest that dendro-dendritic contact between RGCs is cell-type specific. Dendritic contacts were observed even before the alpha cell arbors were completely stratified, suggesting that cell-cell recognition may take place early in their development. For each cell type, the relative overlap of dendritic fields was constant with age, despite a two-fold increase in field area. We suggest that dendro-dendritic contacts may be sites of intercellular signaling that could regulate local extension of dendrites to maintain the relative overlap of RGCs within a mosaic during development.     

Ultrastructure of synapses of the parietal and visceral ganglia of Helix pomatia

Types of synaptic contacts and peculiarities of their distribution in the neuropil of the parietal and visceral ganglia of the edible snail (Helix pomatia) CNS have been studied electron microscopically. Ultrastructure of dendrites and axons has been identified. Dendrites with spinous++ processes, polymorphism of synaptic contacts have been revealed. Besides axo-axonal synapses, axo-dendritic synapses are demonstrated on the trunks and on the spinous processes of the dendrites, as well as dendro-dendritic and serial synapses. Unevenness in distribution of synaptic contacts is shown in the neuropil. The areas of the greatest concentration of the synapses are the "synaptic fields". Peculiarities in distribution of the synaptic contacts are demonstrated in the parietal and visceral ganglia.     

Avesicular intercellular contacts in the neuropil of the ganglia of the subpharyngeal complex and in the nerves of the edible snail

Intercellular interconnections in neuropil of dorsal ganglia in the subpharyngeal complex and nerves have been studied in a mature snail (Helix pomatia), dimensions of the shell--3.0 X 3.5 sm, after double fixation (glutaraldehyde and osmium tetraoxide) and treatment with phosphoric tungsten acid. Zones of the neuropil in the left pleural and parietal ganglia have been investigated--where the nerves leave ganglia and in the area of giant cells; neuropil of the central part of both parietal ganglia; central part of the visceral ganglion and nerves--left palial, intestinal and left pleural commissure. The most spread forms of avesicular intercellular contacts in the neuropil are symmetrical. Besides the latter, processes of the neuropil form fissural and desmosomal junctions. Localization of the avesicular contacts supposes presence of dendro-dendritic junctions of desmosomal and fissural types and axo-axonal ones--of desmosomal type. Various types of the contacts differ from each other by their ultrastructural organization.     

Neuronal ensembles in the cat motor cortex

Vertically oriented bundles of apical dendrites in the cat motor cortex were studied by methods of light and electron microscopy. The presence of desmosome-like and dendro-dendritic contacts in the bundles is regarded as the structural basis for electrotonic interaction between neurons in the same column. Axo-spinous "en passant" contacts between the descending axon of the pyramids of layer III and the apical dendrite of pyramids in layer V, possibly serving to regulate the activity of the principal cortical output elements, are described.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 455–458, September–October, 1976.     

Electrical Transmission between Mammalian Neurons is Supported by a Small Fraction of Gap Junction Channels

Electrical synapses formed by gap junctions between neurons create networks of electrically coupled neurons in the mammalian brain, where these networks have been found to play important functional roles. In most cases, interneuronal gap junctions occur at remote dendro-dendritic contacts, making difficult accurate characterization of their physiological properties and correlation of these properties with their anatomical and morphological features of the gap junctions. In the mesencephalic trigeminal (MesV) nucleus where neurons are readily accessible for paired electrophysiological recordings in brain stem slices, our recent data indicate that electrical transmission between MesV neurons is mediated by connexin36 (Cx36)-containing gap junctions located at somato-somatic contacts. We here review evidence indicating that electrical transmission between these neurons is supported by a very small fraction of the gap junction channels present at cell-cell contacts. Acquisition of this evidence was enabled by the unprecedented experimental access of electrical synapses between MesV neurons, which allowed estimation of the average number of open channels mediating electrical coupling in relation to the average number of gap junction channels present at these contacts. Our results indicate that only a small proportion of channels (~0.1?%) appear to be conductive. On the basis of similarities with other preparations, we postulate that this phenomenon might constitute a general property of vertebrate electrical synapses, reflecting essential aspects of gap junction function and maintenance.     

Dendro-dendritic and reciprocal synapses in the primate motor cortex.

Dendro-dendritic synapses have been observed infrequently in the deep layers of the motor cortex. The presynaptic dendrites are of a varicose type and themselves receive a considerable density of synapses both of the asymmetric and symmetrical type. The ultrastructure of the dendro-dendritic synapse itself shows the typical arrangement of presynaptic and postsynaptic membrane densities, often with presynaptic dense projections, and the membrane specialization is of the symmetrical type. There is the usual cleft containing electron-dense material between the presynaptic and postsynaptic profiles. The synaptic vesicles occur in a small cluster confined to a region close to the presynaptic membrane specialization; some of the vesicles are flattened and were shown by tilt analysis to be of the discoid type. Two examples were found of reciprocal dendro-dendritic synapses, both components being of the symmetrical type. A single axon terminal may make a synapse on to both dendrites involved in a dendro-dendritic synapse.     

Regulation of somatic firing dynamics by backpropagating dendritic spikes

Pyramidal cells of the apteronotid ELL have been shown to display a characteristic mechanism of burst discharge, which has been shown to play an important role in sensory coding. This form of bursting depends on a reciprocal dendro-somatic interaction, in which discharge of a somatic spike causes a dendritic spike, which in turn contributes a dendro-somatic current flow to create a depolarizing afterpotential (DAP) in the soma. We review here our recent work showing how the timing of this DAP influences the somatic firing dynamics, and how the degree of inactivation of dendritic Na⁺ currents can cause an increased delay between somatic and dendritic spikes. This ultimately allows the DAP to become more effective at increasing the excitability of the somatic spike generating mechanism. Further, this delay between dendritic and somatic spiking can be regulated by strongly hyperpolarizing GABA_B mediated dendritic inhibition, allowing the burst dynamics to fall under synaptic regulation. In contrast, a weaker, shunting inhibition due to GABA_A mediated dendritic inhibition can regulate the dendritic spike waveform to decrease the dendro-somatic current flow and the resulting DAP. We therefore show that the qualitative behaviour of an individual cell can depend on the degree of synaptic input, and the exact timing of events across the spatial extent of the neuron. Thus, our results serve to illustrate the complex dynamics that can be observed in cells with significant dendritic arborisation, a nearly ubiquitous adaptation amongst principal neurons.     

Ultrastructural organization of the synapses in ganglia of the hen intestinal nerve under various conditions of its activity

The interneuronal connections in ganglia of the caudal part of the hen intestinal nerve of Remak are presented as axodendritic and axosomatic synapses and symmetric axo-axonal, dendro-dendritic and axodendritic contacts, often forming complicated complexes. Under conditions of preliminary decentralization or under certain disturbances of nervous connections with the intestine, a part of synapses remains, and a part of them degenerates, this demonstrates participation of peripheral afferent neurons in formation of the synaptic apparatus of the ganglia mentioned. The axonal terminals differentiate by composition of the synaptic vesicles: some contain mainly light agranular vesicles, others--a large amount of granular ones. The characteristic peculiarities of the hen intestinal nerve ganglia, in contrast to analogous mammalian ganglia, are abundant axosomatic synapses in some neurons, and presynaptic terminals, containing a large number of granular vesicles.     

The morphology and fine structure of the giant interneurons of the wood cricket Nemobius sylvestris

The structural and ultrastructural characteristics of giant interneurons in the terminal abdominal ganglion of the cricket Nemobius sylvestris were investigated by means of cobalt and fluorescent dye backfilling and transmission electron microscopy.The projections of the 8 eight pairs of the biggest ascending interneurons (giant interneurons) are described in detail. The somata of all interneurons analyzed are located contralateral to their axons, which project to the posterior region of the terminal ganglion and arborise in the cercal glomerulus. Neuron 7-1a is an exception, because its arborisation is restricted to the anterior region of the ganglion. The fine structure of giant interneurons shows typical features of highly active cells. We observed striking indentations in the perineural layer, enabling the somata of the giant interneurons to be very close to the haemolymph. The cercal glomerulus exhibits a high diversity of synaptic contacts (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic), as well as areas of tight junctions. Electrical synapses seem to be present, as well as mixed synapses. The anatomical organization of the giant interneurons is finally discussed in terms of functional implications and on a comparative basis.     

Spatial organization and interneuronal relations in various areas of the neocortex in Cetacea

The data on pyramidal neurons joining in the parietal and temporal areas of the dolphin neocortex by means of apical dendrites fasciculi are presented. The fasciculi also contain dendrites of spindle-like and stellate cells. The vertical fasciculi of the dendrites unite neurons of the layer V and of the sublayer III2. In the sublayer III1, after dichotonic division of the apical dendrites, fasciculi of the second order are formed, to them the dendrites of the pyramidal neurons of the sublayer III1 and the layer II join. Several forms of the interneuronal contacts have been revealed: axo-dendritic, axo-spinous and dendro-dendritic. Synaptic complexes of the converged and divergent types have been shown. A suggestion is made on formation of larger neuronal modules++, having common afferent entrance and demonstrating selectivity to the stimulus properties.     

Neural circuit mechanisms of hierarchical sequence learning tested on large-scale recording data

The brain performs various cognitive functions by learning the spatiotemporal salient features of the environment. This learning requires unsupervised segmentation of hierarchically organized spike sequences, but the underlying neural mechanism is only poorly understood. Here, we show that a recurrent gated network of neurons with dendrites can efficiently solve difficult segmentation tasks. In this model, multiplicative recurrent connections learn a context-dependent gating of dendro-somatic information transfers to minimize error in the prediction of somatic responses by the dendrites. Consequently, these connections filter the redundant input features represented by the dendrites but unnecessary in the given context. The model was tested on both synthetic and real neural data. In particular, the model was successful for segmenting multiple cell assemblies repeating in large-scale calcium imaging data containing thousands of cortical neurons. Our results suggest that recurrent gating of dendro-somatic signal transfers is crucial for cortical learning of context-dependent segmentation tasks.     

Spontaneous activity level and spike responses of cortical neurons to local supply of amino acids for dendrites and soma

In guinea-pig parietal cortex slices, it was shown that neuronal spontaneous activity depended on dendro-somatic propagation of excitations evoked in dendrites. The functional dendritic properties are essentially non-uniform in the population of cortical neurons. Spike responses to direct soma activations are quite stable among neurons with various levels of spontaneous activity.     

Gap junctions between dendrites and somata of neurons in the primate sensori-motor cortex.

Gap junctions have been found infrequently between two dendrites or a dendrite and a cell soma in the deep layers of both the motor and somatic sensory cortices of the primate. At these junctions the outer leaflets of the plasma membranes of both profiles are intimately apposed with a gap of 2 nm between them which shows a structure of hexagonal subunits in tangential sections. These gap junctions occur mainly between the dendrites or dendrites and somata of large stellate cells but are also associated in some examples with a dendro-dendritic synapse and thus occur between large stellate dendrites and presynaptic dendrites; a desmosome may also occur in association with a gap junction and dendro-dendritic synapse. Gap junctions have been identified as sites of electrical transmission between cells in a number of sites and it is therefore suggested that some neurons in the sensori-motor cortex are electrotonically couples.     

Presynaptic dendrites and dendro-dendritic synapses in the superior tubercles of the rat quadrigeminum]

The types of dendro-dendritic synapses and their participation in the synaptic, organization of superficial layers of the quadrigeminum superior tubercles were studied electron microscopically. In addition to simple forms of dendro-dentritic synapses the reciprocal dendro-dendritic synapses were revealed. Presynaptic dendrites formed the synaptic fields and glomerules of the superficial grey layer. The terminals of optical, cortical fibres from the visual cortex and other types of terminals terminated on presynaptic dendrites.     

Dendritic Branch Intersections Are Structurally Regulated Targets for Efficient Axonal Wiring and Synaptic Clustering

Synaptic clustering on dendritic branches enhances plasticity, input integration and neuronal firing. However, the mechanisms guiding axons to cluster synapses at appropriate sites along dendritic branches are poorly understood. We searched for such a mechanism by investigating the structural overlap between dendritic branches and axons in a simplified model of neuronal networks - the hippocampal cell culture. Using newly developed software, we converted images of meshes of overlapping axonal and dendrites into topological maps of intersections, enabling quantitative study of overlapping neuritic geometry at the resolution of single dendritic branch-to-branch and axon-to-branch crossings. Among dendro-dendritic crossing configurations, it was revealed that the orientations through which dendritic branches cross is a regulated attribute. While crossing angle distribution among branches thinner than 1 µm appeared to be random, dendritic branches 1 µm or wider showed a preference for crossing each other at angle ranges of either 50°–70° or 80°–90°. It was then found that the dendro-dendritic crossings themselves, as well as their selective angles, both affected the path of axonal growth. Axons displayed 4 fold stronger tendency to traverse within 2 µm of dendro-dendritic intersections than at farther distances, probably to minimize wiring length. Moreover, almost 70% of the 50°–70° dendro-denritic crossings were traversed by axons from the obtuse angle’s zone, whereas only 15% traversed through the acute angle’s zone. By contrast, axons showed no orientation restriction when traversing 80°–90° crossings. When such traverse behavior was repeated by many axons, they converged in the vicinity of dendro-dendritic intersections, thereby clustering their synaptic connections. Thus, the vicinity of dendritic branch-to-branch crossings appears to be a regulated structure used by axons as a target for efficient wiring and as a preferred site for synaptic clustering. This synaptic clustering mechanism may enhance synaptic co-activity and plasticity.     

Recurrent axon collaterals of lumbar motor neurons in turtles

A combined morphophysiological study was made of connections between motoneurons on the superfused isolated lumbar spinal cord of Testudo horsfieldi. Postsynaptic potentials of motoneurons, followed by antidromic stimulations of ventral root filaments (VR-PSPs), were recorded intracellularly. Depolarizing VP-PSPs had short latencies (1.0-1.5 mc) and amplitudes in the range of 0.3-3.0 mV. At the constant stimulus intensity, the fluctuations of amplitudes were recorded. In some motoneurons, hyperpolarizing VP-PSRs with the latencies 2.5-3.0 mc were observed. A possible structural basis of VR-PSPs was studied by the horseradish peroxidase (HRP) method. After HRP application on thin ventral root filaments the retrograde staining of motoneurons revealed recurrent axon collaterals of labeled motoneurons. Three-dimensional computer reconstructions showed one to three collaterals given off by motoneuron axons. There were up to 19 points of branching in a single collateral. In some cases the full length of collateral trees reached 4.0 mm. The collateral branches had up to 72 "en passant" and terminal axon swellings. The swellings (presumed contacting boutons) were distributed in the ventral and intermedial gray matter and in the ventromedial while matter and revealed on motoneurons and inerneurons. These data suggest the participation of the motor axon collaterals in the motoneuron--motoneuron communication in the turtle spinal cord whereas only dendro-dendritic contacts had been discussed earlier.