Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Neuronal Connectivity”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,423 records · Page 79Linked to original sources
PubMed2003

Synchronization in networks of excitatory and inhibitory neurons with sparse, random connectivity.

In model networks of E-cells and I-cells (excitatory and inhibitory neurons, respectively), synchronous rhythmic spiking often comes about from the interplay between the two cell groups: the E-cells synchronize the I-cells and vice versa. Under ideal conditions-homogeneity in relevant network parameters and all-to-all connectivity, for instance-this mechanism can yield perfect synchronization. We find that approximate, imperfect synchronization is possible even with very sparse, random connectivity. The crucial quantity is the expected number of inputs per cell. As long as it is large enough (more precisely, as long as the variance of the total number of synaptic inputs per cell is small enough), tight synchronization is possible. The desynchronizing effect of random connectivity can be reduced by strengthening the E --> I synapses. More surprising, it cannot be reduced by strengthening the I --> E synapses. However, the decay time constant of inhibition plays an important role. Faster decay yields tighter synchrony. In particular, in models in which the inhibitory synapses are assumed to be instantaneous, the effects of sparse, random connectivity cannot be seen.

Animals↗
PubMed1998

Synaptic targets of pyramidal neurons providing intrinsic horizontal connections in monkey prefrontal cortex.

xũ I sLxxJ In monkey prefrontal cortex, the intrinsic axon collaterals of supragranular pyramidal neurons extend horizontally for considerable distances through the gray matter and give rise to stripe-like clusters of axon terminals (Levitt et al. [1993] J. Comp. Neurol. 338:360-376). Because understanding the functional role of these connections requires knowledge of their synaptic targets, we made injections of biotinylated dextran amine (BDA) into layer 3 of macaque prefrontal area 9 and examined the labeled intrinsic axon collaterals by electron microscopy. Labeled axon terminals formed exclusively asymmetric synapses, and 95.6% of the postsynaptic structures were dendritic spines, presumably belonging to other pyramidal neurons. The remaining postsynaptic structures were dendritic shafts, many of which had the morphological characteristics of local circuit neurons. The prefrontal injections also labeled associational projections that traveled through the white matter to terminate in other areas of prefrontal cortex. All of the synapses formed by these associational axons were asymmetric, and 91.9% were onto dendritic spines. The similarities in synaptic targets of the prefrontal intrinsic and associational axon terminals suggested that these projections might arise from the same neurons, an interpretation confirmed in dual label, retrograde tracing studies. To determine the specificity of the synaptic targets of these prefrontal connections, two additional comparisons were made. In the posterior parietal cortex (area 7a), 94.2% of the synapses furnished by BDA-labeled intrinsic collaterals of supragranular pyramidal neurons were also with dendritic spines. In contrast, only 75.6% of unlabeled asymmetric synapses in the prefrontal cortex were onto dendritic spines. These comparisons suggest that the axons of supragranular pyramidal neurons in primate association cortices are preferentially directed to specific targets. Finally, after injections of BDA, a small number of retrogradely labeled pyramidal neurons were observed within the anterogradely labeled clusters of intrinsic axon terminals. At the ultrastructural level, synapses between anterogradely labeled axon terminals and retrogradely labeled dendritic spines were identified. These findings suggest that reciprocal, monosynaptic connections may exist between pyramidal neurons located in different stripe-like clusters, providing a potential anatomical substrate for reverberating excitatory circuits within the primate association cortices.

Animals↗
PubMedDate not supplied

[Possibilities of detecting the plasticity of monosynaptic excitable connections during the analysis of impulse activity of neurons using cross-correlation histograms and Cox's method].

By the method of mathematical modelling, stationary irregular exogenous impulse activity of two monosynaptically connected neurones (N1 and N2; excitatory connection) was reproduced. The degree of impulse flows N1 and N2 dependence was estimated by the value P12, proportional to the height of crosscorrelation histogram peak, and by Cox's coefficient beta 12. The dynamics was studied of P12, beta 12 and of values P1* and P2*, proportional to mean interpulse intervals of N1 and N2, at changes of connection efficiency, neurones excitability and of influence of afferent synaptic bombardment on them. The following signs of modification of existing between N1 and N2 monosynaptic excitatory connection were established: 1) change of P12 in direction opposite to that of P1* and P2* change; 2) differently directed changes of beta 12 and P2* of postsynaptic N2.

Animals↗
PubMed1995

Bilateral connections from ventral group inspiratory neurons to phrenic motoneurons in the rat determined by cross-correlation.

We examined the functional connections from inspiratory neurons in the ventrolateral medulla to phrenic motoneurons in 11 Sprague-Dawley rats with intact vagi anaesthetized with sodium pentobarbital and paralysed with pancuronium bromide. Cross-correlation histograms were computed between the extracellular activity of 70 of these neurons and the discharge of the phrenic nerves; 38 contralateral only, 10 ipsilateral only and 22 bilaterally. A total of 22 peaks were detected in the cross-correlation histograms. These were classified as indicating common activation or synaptic connection according to their latencies to onset, and those suggesting connections as monosynaptic or paucisynaptic according to their half-amplitude widths. Nine peaks suggesting monosynaptic excitation of phrenic motoneurons were detected for 7 ventral group inspiratory neurons; 4 with contralateral connections and not tested for ipsilateral connections, 2 with bilateral connections, and 1 with a monosynaptic contralateral connection and a paucisynaptic ipsilateral connection. Four peaks suggesting paucisynaptic excitation of phrenic motoneurons were detected for 3 ventral group inspiratory neurons; 1 with bilateral connections, 1 with a contralateral connection only, and 1 (previously mentioned) with a paucisynaptic ipsilateral connection and a monosynaptic contralateral excitation. The remaining 9 peaks were classified as due to a common activation of the phrenic motoneurons and the ventral group inspiratory neurons. Cross-correlation histograms were also computed between the left and right phrenic nerve discharges in 5 rats and all displayed central broad peaks indicative of common activation, possibly due to excitation from bilaterally projecting medullary inspiratory neurons. We concluded that there is a substantial bilateral excitation of phrenic motoneurons by inspiratory neurons in the ventral medullary group of the rat.

Animals↗
PubMed2005

Fine-scale specificity of cortical networks depends on inhibitory cell type and connectivity.

Excitatory cortical neurons form fine-scale networks of precisely interconnected neurons. Here we tested whether inhibitory cortical neurons in rat visual cortex might also be connected with fine-scale specificity. Using paired intracellular recordings and cross-correlation analyses of photostimulation-evoked synaptic currents, we found that fast-spiking interneurons preferentially connected to neighboring pyramids that provided them with reciprocal excitation. Furthermore, they shared common fine-scale excitatory input with neighboring pyramidal neurons only when the two cells were reciprocally connected, and not when there was no connection or a one-way, inhibitory-to-excitatory connection. Adapting inhibitory neurons shared little or no common input with neighboring pyramids, regardless of their direct connectivity. We conclude that inhibitory connections and also excitatory connections to inhibitory neurons can both be precise on a fine scale. Furthermore, fine-scale specificity depends on the type of inhibitory neuron and on direct connectivity between neighboring pyramidal-inhibitory neuron pairs.

Action Potentials↗
PubMedDate not supplied

[Changes in functional connections between adjacent and distant neurons of the motor cortex in the cat during elaboration of an electrodefensive reflexes].

Dynamic reorganization of functional connections between neurones was observed in the motor cortex of cats in the process of electrodefensive conditioning. At a stable conditioned reflex, an increase of number of functional connections was observed more expressed between the neurones of distant microareas (within the range of 450 mcm investigated). In the process of conditioning sometimes one and the same neurone did not change its network properties within its own microarea but became noticeably activated in relation to the neurones of other microareas. The results obtained confirm the suggestion about a multilevel structure of neuronal integration in the process of learning, the first level consisting of consolidation of neighbouring neurones in the microarea, the second one consisting in consolidation of neurones of neighbouring microareas.

Action Potentials↗
PubMedDate not supplied

[Closing an intra-analyzer temporary connection in visual cortex neuronal element microsystems].

The work was aimed at studying the formation of microsystems of learning neurones in the cortical visual zone of immobilized cats during elaboration of a temporary connection based on a combined stimulation of two its points in acute experiment. 18.6 per cent of the cells developed responses to the stimulus which had been ineffective before conditioning. The most numerous group consisted of units which displayed a complex form of elaborated response. Presumably the untis observed are part of a system of learning neurones controlling the intraanalyser interactions by the mechanism of temporary connection.

Animals↗
PubMed1998

Synaptic connections between GABA-immunoreactive neurons and uniglomerular projection neurons within the antennal lobe of the cockroach, Periplaneta americana.

Synapses within deutocerebral glomeruli between GABA-immunoreactive, putatively inhibitory local interneurons and uniglomerular projection (output) neurons were demonstrated by means of a combination of GABA-immunogold labeling and intracellular HRP injection. The following connections were identified. 1) GABA-immunoreactive (GABAir) neurons form output synapses in a dyadic fashion onto a uniglomerular projection neuron and, in addition, a second GABAir neuron. A uniglomerular projection neuron in turn forms dyadic output synapses onto two GABAir neurons. Several examples of reciprocal connections have been identified between, first, GABAir neurons and uniglomerular projection neurons, and, second, GABAir neurons themselves. 2) GABAir neurons are serially connected with uniglomerular projection neurons via interposed GABAir processes. In some cases, also the first GABAir process of such a polysynaptic connection formed an output synapse onto the projection neuron. Such serial connections may form the structural basis for both, the feedforward inhibition as well as the feedforward disinhibition of uniglomerular projection neurons by GABAergic neurons. The reciprocal contacts may serve as control devices that modulate the output activity of the projection neurons.

Animals↗
PubMedDate not supplied

Cell type specificity of local cortical connections.

The function of the cerebral cortex is dependent on the precise organization of the circuits formed by its component neurons. The connections between neurons are not random, but are specific at multiple levels of organization. For example, each cortical area connects to only a selected subset of other areas and within any given area the axonal and dendritic arbors of individual neurons arborize in precise, layer-specific patterns (for review see Felleman & Van Essen, 1991; Callaway, 1998). In each layer there are dendrites from multiple cell types including cells with somata both within and outside that layer. Anatomical studies have shown that axons arborizing in a particular cortical layer can connect selectively onto dendrites of some cell types in the layer, while avoiding the dendrites of other cell types (e.g. Freund & Gulyas, 1991; Hornung & Celio, 1992; Staiger et al., 1996). These cell type specific connections are, however, difficult to elucidate with anatomical methods, so the frequency of such specificity has remained elusive. Recent experimental methods combining intracellular recording of single neurons with focal neuronal stimulation by uncaging glutamate with light ("photostimulation") have made the analysis of cell type specific cortical connections more tractable. These studies show that cell type specificity of connections is prevalent in cortex. Here I review photostimulation-based studies investigating the laminar sources of cortical input to distinct cell types in the visual and somatosensory cortices of rats and the primary visual cortex of monkeys.

Animals↗
PubMed1987

Formation, maintenance, and functional uncoupling of connections between identified Helisoma neurons in situ.

Previous work with identified Helisoma neurons has characterized an array of neuroplastic responses to axotomy that include the generation of new neuritic outgrowth, the reinnervation of target organs, and the formation of new electrical synapses. These responses are not random, but rather occur in a precise, predictable manner under a variety of culture conditions. The present investigation demonstrates that specific identified neurons display similar neuroplastic "behavior" within the living animal. In response to in situ nerve crushes, neurons B4 and B5 generate new neuritic outgrowth, neuron B4 functionally reinnervates the salivary glands, and new electrical synapses form between the left and right neurons B5. The in situ paradigm employed in the present experiments made it possible to examine responses to axotomy over longer periods than in earlier studies with organ cultures. New B5R-B5L connections, previously found to be stable over the short term in culture, gradually decreased in strength in situ, and the cells effectively uncoupled by 8 weeks after axotomy. This uncoupling did not depend upon target reinnervation and occurred in the continued presence of neurites in the buccal commissure. It is suggested that the stability of new connections is related to whether the connection previously existed in the unperturbed nervous system. The similarities between the ability of identified neurons to grow and to form synaptic connections in situ and in culture suggests that neurons are endowed with a specific program of regenerative responses that can be expressed reliably in a wide variety of environmental conditions.

Animals↗
PubMed1988

Peptide B induction of bag-cell activity in Aplysia: localization of sites of action to the cerebral and pleural ganglia.

The bag cells of the marine mollusc Aplysia are model neuroendocrine cells involved in the initiation of egg laying and its associated behaviors, but the natural stimulus triggering bag-cell activity is not known. The atrial gland of A. californica, an exocrine organ in the reproductive tract, contains two structurally related peptides (A and B) which can induce an afterdischarge in vitro, and these peptides can be used to probe the central nervous system for sites where extrinsic excitatory input onto the bag-cell system might occur. These sites were identified in a series of lesion and ablation experiments. The entire central nervous system was removed from an animal and placed in a chamber with two compartments which could be independently perfused, allowing peptides (atrial gland extract or purified peptide B) to be selectively applied to specific regions of the nervous system while bag-cell activity was monitored using extracellular suction electrodes. Afterdischarges were consistently and reliably induced when peptides were applied to the cerebral ganglion, the pleural ganglia, the cerebropleural connectives, or the rostral 10-15% of the pleurovisceral connectives, provided that an intact neuronal pathway connected the site of peptide application with the bag cells. In contrast, afterdischarges were never observed when peptides were selectively applied to the buccal or pedal ganglia and only rarely observed when applied to the abdominal ganglion and caudal pleurovisceral connectives. These results support the hypothesis that bag-cell processes and/or neuron(s) presynaptically excitatory to the bag cells are located in the pleural and cerebral ganglia and narrow the region of the central nervous system where the critical initiator element(s) can be identified.

Amino Acids↗
PubMed1983

Fine structural studies on a type of somatostatin-immunoreactive neuron and its synaptic connections in the rat neostriatum: a correlated light and electron microscopic study.

Somatostatin-immunoreactive neurons in the rat neostriatum were studied by correlated light and electron microscopy using the peroxidase-antiperoxidase immunocytochemical technique. Immunoreactivity was localized in neuronal perikarya and processes. The perikarya were of spindle or fusiform shape (average length 16.9 microns) and were found in all parts of the neostriatum. From each neuron there arose two to four straight immunoreactive dendritelike processes, which could frequently be traced as far as about 130 microns from their perikaryon. Immunoreactive varicose axonlike processes were occasionally found, some of which were proximal axons of identified immunoreactive cells. Nine of the light microscopically identified neurons showing somatostatin-immunoreactivity were studied in the electron microscope; two of them had proximal axons with varicosities. Each neuron had an oval or elongated nucleus, which was always indented. These morphological features correspond well to those of certain "medium-size aspiny" neurons classified by Golgi studies. Although the immunoreactive endproduct was diffusely located throughout the neuron, it was characteristically located in the saccules and large granules (diameter 133 nm) of the Golgi apparatus, and large immunoreactive vesicles of similar size to those in the Golgi apparatus frequently occurred in all parts of axon. Very little synaptic input was found on the perikarya and dendrites of somatostatin-immunoreactive neurons. The perikarya and proximal dendrites received both symmetrical and asymmetrical synaptic input, while the distal dendrites usually received boutons that formed asymmetrical contacts. The somatostatin-immunoreactive boutons contained pleomorphic electron-lucent vesicles (diameter 39.3 nm) and a few large immunoreactive granular vesicles; these boutons always formed symmetrical synapses. Their postsynaptic targets were dendritic shafts, spines, and unclassified dendritic profiles. On the other hand, the varicosities of identified proximal axons of somatostatin-positive neurons did not form typical synapses, since they lacked clusters of small vesicles, but some of them were in direct apposition (via membrane specializations) to unlabelled perikarya or dendrites. It is concluded that somatostatin is a useful marker for a particular type of neuron in the neostriatum. The presence of somatostatin immunoreactivity in synaptic boutons is consistent with the view that somatostatin could be a neurotransmitter in the neostriatum.

Animals↗
PubMed1987

Physiological studies of brainstem reticular connectivity. I. Responses of mPRF neurons to stimulation of bulbar reticular formation.

The connectivity between medial pontine reticular formation (mPRF) and bulbar reticular formation (BRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of BRF in unanesthetized, undrugged cats. There was a very high percentage (75-90%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of 3 BRF areas: the magnocellular tegmental field (FTM), the bulbar gigantocellular tegmental field (BFTG), and bulbar lateral tegmental field (BFTL). The type of initial orthodromic response produced in mPRF neurons by BRF stimulation was predominantly (75-95%) a monosynaptic excitatory PSP (EPSP) which was characterized by a rapid rise time, a nearly constant latency, and often led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (IPSPs) was much lower for FTM (12.3%), for BFTG (12.5%) and was zero for BFTL. While microstimulation techniques alone cannot differentiate between excitation of fibers of passage and neuronal somata, the very high percentage of initial EPSPs in our data and the anatomical evidence for dense BRF to mPRF neuronal projections as compared with less dense projections from fibers passing through BRF to mPRF suggest that excitatory BRF-mPRF connections are predominant. The high degree of connectivity between BRF and mPRF may furnish an important substrate for functional interaction. Comparison of the mPRF neuronal population that was not antidromically activated by FTM microstimulation vs the mPRF neuronal population that was antidromically activated from FTM and also studied for orthodromic responsiveness showed no statistically significant differences between these populations on the parameters of percentage of monosynaptic input, monosynaptic initial EPSPs, monosynaptic initial IPSPs and presence of a PSP with a latency of less than 5 ms. For BRF connectivity this suggests an identity of mPRF input and output neurons with respect to synaptic response properties.

Animals↗
PubMed1986

Synaptic connections of callosal projection neurons in the vibrissal region of mouse primary motor cortex: an electron microscopic/horseradish peroxidase study.

Reciprocal axonal projections between homotypic areas of the vibrissal region of mouse primary motor cortex (MsI) (Porter and White: Neurosci. Lett. 47:37-40, '84) suggested the existence of reciprocal synaptic connections between callosal projection neurons and callosal afferents. In the present study, the retrograde transport of horseradish peroxidase (HRP) was combined with lesion-induced degeneration to identify synapses between callosal afferents and callosal neurons in the corresponding region of the contralateral cortex. The procedure was as follows: MsI was injected with HRP and aspirated on the following day. After 4 days, the animals were perfused and motor cortex was processed for HRP according to a variation of the Adams (Brain Res. 176:33-47,'77) technique, and postfixed in OsO4. The methods used consistently filled fine dendritic branches and spines with dense reaction product, thus allowing examination of synaptic contacts with these processes. All callosal projection neurons were identified as pyramidal neurons, having somata in cortical layers II/III and V. Labeled cells from each of the two levels were prepared for electron microscopy, and that part of each cell's apical dendrite that traversed the superficial cortical layers, where most callosal axons terminate, was cut in an unbroken series of thin sections. Micrographs were taken of all labeled profiles in each thin section, and tracings of the profiles were assembled to reconstruct the apical dendrites. Data on the distribution, type, and amount of callosal and other synapses with the shaft and spines of the apical dendrites were obtained by examining the reconstructions. In addition, profiles of basal dendrites of layer II/III cells were examined in thin sections to ascertain the numbers of callosal and other synapses formed with their shafts and spines. The proportion of synapses that each dendrite formed with callosal axon terminals was compared to the concentration of callosal afferents in the neuropil. Dendrites of both layer II/III and layer V pyramidal cells synapsed with callosal axon terminals. The apical and basal dendrites of layer II/III neurons formed a similar proportion of their synapses with callosal afferents, and this was similar to the concentration of callosal synapses in the surrounding neuropil. Segments of apical dendrites belonging to layer V and layer II/III neurons course through neuropil containing nearly the same concentration of degenerating callosal terminals, but the layer V cells form fewer callosal synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1997

Quantitative and neurogenic analysis of the total population and subpopulations of neurons defined by axon projection in the superficial dorsal horn of the rat lumbar spinal cord.

The total neuron population of the superficial dorsal horn (SDH), i.e., laminae I and II, was quantitated in Nissl preparations of spinal segment L1 in the rat. Subpopulations of the SDH, defined by axon projection, were quantitated following strategic intraspinal injections of dual retrograde tracers (Fluoro-Gold and true blue). These methods were used in conjunction with [3H]thymidine (delivered in utero) autoradiography for neurogenic pattern analysis. Following stereological correction, each dorsal horn in spinal segment L1 contained 11 neurons in lamina I and 42.6 neurons in lamina II per 10-microm transverse section. Neurons with long projections, i.e., neurons with projections rostral to spinal segment T5, were only slightly more numerous in lamina I than in lamina II. These neurons made up 34% of the total neuron population in lamina I and 7.0% in lamina II. Most of these neurons did not demonstrate descending connections, and many (presumed supraspinal projection neurons) did not demonstrate short, ascending, intersegmental connections. Neurons with short propriospinal projections, i.e., neurons with connections caudal to spinal segment T5, made up approximately half of the total neuron population in both lamina I and lamina II: 55% and 52%, respectively. Of these, 79% had both short ascending and descending projections; the remaining 21% had only descending projections. Neurons that were not labeled with retrograde tracers (presumed local circuit cells) represented 11% of the neurons in lamina I and 41% in lamina II. Neurogenesis in the SDH proceeded along an axon-length gradient, whereby neurons with the longest axons completed neurogenesis first, and those with the shortest completed neurogenesis last. The generation of both propriospinal and supraspinal projection neurons began on embryonic day 13 (E13). Nearly equal numbers of neurons in this group were generated in laminae I and II through E14. On E15, neuron production slowed in lamina I and accelerated in lamina II as local circuit neurons and the remaining propriospinal neurons were generated. Neuron production ceased simultaneously in both lamina I and lamina II on E16.

Animals↗
PubMedDate not supplied

Cytoplasmic dynein and microtubule transport in the axon: the action connection.

The neuron uses two families of microtubule-based motors for fast axonal transport, kinesin, and cytoplasmic dynein. Cytoplasmic dynein moves membranous organelles from the distal regions of the axon to the cell body. Because dynein is synthesized in the cell body, it must first be delivered to the axon tip. It has recently been shown that cytoplasmic dynein is moved from the cell body along the axon by two different mechanisms. A small amount is associated with fast anterograde transport, the membranous organelles moved by kinesin. Most of the dynein is transported in slow component b, the actin-based transport compartment. Dynactin, a protein complex that binds dynein, is also transported in slow component b. The dynein in slow component b binds to microtubules in an ATP-dependent manner in vitro, suggesting that this dynein is enzymatically active. The finding that functionally active dynein, and dynactin, are associated with the actin-based transport compartment suggests a mechanism whereby dynein anchored to the actin cytoskeleton via dynactin provides the motive force for microtubule movement in the axon.

Animals↗
PubMed1973

Kitten visual cortex: short-term, stimulus-induced changes in connectivity.

Single neurons in the kitten visual cortex can be induced to increase their responsiveness to a repeated stimulus applied while the neurons are under observation. These short-term changes are in the same direction as the permanent modifications produced in whole populations of neurons following environmental manipulations during the "critical period" of cortical development, but are less pronounced and probably transient.

Age Factors↗