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PubMed1991

Silent synaptic connections and their modifiability.

Comparison of the two afferent systems illustrates certain features common to synaptic transmission as well as differences that might be important for synaptic plasticity. Transmission at both the inhibitory and excitatory connections is satisfactorily described by a simple binomial model that considers the average probability of release to be the same at each active site, although it should be stressed that the best evidence derives from the first set of afferents. Another similarity between the two systems is that short-term changes in synaptic efficacy, namely, facilitation and depression, appear to be due to changes in p. We previously suggested that both phenomena occur during repetitive stimulation, with the dominant effect depending upon the initial probability of release. It remains to be seen if depression dominates at other inhibitory connections, although it is already clear that one cannot generalize about excitation, because some excitatory junctions have an initial high p and exhibit a marked depression rather than the facilitation described here. We have found no evidence for the notion that some synapses within a connection may be silent. That idea has been proposed, but not proven, for other synaptic connections in the vertebrate central nervous system. Indeed, it will be difficult to assess as long as quantal release cannot be reliably detected at these junctions, and morphological confirmation at the ultrastructural level will also be required. On the other hand, evidence from a few peripheral junctions where one presynaptic afferent establishes hundreds of contacts with its target cell, does suggest the possibility of silent synapses, or at least an extremely low probability of release in those cases. These situations may correspond to extremes of our finding that as the number of release sites increases, p decreases. Regardless, the inverse relation between n and p suggests caution should be exercised in interpreting data indicating that synaptic plasticity is associated with increased numbers of synapses between two cells. Although we have not detected silent synapses within a transmitting connection, we have observed chemically silent connections between neurons, and the evidence reviewed here suggests transmission may be blocked postsynaptically, as with the inhibitory connections, or presynaptically, as with the excitatory ones. Although the underlying mechanisms are only partially elucidated, it is also clear that such connections can be switched into a transmitting mode. Consequently, they may provide a significant reserve that might well become functional in different behavioral states or in response to certain patterns of activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed2005

Factors determining the precision of the correlated firing generated by a monosynaptic connection in the cat visual pathway.

Across the visual pathway, strong monosynaptic connections generate a precise correlated firing between presynaptic and postsynaptic neurons. The precision of this correlated firing is not the same within thalamus and visual cortex. While retinogeniculate connections generate a very narrow peak in the correlogram (peak width < 1 ms), the peaks generated by geniculocortical and corticocortical connections have usually a time course of several milliseconds. Several factors could explain these differences in timing precision such as the amplitude of the monosynaptic EPSP (excitatory postsynaptic potential), its time course or the contribution of polysynaptic inputs. While it is difficult to isolate the contribution of each factor in physiological experiments, a first approximation can be done in modelling studies. Here, we simulated two monosynaptically connected neurons to measure changes in their correlated firing as we independently modified different parameters of the connection. Our results suggest that the precision of the correlated firing generated by strong monosynaptic connections is mostly determined by the EPSP time course of the connection and much less by other factors. In addition, we show that a polysynaptic pathway is unlikely to emulate the correlated firing generated by a monosynaptic connection unless it generates EPSPs with very small latency jitter.

Animals↗

PubMed1997

Connectivity in human cancellous bone by three-dimensional magnetic resonance imaging.

Bone architecture affects strength and resistance to fracture. Trabecular connectivity is now recognized as an important measure of bone quality, and could be useful as an indicator of the osteoporotic condition, as well as a tool for measuring the effectiveness of therapies. We have applied three-dimensional magnetic resonance imaging microscopy to human cancellous bone biopsies, and report the results of connectivity measurements. Sample heterogeneity was examined on the basis of connectivity density for subvolumes. The choice of examination volume had a significant effect on connectivity density measurements, but sample volumes greater than 100 mm3 were found to give stable results. Connectivity density was strongly correlated with nodal density, and two-dimensional estimates of connectivity, but not bone volume fraction. Repeat measurement at constant resolution (69 x 138 x 109 microns, signal-to-noise ratio of about 35) showed reproducibility of about 5% for connectivity density. Our most recent results have significantly enhanced resolution (69 x 69 x 43 microns); bone fraction remained constant, but apparent connectivity density is greater.

Algorithms↗

PubMed2004

Functional connectivity in layer IV local excitatory circuits of rat somatosensory cortex.

There are two types of excitatory neurons within layer IV of rat somatosensory cortex: star pyramidal (SP) and spiny stellate cells (SS). We examined the intrinsic properties and connectivity between these neurons to determine differences in function. Eighty-four whole cell recordings of pairs of neurons were examined in slices of rat barrel cortex at 36 +/- 1 degrees C. Only minimal differences in intrinsic properties were found; however, differences in synaptic strength could clearly be shown. Connections between homonymous pairs (SS-SS or SP-SP) had a higher efficacy than heteronymous connections. This difference was mainly a result of quantal content. In 42 pairs, synaptic dynamics were examined. Sequences of action potentials (3-20 Hz) in the presynaptic neuron consistently caused synaptic depression (E2/E1=0.53+/-0.18). The dominant component of depression was release-independent; this depression occurred even when preceding action potentials had failed to cause a response. The release-dependence of depression was target specific; in addition, release-independence was greater for postsynaptic SPs. In a subset of connections formed only between SP and any other cell type (43%), synaptic efficacy was dependent on the presynaptic membrane potential (Vm); at -55 mV, the connections were almost silent, whereas at -85 mV, transmission was very reliable. We suggest that, within layer IV, there is stronger efficacy between homonymous than between heteronymous excitatory connections. Under dynamic conditions, the functional connectivity is shaped by synaptic efficacy at individual connections, by Vm, and by the specificity in the types of synaptic depression.

Algorithms↗

PubMed1988

Connectivity pattern of the cercal-to-giant interneuron system of the American cockroach.

1. The pattern of connectivity between identified cercal afferents and the three largest giant interneurons (GIs 1, 2, and 3) of the American cockroach was investigated by intracellular methods. These three GIs all have different directional response sensitivities and appear to be especially important in initiating the short latency escape behavior of the American cockroach. 2. One of the interneurons, GI 1 responds to wind from all four quadrants of space about the animal. However, it clearly shows a greater ipsilateral versus contralateral (relative to the GI's axon within the nerve cord) wind sensitivity. In contrast, the directional sensitivity of GI 2 is more nearly bilaterally symmetrical. Both of these interneurons receive excitatory synaptic input from the sensory cells of the nine most prominent columns (a, d, g, f, h, i, k, l, and m) of filiform hairs of the ipsilateral cercus. 3. The nine ipsilateral inputs all made roughly equivalent strength excitatory connections with GI 1. The connectivity pattern to GI 2 was the same as that to GI 1 except that the connection strength for two of the nine columns, h and i, was substantially stronger to GI 2 than to GI 1. The remaining seven sensory columns all make equivalent strength connection with GI 2. 4. Only select columns of contralateral sensory cells made synaptic connection with GIs 1 and 2. All detectable connections produced subthreshold depolarizations. 5. The response curve of GI 3 is more sharply restricted in space than that of either GI 1 or 2 and this interneuron only responds to wind stimuli originating from in front of the animal. GI 3 received excitatory synaptic input only from ipsilateral columns d, f, g, i, and k, all of which have their best excitatory directions well within the boundaries of the response curve of GI 3. Columns a and l with best excitatory directions near the edges of the response curve of GI 3 made no detectable connection. The remaining two columns (h and m) with best excitatory directions well outside the boundaries of the response curve of GI 3 provided inhibitory input. 6. GI 3 received synaptic input from contralateral columns d, f, g, h, i, k, and m.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1997

Synaptic connectivity in cultured hypothalamic neuronal networks.

We have developed a novel approach to analyze the synaptic connectivity of spontaneously active networks of hypothalamic neurons in culture. Synaptic connections were identified by recording simultaneously from pairs of neurons using the whole cell configuration of the patch-clamp technique and testing for evoked postsynaptic current responses to electrical stimulation of one of the neurons. Excitatory and inhibitory responses were distinguished on the basis of their voltage and time dependence. The distribution of latencies between presynaptic stimulation and postsynaptic response showed multiple peaks at regular intervals, suggesting that responses via both monosynaptic and polysynaptic paths were recorded. The probability that an excitatory event is transmitted to another excitatory neuron and results in an above-threshold stimulation was found to be only one in three to four. This low value indicates that in addition to evoked synaptic responses other sources of excitatory drive must contribute to the spontaneous activity observed in these networks. The various types of synaptic connections (excitatory and inhibitory, monosynaptic, and polysynaptic) were counted, and the observations analyzed using a probabilistic model of the network structure. This analysis provides estimates for the ratio of inhibitory to excitatory neurons in the network (1:1.5) and for the ratio of postsynaptic cells receiving input from a single GABAergic or glutamatergic neuron (3:1). The total number of inhibitory synaptic connections was twice that of excitatory connections. Cell pairs mutually connected by an excitatory and an inhibitory synapse occurred significantly more often than predicted by a random process. These results suggests that the formation of neuronal networks in vitro is controlled by cellular mechanisms that favor inhibitory connections in general and specifically enhance the formation of reciprocal connections between pairs of excitatory and inhibitory neurons. These mechanisms may contribute to network formation and function in vivo.

Animals↗

PubMed2000

Monosynaptic connections between identified A and B photoreceptors and interneurons in Hermissenda: evidence for labeled-lines.

The cellular and synaptic organization of the eye of the nudibranch mollusk Hermissenda is well-documented. The five photoreceptors within each eye are mutally inhibitory and can be classified into two types: A and B based on electrophysiological and anatomical criteria. Two of the three type B and two type A photoreceptors can be further identified according to their medial or lateral positions within each eye. In addition to reciprocal synaptic connections between photoreceptors, photoreceptors also project to second-order neurons in the cerebropleural ganglion. The second-order neurons receive convergent synaptic input from two additional sensory pathways; however, it has not been previously established if lateral A, lateral B, or medial B photoreceptors converge onto the same second-order neurons. To determine the specific synaptic organization of these components of the visual system, we have examined monosynaptic connections between identified lateral and medial type A and B photoreceptors and second-order cerebropleural (CP) interneurons. We found that monosynaptic connections between identified lateral A and lateral and medial B photoreceptors and CP interneurons follow a labeled-line principle. Illumination of the eyes or extrinsic depolarizing current applied to identified photoreceptors evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different CP interneurons. The PSPs in CP interneurons followed one-for-one spikes in the photoreceptors and could be elicited in artificial seawater solutions containing high divalent cations. Identified photoreceptors projected to more than one CP interneuron and expressed both excitatory and inhibitory connections with the different CP interneurons. In examples where a monosynaptic connection between a lateral B photoreceptor and a CP interneuron was identified, lateral A, medial A, or medial B photoreceptors did not project to the same CP interneuron. Moreover, when connections between medial B and CP interneurons were identified, lateral A, medial A, and lateral B connections were not found to project to the same CP interneuron. Similar results were obtained for a lateral A and CP interneuron connection. These results indicate that divergent labeled-lines exist between specific photoreceptors and second-order CP interneurons and potential convergence of synaptic input from primary and secondary elements of the visual system must occur at sites that are postsynaptic to the CP interneurons.

Action Potentials↗

PubMed1992

Phylogenetic changes in the connections of the lateral preglomerular nucleus in ostariophysan teleosts: a pluralistic view of brain evolution.

The connections of the lateral preglomerular nucleus were examined with the fluorescent tracer DiI in a cyprinid, a characin, a catfish and a gymnotoid species. In all of these taxa the lateral preglomerular nucleus receives inputs from the torus semicircularis and projects to the telencephalon. Cytoarchitectural and topological data further support the hypothesis that the lateral preglomerular nucleus is homologous among the species examined. A cladistic analysis of species differences, however, reveals that some connections of the lateral preglomerular nucleus have changed dramatically during the course of evolution. Some of the cell groups that are interconnected with the lateral preglomerular nucleus have increased in size, while others have decreased. Increases in the number of subdivisions of cell groups are frequently associated with size increases, but they may also be associated with size decreases. Some of the connections between cell groups have increased in strength, whereas others have decreased in strength, but these connectional changes bear no simple relationship to phylogenetic changes in the size of the cell groups. Phylogenetic increases in the number of subdivisions within a cell group may be associated with selective gains of connections, but they may be associated also with selective losses of connections or with no major changes in connections. Furthermore, several cell groups and connections appear to have evolved 'de novo', whereas others have disappeared during the course of evolution. Finally, the individual cell groups within this system of interconnected nuclei have changed largely independently of one another. These findings establish that brain evolution is not dominated by a single type of phylogenetic change.

Animals↗

PubMed1985

The size-principle: a deterministic output emerges from a set of probabilistic connections.

Orderly, size-related recruitment of motoneurones (MNs) illustrates how hundreds of cells operate as a functional entity to produce a highly deterministic output. The coherent action of the pool depends largely on the distribution of input to its members through the connections of afferent fibres. Three types of spike-triggered averaging have been utilized to study these connections. Impulses in individual Ia afferents elicit excitatory postsynaptic potentials ('single-fibre' EPSPs) in about 80% of homonymous MNs. After spinal transection 100% may respond, suggesting that Ia fibres project anatomically to all homonymous MNs. Functionally absent Ia connections are due to transmission failure. The sum of all the EPSPs elicited in a large population of MNs was recorded electrotonically from ventral roots. The mean amplitudes of these 'postsynaptic population potentials' (PSPPs) were correlated with the conduction velocities (CVs) of the Ia or spindle group II fibres. The greater the distance between the spinal entry point of a Ia fibre and the ventral root, the smaller was the PSPP. Tape recording of multiple afferents and the responses of up to 24 MNs permitted study of as many as 264 possible connections in a single, acute experiment. Construction of wiring diagrams and connectivity matrices from the data showed that functional connectivity is influenced by afferent fibre size, the effect of branching on fibre size, MN size and probably transmission failure, but that on a cell-to-cell level, connectivity does not follow strict, deterministic rules. The results raise the question of how probabilistic connections between afferent fibres and MNs give rise to deterministic outputs from the whole pool.

Animals↗

PubMed1997

Diversity of connections of the temporal neocortex with amygdaloid nuclei in the dog (Canis familiaris).

Reciprocal connections of amygdaloid nuclei with the temporal neocortex in the dog were investigated. Injections of fluorescent tracers and BDA into particular temporal areas were made in eleven dogs. The topographical arrangement of connections and variations in their density differentiate the temporal neocortex in the dog into a few regions. Among them, the cortex involving the anterior part of the ectosylvian gyrus did not send any amygdalopetal projection. The middle ectosylvian, dorsal zone of the posterior ectosylvian and the anterior part of the Sylvian gyrus were weakly connected with the amygdala. The cortical region involving the ventral zone of the posterior ectosylvian and composite posterior areas, as well as posterior Sylvian gyrus, was characterized by profuse connections with the amygdaloid complex. Cortico-amygdaloid connections originate in the wide cortical area of the auditory cortex of the middle and dorsal part of the posterior ectosylvian gyrus as well as in the auditory association cortex located in the ventral ectosylvian, composite posterior and posterior Sylvian gyri. The connections showed a dorso-ventral gradient of increasing density, in the direction of association fields. The most substantial projection taking rise from the ectosylvian posterior and posterior composite gyri terminated preferentially in the pericapsular sector of the lateral amygdaloid nucleus and, to a lesser degree, in its medial sector. Terminals of connections originating in the Sylvian gyrus occupied preferentially the intermediate part of the lateral nucleus, slightly more medially than that from the ectosylvian and posterior composite areas. Additionally, axonal terminals derived from the composite posterior and Sylvian posterior areas were observed in the basal parvocellular and magnocellular nuclei. Neocortical projections were reciprocated by amygdalofugal connections with two exceptions: the basal magnocellular nucleus was distinguished by a substantial amygdalofugal projection to the temporal neocortex focused on the dorsal Sylvian gyrus, and the central nucleus of the amygdala, in contrast, received an exclusively corticofugal projection.

Amygdala↗

PubMedDate not supplied

Surgical considerations of univentricular heart with total anomalous pulmonary venous connection.

Out of the 600 patients undergoing univentricular repair during the last 11 years, 20 children had associated total anomalous pulmonary venous connection. The objective was to outline the clues to establish the diagnosis of this rare disease combination and the various surgical options available to manage the same. Bidirectional Glenn, bilateral bidirectional Glenn, total cavopulmonary connection and atriopulmonary connection were performed in combination with rechannelling of various types of total anomalous pulmonary venous connection in 20 children aged 6 months to 36 months (mean +/- SD 17.65 +/- 9.02 months). Diagnosis could be established pre-operatively in only 13 (65%) patients. Out of 6 early deaths (30%), 4 were directly attributable to missed diagnosis. No late deaths occurred over a follow-up period ranging from 1 month to 132 months. None of the surviving children required reoperation and all are in NYHA functional class I. Doppler echocardiography of the surviving children revealed unrestricted atrio/cavopulmonary anastomosis and pulmonary vein to atrium connection in all survivors. Our own experience, coupled with a review of the literature, indicates that a missed diagnosis increases the hospital mortality. Cross sectional 2D echocardiography is a superior method of detection of associated total anomalous pulmonary venous connection compared to angiocardiography. Exclusion of the diagnosis of anomalous pulmonary venous connection is imperative in all univentricular hearts pre-operatively and on operation table. Failure to recognise this disease combination results in formation of a closed systemic circuit after bidirectional Glenn or a modified Fontan of connection and is lethal as happened in our early experience. It is suggested that one-stage Fontan operation should be performed only if other criteria for Fontan procedure are satisfied.

Cardiac Surgical Procedures↗

PubMed2001

Postnatal growth of intrinsic connections in mouse barrel cortex.

Surprisingly little is known about the development of connections within a functional area of the cerebral cortex. We examined the postnatal growth of connections in mouse barrel cortex during the second and third weeks after birth, coinciding with the period of rapid synaptogenesis that occurs just after the barrels first form. A barrel is a group of neurons in layer 4 of somatosensory cortex that is part of a cortical column. Each whisker/barrel column is linked anatomically and functionally to a homotopic whisker on the contralateral face. Radial groups of cortical neurons were labeled with the neuronal tracer biotinylated dextran amine in mice ranging in age from postnatal day 8 (P8; P0 is the date of birth) to adulthood. The spatial distributions of retrogradely labeled neurons in different laminae were analyzed. The barrel map in layer 4 was used as a template to compare quantitative data from different animals and to account for substantial changes in barrel and barrel field size during development. Intrinsic projections 1) innervate increasingly more distant targets within barrel cortex up to 3 weeks of age; 2) continue to form in targets after 3 weeks, effectively strengthening existing connections; 3) follow a timetable for growth that is layer-specific; 4) link more distant barrel columns in layer 4 from neurons that are found preferentially in the barrel side and the septa between barrels; and 5) form over the shortest distances between the barrel columns. These data indicate that intrinsic connections in mouse barrel cortex develop by the progressive addition of neuronal connections rather than by sculpting preliminary connections. We describe statistically significant changes in connectivity during development that may be applied to model and assess the development of connections after a variety of experimental perturbations, such as to the environment and/or the genome.

Aging↗

PubMedDate not supplied

[An analysis of interneuronal connections in the auditory cortex of awake cats].

A statistical analysis has been made of the interaction of the auditory cortex units in alert cats with chronically implanted electrodes. Three neurones with an amplitude ratio of 4:2:1 were singled out from the multineuronal activity. The dependence between the firing of two neurones was determined by the cross interval histograms. The relationships between 78 pairs of units were studied in 26 three units microsystems. About a third of the studied pairs functioned independently. The number of pairs with one-way and two-way connections was about equal (26 and 30 respectively). The neurones which generated spikes of high and medium amplitude, had the largest number of two-way connections. One-way connections were equally represented in all the three neurones, though with regard to direction they depended on the amplitude characteristics of the spikes. In neurones with large and medium spikes, output connections predominated, while in neurones with small spikes input connections predominated considerably. The connection could be of inhibitory, excitatory or mixed type. The inhibitory type of connections was the most frequent occurrence (57 out of 86). At prolonged recording (6 to 16 min) of spike activity, most of the functional connections persisted.

Animals↗

PubMed2003

Role of the corpus callosum in functional connectivity.

BACKGROUND AND PURPOSE: Regional cerebral blood flow fluctuates synchronously in corresponding brain regions between the hemispheres. This synchrony implies neuronal connections between brain regions. The synchrony of blood flow changes is defined operationally as functional connectivity. Our purpose was to measure functional connectivity in patients with corpus callosal agenesis, in whom the interhemispheric connectivity is hypothetically diminished. METHODS: In three patients with agenesis of the corpus callosum, functional MR imaging was performed while patients performed text-listening and finger-tapping tasks. Functional images were also acquired while the patients performed no specific task (resting state). Regions of activation temporally correlated with the performance of the tasks were identified by cross-correlating the task data with a reference function. Voxel clusters (seed voxels) that corresponded to regions of activation in the task-activation data set were selected in the resting data set. All the voxels in the resting 3D data set that had a correlation coefficient exceeding 0.4 were identified. The number of these voxels in the ipsilateral and contralateral hemispheres was tabulated. RESULTS: In all patients, technically adequate functional MR and functional connectivity MR maps were obtained. For both tasks, activation was found in both hemispheres. For all of the seed voxels, significantly more functionally connected voxels were found in the ipsilateral hemisphere than in the contralateral hemisphere. For most seed voxels, no functionally connected voxels were found in the contralateral hemisphere. CONCLUSION: Interhemispheric functional connectivity in the motor and auditory cortices is diminished in patients with agenesis of the corpus callosum compared with that of healthy subjects.

Adult↗

PubMed2004

[CT findings of pulmonary hamartoma with special reference to epithelial-lined clefts and connection with pulmonary arteries].

PURPOSE: The purpose of this study was to clarify the characteristic CT findings of pulmonary hamartoma. MATERIALS AND METHODS: The thin-section CT and multiplanar reformation images of 19 pulmonary hamartomas diagnosed by surgical resection were analyzed and correlated with the pathological findings. RESULTS: Most hamartomas presented lobulated nodules apart from pleura. There was no case in which fat density was recognized. Only one case was recognized as having calcification. Air density in the connection of the side or the inside was pointed out in 5 cases (26%). Air density reflected epithelial-lined cleft. The connection with the bronchus was recognized in 4 cases (21%), and the connection with the pulmonary artery branch was recognized in 10 cases (53%). However, the connection of the pulmonary artery and bronchus could not be pathologically confirmed in most cases. The connection with the pulmonary vein was not recognized in any of the cases. CONCLUSION: Air density in the connection of the side or the inside is characteristic of pulmonary hamartoma. Pulmonary artery branches connect beyond half of hamartomas. This finding suggests close relations in the bronchus along the artery. It is important that there is no connection of the pulmonary vein, to differentiate it from lung cancer.

Calcinosis↗

PubMed2006

Psychosis related to neurological conditions: pro and cons of the dis- / mis-connectivity models of schizophrenia.

Schizophrenia is still a condition with obscure causes and psychopathology. This paper aims to discuss the "disconnectivity" hypothesis in relation to some neurological conditions which are known to alter brain connectivity, as well as mimicking some aspects of the disorder. After a short historical introduction to the concept, we will examine the evidence for connectivity problems in schizophrenia, separating the anatomical level from the functional level. Then, we will discuss three different issues concerning connectivity: i) local reduction in connectivity without neuronal loss (within the gray matter); ii) reduction in or alteration of long-range connectivity (within the white matter); and iii) abnormal targets for connections. For each of these aspects, we will look at the conditions able to reproduce anomalies capable of increasing susceptibility to schizophrenia. We conclude that psychosis is more likely to occur: i) when long-range connectivity is concerned; ii) when lesions result in lengthening and scattering of conduction times; and iii) when there are high dopamine levels, shedding light on or adding weight to the idea of an interaction between dopamine and connectivity.

Age Factors↗

PubMed1989

Patterns of connections in rat visual cortex.

The definition of visual areas is one of the central problems in visual cortex research. Rodent extrastriate cortex offers a striking example of the complexity of this issue, in that different parcelation schemes identify within it from 2 to as many as 13 separate visual areas. In the experiments reported here, patterns of connections within rat visual cortex were studied in an effort to better define its organizational layout. The experimental paradigm used consisted of the following steps: first, the pattern of callosal connections was revealed in vivo with the fluorescent tracer bisbenzimide. Then, using the callosal pattern as a landmark, single injections of WGA-HRP were placed at various sites in striate and extrastriate cortex. Subsequently, the relation between the tangential distribution of ipsilateral corticocortical connections, the callosal connections, and the borders of striate cortex were examined in the flattened cortex preparation. The experiments revealed widespread, patchy connections within rat visual cortex. These connections appeared to reflect 3 organizational trends. First, neighboring sites were more extensively connected than distant ones. Second, extrastriate sites receiving common striate cortex inputs tended to be interconnected. Finally, projections from opposite poles in striate cortex tended to form interdigitating patterns of connections in regions of overlap. Altogether these trends suggest that the extrastriate band adjoining striate cortex has a single, global map organization. However, within the global map, a clear modular organization was evident, which appeared to correspond to the multiple visuotopic representations reported for this region. Based on its location, and some organizational similarities. it is suggested that the global map may constitute the rat homolog of area V2 in cat and monkey.

Animals↗

PubMed1986

[The organization of thalamic connections].

Connections ascending to the thalamus. Contrary to classical opinion, all thalamic nuclei receive extrathalamic afferents. Segregation or convergence within a topographically defined nucleus represent two modalities of thalamic afferents. In addition, certain topographically organized thalamic afferents possess "privileged" or primary "targets" in the thalamic nucleus while others possess supplementary "targets" in other thalamic nuclei (see cerebellar, pallidal and spinothalamic projections). Ascending connections from several brain stem structures can converge on the same nucleus or diverge to several thalamic nuclei. Thalamic connections with the telencephalon. Methods for determining axonal transport have demonstrated that all thalamic nuclei, with the exception of the reticular nucleus and the ventral part of the lateral geniculate body, project towards the cerebral cortex. Four nuclear complexes can be recognized in the cat as a function of the different modalities of localization, concentration and lamination of the projections towards the cortex and the central grey nuclei. In general, the thalamocortical connections have reciprocal ipsilateral corticothalamic projections originating in the infragranular layers of the cerebral cortex. The reticular nucleus and the ventral part of the lateral geniculate body, which is not projected to the cerebral cortex, are exceptions. Each cortical area receives a "privileged" connection from a thalamic nucleus and a supplementary connection- from one or several other thalamic nuclei. The "privileged" connections usually pass to the fourth and third layers of the neocortex, and sometimes also to the first layer. In contrast, the supplementary connections pass to different superficial or deep cortical layers. Each nucleus is formed of subunits which possess different hodologic and topographic characteristics as a function of the nucleus considered. Convergence or divergence of thalamocortical and corticothalamic projections on the different thalamic nuclei, as well as the laminar distribution of efferents in the cerebral cortex, are related strictly to the hodologic organization of different cellular subunits constituting the nuclei. Concentration or diffusion of thalamic projections on cerebral cortex is related more to the single or multiple projection of cell populations belonging to a thalamic nucleus than to widespread collateralization of thalamocortical axons.

Afferent Pathways↗