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At least 1,369 records · Page 76Linked to original sources
PubMed1997

Physiology and anatomy of synaptic connections between thick tufted pyramidal neurones in the developing rat neocortex.

1. Dual voltage recordings were made from pairs of adjacent, synaptically connected thick tufted layer 5 pyramidal neurones in brain slices of young rat (14-16 days) somatosensory cortex to examine the physiological properties of unitary EPSPs. Pre- and postsynaptic neurones were filled with biocytin and examined in the light and electron microscope to quantify the morphology of axonal and dendritic arbors and the number and location of synaptic contacts on the target neurone. 2. In 138 synaptic connections between pairs of pyramidal neurones 96 (70%) were unidirectional and 42 (30%) were bidirectional. The probability of finding a synaptic connection in dual recordings was 0.1. Unitary EPSPs evoked by a single presynaptic action potential (AP) had a mean peak amplitude ranging from 0.15 to 5.5 mV in different connections with a mean of 1.3 +/- 1.1 mV, a latency of 1.7 +/- 0.9 ms, a 20-80% rise time of 2.9 +/- 2.3 ms and a decay time constant of 40 +/- 18 ms at 32-24 degrees C and -60 +/- 2 mV membrane potential. 3. Peak amplitudes of unitary EPSPs fluctuated randomly from trial to trial. The coefficient of variation (c.v.) of the unitary EPSP amplitudes ranged from 0.13 to 2.8 in different synaptic connections (mean, 0.52; median, 0.41). The percentage of failures of single APs to evoke a unitary EPSP ranged from 0 to 73% (mean, 14%; median, 7%). Both c.v. and percentage of failures decreased with increasing mean EPSP amplitude. 4. Postsynaptic glutamate receptors which mediate unitary EPSPs at -60 mV were predominantly of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor type. Receptors of the N-methyl-D-aspartate (NMDA) type contributed only a small fraction (< 20%) to the voltage-time integral of the unitary EPSP at -60 mV, but their contribution increased at more positive membrane potentials. 5. Branching patterns of dendrites and axon collaterals of forty-five synaptically connected neurones, when examined in the light microscope, indicated that the axonal and dendritic anatomy of both projecting and target neurones and of uni- and bidirectionally connected neurones was uniform. 6. The number of potential synaptic contacts formed by a presynaptic neurone on a target neurone varied between four and eight (mean, 5.5 +/- 1.1 contacts; n = 19 connections). Synaptic contacts were preferentially located on basal dendrites (63%, 82 +/- 35 microns from the soma, n = 67) and apical oblique dendrites (27%, 145 +/- 59 microns, n = 29), and 35% of all contacts were located on tertiary basal dendritic branches. The mean geometric distances (from the soma) of the contacts of a connection varied between 80 and 585 microns (mean, 147 microns; median, 105 microns). The correlation between EPSP amplitude and the number of morphologically determined synaptic contacts or the mean geometric distances from the soma was only weak (correlation coefficients were 0.2 and 0.26, respectively). 7. Compartmental models constructed from camera lucida drawings of eight target neurones showed that synaptic contacts were located at mean electrotonic distances between 0.07 and 0.33 from the soma (mean, 0.13). Simulations of unitary EPSPs, assuming quantal conductance changes with fast rise time and short duration, indicated that amplitudes of quantal EPSPs at the soma were attenuated, on average, to < 10% of dendritic EPSPs and varied in amplitude up to 10-fold depending on the dendritic location of synaptic contacts. The inferred quantal peak conductance increase varied between 1.5 and 5.5 nS (mean, 3 nS). 8. The combined physiological and morphological measurements in conjunction with EPSP simulations indicated that the 20-fold range in efficacy of the synaptic connections between thick tufted pyramidal neurones, which have their synaptic contacts preferentially located on basal and apical oblique dendrites, was due to differences in transmitter release probability of the projecting neurones and, to a lesser extent, to differenc

Action Potentials↗
PubMed1992

Synaptic connections between nonspiking afferent neurons and motor neurons underlying phase-dependent reflexes in crayfish.

1. This paper analyzes the synaptic connections made by nonspiking afferent neurons of the thoracocoxal muscle receptor organ (TCMRO) with basal limb motor neurons in the crayfish. The T fiber, a dynamically sensitive afferent, monosynaptically excites promotor motor neurons. Evidence suggests that both tonic graded chemical transmission and electrical synaptic transmission may be involved, depending on the motor neuron under consideration. 2. In preparations in the active state (spontaneously producing reciprocal motor patterns), the T fiber also inhibits promotor motor neurons in a phase-dependent manner. This inhibitory pathway is probably indirect, because it involves additional synaptic delay. 3. The statically sensitive S fiber also excites promotor motor neurons, but phase-dependent inhibition of promotor motor neurons by the S fiber was not seen. 4. The T fiber excites a subclass of remotor motor neurons (group 1) by a combination of direct chemical input and electrical input. This connection underlies the positive feedback reflex that excites these remotor motor neurons, in a phase-dependent manner, on stretch of the TCMRO during the active state. In inactive preparations, this connection remains subthreshold. 5. Central synaptic outputs of group 1 remotor motor neurons can also inhibit promotor motor neurons. This pathway may contribute to the phase-dependent reflex inhibition of promotor motor neurons that occurs in the active state.

Afferent Pathways↗
PubMed1995

Long-lasting potentiation of a direct central connection between identified motor neurons in the locust.

The plasticity of the direct central connection between the fast extensor and the posterior fast flexor tibiae motor neurons in the locust (Schistocerca gregaria) metathoracic ganglion was studied. An action potential in the fast extensor results in a monosynaptic excitatory postsynaptic potential (EPSP) in the flexor motor neuron. Antidromic stimulation of the fast extensor at 100 Hz for 3.5 s resulted in a long-lasting potentiation of the EPSP amplitude. The potentiation was not dependent on feedback caused by movement of the tibia, and was associated with an increase in the input resistance of the flexor motor neuron. The potentiation was heterosynaptic, and was not affected by bath application of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid. The potentiation was voltage dependent, as hyperpolarizing the flexor motor neuron during the stimulation blocked the development of the potentiation whereas depolarizing the flexor in the absence of presynaptic activity caused potentiation of subsequent fast extensor-evoked EPSPs. The depolarization-induced potentiation was calcium dependent. Antidromic stimulation of the fast extensor at 100 Hz for 3.5 s also caused modulation of the presynaptic action potential. The spike duration was increased and the amplitude of the afterhyperpolarization reduced. These effects were dependent on movement of the tibia. Bath application of the 5-hydroxytryptamine (5-HT) receptor antagonist ketanserin blocked the changes in the presynaptic spike. The modulation was probably due to the release of 5-HT from proprioceptive afferents that monitor movement of the tibia about the femur. The modulation of the presynaptic action potential increases transmitter release onto the flexor motor neurons, and this acts in synergy with the postsynaptic modulation to potentiate the connection.

Action Potentials↗
PubMed1999

A new subdivision of mammalian neostriatum with functional implications to learning and memory.

A new subdivision with distinctive morphological and functional attributes has been identified at the caudomedial margin of the neostriatum and surrounding the rostrolateral border of the globus pallidus in the brains of the rat, cat, monkey, and human. The subdivision is termed marginal division (MrD) based on its location. It is readily distinguishable from the rest of striatum by consisting of spindle-shaped neurons, special connections, and intensely expressed immunoreactivities of many neuropeptides and some monoamines in the fibers, terminals, and neuronal somata. Three-dimensional reconstruction from Nissl-stained sections of the rat brain revealed that the MrD is a flat, pan-shaped zone between the neostriatum and globus pallidus. Functional neuronal connections were delineated by chemical-induced c-Fos expression between the MrD and hippocampus, amygdala, as well as the basal nucleus of Meynert. In rats with chemical lesions of bilateral MrD, learning and memory functions were severely impaired as demonstrated by double blind Y-maze test. Our results thus suggested that the MrD is a distinct part and a universal structure in the neostriatum of the mammalian brain and might play an important role in the mechanism of learning and memory.

Animals↗
PubMedDate not supplied

Anatomy of cochlear innervation.

In this review of cochlear innervation, the differences in the innervation of outer and inner hair cells are emphasized. Of the afferent neurons, 90 to 95 per cent are large, myelinated type I neurons, exclusively connected in an essentially radial unbranched manner to the inner hair cells; 5 to 10 per cent are small, mostly unmyelinated type II neurons connected to the outer hair cells with considerable spiral extension and branching. The few small type II neurons, with their thin unmyelinated axons, probably have a minor functional importance for centripetal information transfer. The functional emphasis of the outer hair cell system is likely at the level of the receptor cells where the outer hair cells monitor receptor function. The efferent innervation also consists of at least two types of neurons. Small neurons from the lateral superior olivary nucleus project to the inner hair cell area in a predominantly homolateral fashion, making almost exclusively synaptic contacts with the afferent dendrites associated with the inner hair cells. Larger neurons from the medial nucleus of the trapezoid body and periolivary nucleus provide the abundant efferent nerve supply of the outer hair cells, predominantly contralateral. They have mostly large synaptic contacts, and, in some species exclusively, with the receptor cells, indicating again the functional emphasis of the outer hair cell system at the receptor cell level.

Animals↗
PubMed2003

Early establishment of multiple release site connectivity between interneurons and pyramidal neurons in the developing hippocampus.

The strength of the synaptic transmission between two neurons critically depends on the number of release sites connecting the neurons. Here we examine the development of connectivity between gamma-aminobutyric acid (GABA)ergic interneurons and CA1 pyramidal neurons in the hippocampus. GABAergic postsynaptic currents (PSCs) were recorded in whole-cell voltage-clamped CA1 pyramidal neurons. By comparing spontaneous and miniature (action potential-independent) GABAergic PSCs, we found that multiple release site connectivity is established already at the first postnatal day and that the degree of connectivity remains unaltered into adulthood. During the same time there is a dramatic increase in the number of GABAergic synapses on each pyramidal neuron as indicated by the increase in frequency of miniature GABAergic PSCs. These results indicate that during development a given interneuron contacts an increasing number of target pyramidal neurons but with the same multiple release site connectivity. It has been shown previously that the connectivity between CA3 and CA1 pyramidal neurons is initially restricted to one release site, and develops gradually. The present result thus suggests different mechanisms to govern the maturation of excitatory and inhibitory synaptic transmissions.

Animals↗
PubMed1993

Dependence of functional synaptic connections of hippocampal and neocortical neurons on types of memory.

Different types of memory are thought to be processed by different neuronal networks. The present study evaluated the functional synaptic connections among neurons by means of cross-correlation analysis when two types of memory, working and reference memory, were being processed in behaving rats. Some of the synaptic connections of neurons in the hippocampal CA1, CA3, dentate gyrus and auditory cortex functioned when the rat was performing only one of the memory tasks. Working memory needed more of these memory-type dependent synapses in the hippocampal regions than reference memory.

Animals↗
PubMed2004

Neuronal activation of Ras regulates synaptic connectivity.

A synRas mouse model was used expressing constitutively activated Ha-Ras (Val12 mutation) in neurons to investigate the role of Ras-MAPkinase signalling for neuronal connectivity in adult brain. Expression of the transgene in the cortex of these mice starts after neuronal differentiation is completed and allows to directly investigate the effects of enhanced Ras activity in differentiated neurons. Activation of Ha-Ras induced an increase in soma size which was sensitive to MEK inhibitor in postnatal organotypic cultures. Adult cortical pyramidal neurons showed complex structural rearrangements associated with an increased size and ramification of dendritic arborization. Dendritic spine density was elevated and correlated with a twofold increase in number of synapses. In acute brain slices of the somatosensory and of the visual cortex, extracellular field potentials were recorded from layer II/III neurons. The input-output relation of synaptically evoked field potentials revealed a significantly higher basal excitability of the transgenic mice cortex compared to wild-type animals. In whole cell patch clamp preparations, the frequency of AMPA receptor-mediated spontaneous excitatory postsynaptic currents was increased while the ratio between NMDA and AMPA-receptor mediated signal amplitude was unchanged. A pronounced depression of paired pulse facilitation indicated that Ras contributes to changes at the presynaptic site. Furthermore, synRas mice showed an increased synaptic long-term potentiation, which was sensitive to blockers of NMDA-receptors and of MEK. We conclude that neuronal Ras is a common switch of plasticity in adult mammalian brain sculpturing neuronal architecture and synaptic connectivity in concert with tuning synaptic efficacy.

2-Amino-5-phosphonovalerate↗
PubMed2005

Transynaptic effects of tetanus neurotoxin in the oculomotor system.

The question whether general tetanus arises from the independent sum of multiple local tetani or results from the actions of the transynaptic tetanus neurotoxin (TeNT) in higher brain centres remains unresolved. Despite the blood-borne dissemination of TeNT from an infected wound, the access to the central nervous system is probably prevented by the blood-brain barrier. However, several long-term sequelae (e.g. autonomic dysfunction, seizures, myoclonus, and sleep disturbances) present after the subsidence of muscle spasms might be indicative of central actions that occur farther away from lower motoneurons. Subsequently, the obvious entry route is the peripheral neurons followed by the transynaptic passage to the brain. We aimed at describing the pathophysiological correlates of TeNT translocation using the oculomotor system as a comprehensive model of cell connectivity and neuronal firing properties. In this study, we report that injection of TeNT into the medial rectus muscle of one eye resulted in bilateral gaze palsy attributed to firing alterations found in the contralaterally projecting abducens internuclear neurons. Functional alterations in the abducens-to-oculomotor internuclear pathway resembled in part the classically described TeNT disinhibition. We confirmed the transynaptic targeted action of TeNT by analysing vesicle-associated membrane protein2 (VAMP2) immunoreactivity (the SNARE protein cleaved by TeNT). VAMP2 immunoreactivity decreased by 94.4% in the oculomotor nucleus (the first synaptic relay) and by 62.1% presynaptic to abducens neurons (the second synaptic relay). These results are the first demonstration of physiological changes in chains of connected neurons that are best explained by the transynaptic action of TeNT on premotor neurons as shown with VAMP2 immunoreactivity which serves as an indicator of TeNT activity.

Abducens Nerve↗
PubMed2005

Neuroeffector connections of giant multimodal neurons in the African snail Achatina fulica.

A new method of making preparations was used to analyse the neuroeffector connections of the paired giant neurons of the African snail Achatina fulica. These neurons were found to induce postsynaptic potentials in the muscles of the mantle, heart, the wall of the pulmonary cavity, and the muscular elements of the renal complex, the pericardium, the sexual apparatus, the walls of the cerebral arteries, the filaments of the columellar muscles, the wall of the abdomen, and the tentacle retractor muscles. Rhythmic neuron activity led to the development of marked facilitation and long-term potentiation of synaptic potentials. The possible significance of the multiple neuroeffector connections of giant neurons is discussed.

Adaptation, Physiological↗
PubMed1989

Direct inhibitory synaptic connections of pontine omnipause neurons with burst neurons in Forel's field H related to vertical saccades in the cat.

This study investigated synaptic connections of omnipause neurons (OPNs) in the midline pontine tegmentum with medium-lead burst neurons (BNs) in Forel's field H (FFH), using the spike-triggered averaging technique in chronically prepared alert cats. OPNs were identified behaviorally by tonic firing during the intersaccadic period, whereas medium-lead BNs were identified by spike bursts during upward or downward saccades. A positive shift of field potential in the BN area, associated with suppression of the BNs' bursts, was observed during tonic firing of OPNs and during microstimulation in the OPN area. Averaging of BN area field potentials triggered by spikes of single OPNs on both sides, which were antidromically evoked from the BN area, revealed positive waves with monosynaptic latencies. These results strongly suggest that OPNs on both sides make direct inhibitory synaptic connections with vertical eye movement-related medium-lead BNs in the FFH.

Action Potentials↗
PubMedDate not supplied

Neuronal composition and interneuronal connection of area 5 in the cat parietal association cortex.

Area 5 of the cat cortex was studied by Nissl's method and by Golgi's chromate-silver impregnation method. Its typical six-layered structure with well-developed layers of pyramidal cells was revealed. The characteristic features of area 5 are: predominance of pyramidal cells in layers II-III and the presence of large forms (40 x 26 mu) among them (in layer III); giant pyramidal neurons (70 x 23 mu) arranged singly or nidally in layer V; large (diameter 25-30 mu) and giant (diameter 40-45 mu) stellate cells with radial dendrites, arranged singly or in groups in layers V-VI; infrequent efferent fusiform neurons (40 x 20 mu) in layers V-VL Stellate cells connecting pyramidal neurons in the same or in different layers were found in layers II-VI. Some stellate cells in layers II-=III form long horizontal connections within area 5. Interneuronal connections are effected by axosomatic and axodendritic terminals, the latter being more numerous; Dendrodendritic and axoaxonal synapses are less common.

Afferent Pathways↗
PubMed2000

Synaptic assembly of the brain in the absence of neurotransmitter secretion.

Brain function requires precisely orchestrated connectivity between neurons. Establishment of these connections is believed to require signals secreted from outgrowing axons, followed by synapse formation between selected neurons. Deletion of a single protein, Munc18-1, in mice leads to a complete loss of neurotransmitter secretion from synaptic vesicles throughout development. However, this does not prevent normal brain assembly, including formation of layered structures, fiber pathways, and morphologically defined synapses. After assembly is completed, neurons undergo apoptosis, leading to widespread neurodegeneration. Thus, synaptic connectivity does not depend on neurotransmitter secretion, but its maintenance does. Neurotransmitter secretion probably functions to validate already established synaptic connections.

Animals↗
PubMed1983

[Study of intracortical connections between groups of neurons in the somatosensory region of the cortex in the rat using the technic of retrograde axonal transport of horseradish peroxidase].

Intercortical connections between different groups of neurons, the so-called "barrels" located in the somatic sensory cortex of the rat brain were identified by the retrograde horseradish transport technique. It is shown that HRP-labelled cells were found at a distance up to 400 microns from the place of injection. 90% of them are pyramidal neurons located mainly in laminae V and III of the cortex. The number of intercortical connections in labelled neurons was 1.6 times as high as that of thalamocortical ones. It is supposed that interaction between neurons of neighbouring groups is realized by the pyramidal cells of laminae V and III.

Animals↗
PubMed1994

Cortical cell assemblies: a possible mechanism for motor programs.

The concept of a motor program has been used to interpret a diverse range of empirical findings related to preparation and initiation of voluntary movement. In the absence of an underlying mechanism, its exploratory power has been limited to that of an analogy with running a stored computer program. We argue that the theory of cortical cell assemblies suggests a possible neural mechanism for motor programming. According to this view, a motor program may be conceptualized as a cell assembly, which is stored in the form of strengthened synaptic connections between cortical pyramidal neurons. These connections determine which combinations of corticospinal neurons are activated when the cell assembly is ignited. The dynamics of cell assembly ignition are considered in relation to the problem of serial order. These considerations lead to a plausible neural mechanism for the programming of movements and movement sequences that is compatible with the effects of precue information and sequence length on reaction times. Anatomical and physiological guidelines for future quantitative models of cortical cell assemblies are suggested. By taking into account the parallel re-entrant loops between the cerebral cortex and basal ganglia, the theory of cortical cell assemblies suggests a mechanism for motor plans that involve longer sequences. The suggested model is compared with other existing neural network models for motor programming.

Journal Article↗
PubMed1994

Developmental changes in electrophysiological properties of LGNd neurons during reorganization of retinogeniculate connections.

Changes in electrophysiological properties of neurons in the ferret dorsal LGN (LGNd) were studied during early postnatal life, a critical developmental period when changes occur in morphology, connectivity, and response properties of LGNd neurons. Using the patch-clamp technique to obtain whole-cell recordings from cells maintained as in vitro slices of thalamus, several distinctive properties were observed in the immature LGNd. Relatively low resting membrane potentials were present that became more negative during the first 2 postnatal weeks. In addition, immature neurons exhibited high input resistances that decreased during early postnatal development. At all ages postnatally, neurons were capable of generating a train of Na(+)-dependent action potentials in response to intracellular injection of a depolarizing current pulse. Moreover, immature neurons resembled older cells in that little spike frequency adaptation was present during a train of action potentials. Action potential activity in immature neurons was nevertheless distinctive in several respects: (1) during the first 2-3 postnatal weeks action potentials became shorter in duration and larger in amplitude; (2) during the same period, thresholds for generation of action potentials changed in conjunction with the changes in resting membrane potential, becoming more negative; and (3) plots of frequency versus injected current revealed that thresholds for generation of trains of action potentials were reached with intracellular injection of lower current levels at earlier ages. These findings raise the possibility that relatively weak ionic currents generated at immature synapses have unexpectedly strong effects on the young LGNd neuron.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1978

Connections of behaviorally identified cat omnipause neurons.

The afferent and efferent connections of omnipause neurons involved in saccadic eye movements were studied electrophysiologically in ketamine anesthetized cats. Omnipause neurons were identified by their characteristic discharge pattern which was qualitatively unaltered by ketamine anesthesia.

Action Potentials↗
PubMedDate not supplied

Selectivity in the inter-laminar connections made by neocortical neurones.

In this brief review of inter-laminar synaptic connections in neocortex a case is made for the hypothesis that axons select their targets, or neurones select their inputs with great specificity. A large part of the data discussed was obtained from dual intracellular recordings in slices of adult neocortex. The neurones innervated in a given layer by a given type of axon were not found to be a random selection of the cells in that layer. Rather in a layer to layer specific fashion, certain types of target cells are densely innervated while others are not contacted or receive only weak inputs. In another layer, the same axons may select other targets.

Animals↗