[Structure of neuronal and interneuronal connections in field 7 of the cat cerebral cortex].
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Identified neurones isolated from leech CNS make selective connections in culture.
Neurones cultured in vitro offer distinct advantages for studying how processes grow towards their targets and form synaptic connections. In contrast to the complex events occurring during the development of the nervous system, synapse formation in culture can be analysed in a few neurones at a time and under controlled conditions. We have now dissected out and cultured single identified neurones from the central nervous system (CNS) of the adult leech. Various types of sensory cells, motor cells, and interneurones can be identified in leech ganglia--each with a stereotyped set of properties, including: (1) the electrical characteristics of its membrane, (2) the arborisation of its branches and the morphology of its terminals and (3) the pattern of connections it makes with other identified neurones, skin or muscle. Thus, cultured cells can be compared in detail with their counterparts in situ. We have found that isolated cells survive for several weeks, maintain their membrane properties, sprout and form selective connections.
Animals↗
[Elementary polysynaptic potentials of hippocampal neurons].
Synaptic connections of 26 pairs of hippocampal neurons were studied in nonanesthetized rabbits by spike-triggered averaging of intracellularly recorded activity. Synchronized activity was detected in 5 pairs and considered to represent common inputs to the neurons recorded. Hyperpolarizing or depolarizing potentials with 3--4 ms latency were revealed in 3 additional pairs. These potentials are considered to be individual postsynaptic potentials (PSPs) evoked in the target neuron by spikes of the adjacent (0.5--2 mm apart) neuron. A quantum analysis of the individual PSPs was performed. The mean quantum content (0.27--0.65) and quantum size (35--200 mkV) were found to be of the same order as those of the excitatory PSPs previously recorded after intracerebral stimulation. It is concluded that most hippocampal synapses are of low efficacy.
Animals↗
Peripheral control of the gain of a central synaptic connection between antagonistic motor neurones in the locust
The metathoracic fast extensor tibiae (FETi) motor neurone of locusts is unusual amongst insect motor neurones because it makes output connections within the central nervous system as well as in the periphery. It makes excitatory chemical synaptic connections to most if not all of the antagonist flexor tibiae motor neurones. The gain of the FETi-flexor connection is dependent on the peripheral conditions at the time of the FETi spike. This dependency has two aspects. First, sensory input resulting from the extensor muscle contraction can sum with the central excitatory postsynaptic potential (EPSP) to augment its falling phase if the tibia is restrained in the flexed position (initiating a tension-dependent reflex) or is free to extend (initiating a movement-dependent resistance reflex). This effect is thus due to simple postsynaptic summation of the central EPSP with peripheral sensory input. Second, the static tibial position at the time of the FETi spike can change the amplitude of the central EPSP, in the absence of any extensor muscle contraction. The EPSP can be up to 30 % greater in amplitude if FETi spikes with the tibia held flexed rather than extended. The primary sense organ mediating this effect is the femoral chordotonal organ. Evidence is presented suggesting that the mechanism underlying this change in gain may be specifically localised to the FETi-flexor connection, rather than being due to general position-dependent sensory feedback summing with the EPSP. The change in the amplitude of the central EPSP is probably not caused by general postsynaptic summation with tonic sensory input, since a diminution in the amplitude of the central EPSP caused by tibial extension is often accompanied by overall tonic excitation of the flexor motor neurone. Small but significant changes in the peak amplitude of the FETi spike have a positive correlation with changes in the EPSP amplitude, suggesting a likely presynaptic component to the mechanism of gain control. The change in amplitude of the EPSP can alter its effectiveness in producing flexor motor output and, thus, has functional significance. The change serves to augment the effectiveness of the FETi-flexor connection when the tibia is fully flexed, and thus to increase its adaptive advantage during the co-contraction preceding a jump or kick, and to reduce the effectiveness of the connection when the tibia is partially or fully extended, and thus to reduce its potentially maladaptive consequences during voluntary extension movements such as thrusting.
Journal Article↗
Ectopic neurons and aberrant connections during neural development.
During the normal development of the nucleus of origin of centrifugal fibers to the chick retina (the so-called isthmo-optic nucleus) a significant number of neurons are misrouted during their migration from the neural epithelium in which they are generated. Like the cells in the isthmo-optic nucleus, these ectopic neurons can be identified by the retrograde transport of horseradish peroxidase (donor: hydrogen peroxide oxidoreductase, EC 1.11.1.7) injected into the eye. Between 500 and 600 such cells have been identified on one side of the brain of 12 to 15-day-old embryos, but only about 10% of this number is seen beyond hatching. In addition, during development some of the cells in the nucleus send their axons to the ipsilateral retina, whereas in post-hatched animals they all project to the contralateral retina. The number of such neurons with aberrant axons is greatly increased (in some cases by as much as two orders of magnitude) in chicks from which one eye was removed early in development. Although normally about 60% of the neurons in the isthmo-optic nucleus degenerate between the 13th and 17th days of incubation, they can almost all be labeled by intraocular injections of horseradish peroxidase on the 12th day, indicating that the axons of virtually all the cells, including those neurone that subsequently die, reach the retina.
Age Factors↗
The neuron of the fast conducting system in hirudo medicinalis: identification and synaptic connections with primary afferent neurons.
A single neuron, located in the center of each segmental ganglion of H. medicinalis is antidromically activated by electrical stimulation of the ventral cord anteriorly and posteriorly to the ganglion, at the same threshold as the fast conducting system (FCS) and with a latency equal to the FCS conduction time. This neuron is activated trans-synaptically by tactile and photic stimulation of the skin and by stimulation of high-threshold fibres running along the cord. A spike evoked by intracellular stimulation of this neuron propagates along the FCS. Intracellular staining shows that this neuron sends two axonal branches in the anterior and posterior median connectives. Direct electrical stimulation of touch cells (T cells), as well as mechanical stimulation of the skin, lowers the threshold of and may eventually fire, the FCS neurons, not only at the level of the ganglion to which they belong, but also at the level of the neighbouring ganglia. This effect is mediated by bilateral pathwasy located in the lateral connectives. It is concluded that the FCS consists of a chain of single neurons, located in each ganglion and electrotonically coupled to each other. Touch cells project with excitatory synapses on the FCS neurons.
Animals↗
Multiple synaptic connections of a single neuron change differentially with age.
The efficacy of chemical synaptic connections of a single identified interneuron with different types of follower neurons was studied throughout the adult life of the pond snail Lymnaea stagnalis. Simultaneous intracellular recordings were made from the interneuron RPeD1 and its follower neurons in isolated CNS preparations from animals of different age groups (3-18 months of age). The presence of postsynaptic responses to RPeD1 action potentials was tested. With increasing age, the number of A-group neurons that was found with a response to evoked RPeD1 action potentials decreased, yet the number of HIJK-group neurons responding to RPeD1 input increased. The number of G-group neurons and the number of individual neurons VD2/3 and VD4 with RPeD1 input did not differ significantly between age groups. However, there was variability in the presence of responses in these individual neurons. Thus, synaptic connections of the single interneuron RPeD1 change differentially throughout the adult life of L. stagnalis. Within the A-group we found indications that changes in RPeD1 input apply to the entire A-group. In the A-group neurons changes in several electrical properties could not account for the observed age-related changes in the number of neurons responding to RPeD1 action potentials.
Action Potentials↗
Behavioral function of glutamatergic interneurons in the feeding system of Lymnaea: plateauing properties and synaptic connections with motor neurons.
Intracellular recording techniques were used to examine the electrical properties and behavioral function of a novel type of retraction phase interneuron, the N2 ventral (N2v) cells in the feeding network of the snail Lymnaea. The N2vs were compared with the previously identified N2 cells that now are renamed the N2 dorsal (N2d) cells. The N2vs are a bilaterally symmetrical pair of electrotonically coupled plateauing interneurons that are located on the ventral surfaces of the buccal ganglia. Their main axons project to the opposite buccal ganglion, but they have an additional fine process in the postbuccal nerve. N2v plateaus that outlast the duration of the stimulus can be triggered by depolarizing current pulses and prematurely terminated by applied hyperpolarizing pulses. Gradually increasing the amplitude of depolarizing pulses reveals a clear threshold for plateau initiation. N2v plateauing persists in a high Mg2+/nominally zero Ca2+ saline that blocks chemical synaptic connections, suggesting an endogenous mechanism for plateau generation. The N2vs fire sustained bursts of action potentials throughout the N2/rasp phase of the fictive feeding cycle and control the retraction phase feeding motor neurons. The N2vs excite the B3 and B9 feeding motor neurons to fire during the rasp phase of the feeding cycle. They also inhibit the B7 and B8 feeding motor neurons. The B8 cells recover from inhibition and fire during the following swallowing phase. These synaptic connections appear to be monosynaptic as they persist in high Mg2+/high Ca2+ (HiDi) saline that blocks polysynaptic pathways. Strong current-induced plateaus in the N2vs generate brief inhibitory postsynaptic responses in the B4CL rasp phase motor neurons, but this was due to the indirect N2v --> N2d --> B4CL pathway. The N2vs are coupled electrotonically to the N2d cells, and triggering plateau in a N2v usually induced one or two spikes in a N2d. Previous experiments showed that the N2ds generate plateau potentials during a fictive feeding cycle. Here we show that the main component of the "plateauing" waveform is due to the electrotonic coupling with the N2v cells. The differential synaptic connections of the N2v and N2d cells with retraction phase motor neurons results in a sequence of motor neuron burst activity B9 --> B4CL --> B8 that produces the full retraction (rasp --> swallow) movements of the feeding apparatus (buccal mass). We conclude that the N2v cells are an essential component of the interneuronal network required to produce feeding motor neuron activity.
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Host Corticostriatal Fibres Establish Synaptic Connections with Grafted Striatal Neurons in the Ibotenic Acid Lesioned Striatum.
The connections between host corticostriatal afferents and neurons in intrastriatal grafts of foetal striatal tissue have been studied with electron microscopic immunocytochemistry using Phaseolus vulgaris leucoagglutinin (PHA-L) as a label of the host corticostriatal fibres. Adult rats with unilateral ibotenic acid lesions of the head of the caudate putamen received foetal cell suspension grafts from E14-15 rat embryos into the lesioned striatal area. Ten months after transplantation, multiple iontophoretic injections of PHA-L were made into the host frontal cortex. These injections labelled large numbers of corticostriatal fibres which extended across the graft - host border to form a rich axonal network mainly in the peripheral portions of the grafts. At the ultrastructural level a total of 134 PHA-L-labelled terminals were identified to form asymmetric synaptic contacts with neurons within the grafts. Of these contacts, 83% were in contact with dendritic spines, 12% with dendritic shafts, and 5% with small shafts or spines. The synaptic contacts were similar to those identified in intact regions of the host striatum that were spared by the lesion. However, the synapses in the host striatum were almost exclusively in contact with spines (98%). These results demonstrate that the corticostriatal projection, which constitutes a major source of afferent control in the normal striatum, not only extends axons into the intrastriatal striatal grafts, but also establishes synaptic connections with the implanted neuronal elements.
Journal Article↗
Connections between pyramidal neurons in layer 5 of cat visual cortex (area 17).
The structural features of two physiologically-characterised pyramidal neurons (PC1 and PC2) closely situated in layer 5b in the visual cortex (area 17) of a single cat were studied using a combination of electrophysiological and anatomical techniques. Both PC1 and PC2 had exceptionally large somata (30-40 microns in diameter). On the basis of this and other morphological features cell PC1 was classified as a Meynert cell. PC1 possessed a very large (2.75 degrees X 4.50 degrees) binocularly driven standard complex receptive field. PC2 was also binocularly driven with a small, B-type receptive field. Both cells had the same preference for the direction and orientation of visual stimuli. PC1 and PC2 could be antidromically activated from stimulating electrodes positioned above the dorsal lateral geniculate nucleus with a response latency indicating that these cells probably innervated the visual tectum or pretectum. In addition to corticoefferent axons, the two neurons possessed extensive intracortical axon arbors that ramified extensively in layers 5 and 6 of the medial and lateral banks of the lateral gyrus in area 17. Axon collaterals from both PC1 and PC2 also innervated a small common target region in area 18. A total of 313 boutons from the axonal arbors of PC1 and PC2 were examined in the electron microscope. All of the identified synaptic junctions were found to establish Gray type 1 asymmetrical contacts. The combined ultrastructural data for both neurons indicated that 80% of boutons were onto dendritic spine heads, with 14%, 6%, and 1% onto small-, medium-, and large-calibre dendritic shafts, respectively. The spectrum of postsynaptic targets showed little variation with respect to lamina, distance from somata, or cortical area. Other large pyramidal neurons in layer 5 and spiny neurons in layer 6 were identified as receiving synaptic input from either PC1 or PC2. Using a computer graphics system, rotations of the bouton distributions revealed the existence of a clustered innervation of layers 5 and 6 in areas 17 and 18 derived from the two identified neurons. The bouton distributions strongly resembled the tangential pattern described previously for the functional slab-like organisation of the cortex. The results provide a morphological basis for the clustered intrinsic connectivity of pyramidal cells in layers 5 and 6 of the cat visual cortex. Furthermore, the results indicate the widespread excitatory influence of large pyramidal neurons on other cells projecting subcortically to sites dealing with visually guided behavior.
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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.
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Formation of novel central and peripheral connections between molluscan central neurons in organ cultured ganglia.
An in vitro organ culture system for buccal ganglia of the adult snail, Helisoma, is described. The system supports: (1) maintenance of characteristic electrophysiological parameters of identified neurons over seven days of culture; (2) choline metabolism including uptake and synthesis over the same duration; (3) sprouting and growth of neurons in response to axotomy; (4) the formation of novel central electrotonic connections between identified neurons as a result of sprouting and growth. These observations on neuronal growth and the formation of connections are similar to those made with in vivo culture. The use of in vitro culture allows precise manipulations not previously possible. When buccal ganglia are cultured in vitro with the cut distal ends of peripheral nerve trunks held closely apposed, axons of neurons 5R and 5L in the nerve trunks are capable of forming electrotonic connections similar to central connections. The capability of these neurons to form electrotonic connections via their peripheral axons implies that special structures (i.e., central neurites) are not required for the formation of connections; and neither are special environments (i.e., the central neuropile) required for these connections.
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Response properties and synaptic connections of mechanoafferent neurons in cerebral ganglion of Aplysia.
1. The cells of two clusters of small neurons on the ventrocaudal surface of each hemicerebral ganglion of Aplysia were found to exhibit action potentials following tactile stimuli applied to the skin of the head. These neurons appear to be mechanosensory afferents since they possess axons in the nerves innervating the skin and tactile stimulation evokes spikes with no prepotentials, even when the cell bodies are sufficiently hyperpolarized to block some spikes. The mechanosensory afferents may be primary afferents since the sensory response persists after chemical synaptic transmission is blocked by bathing the ganglion and peripheral structures in seawater with a high-Mg2+ and low-Ca2+ content. 2. The mechanosensory afferents are normally silent and are insensitive to photic, thermal, and chemical stimuli. A punctate tactile stimulus applied to a circumscribed region of skin can evoke a burst of spikes. If the stimulus is maintained at a constant forces, the mechanosensory response slowly adapts over a period of seconds. Repeated brief stimuli have little or no effect on spike frequency within a burst. 3. Approximately 81% of the mechanoafferent neurons have a single ipsilateral receptive field. The fields are located on the lips, the anterior tentacles, the dorsal portion of the head, the neck, or the perioral zone. Because many cells have collateral axons in the cerebral connectives, receptive fields elsewhere on the body are a possibility. The highest receptive-field density was associated with the lips. Within each area, receptive fields vary in size and shape. Adjacent fields overlap and larger fields frequently encompass several smaller ones. The features of some fields appear invariant from one animal to the next. A loose form of topographic organization of the mechanoafferent cells was observed. For example, cells located in the medial cluster have lip receptive fields, and most cells in the posterolateral portion of the lateral clusters have tentacle receptive fields. 4. Intracellular stimulation of individual mechanoafferents evokes short and constant-latency EPSPs in putative motor neurons comprising the identified B-cell clusters of the cerebral ganglion. On the basis of several criteria, these EPSPs appear to be several criteria, these EPSPs appear to be chemically mediated and are monosynaptic. 5. Repetitive intracellular stimulation of individual mechanoafferent neurons at low rates results in a gradual decrement in the amplitude of the EPSPs evoked in B cluster neurons. EPSP amplitude can be restored following brief periods of rest, but subsequent stimulation leads to further diminution of the response. 6. A decremented response cannot be restored by strong mechanical stimulation outside the receptive field of the mechanoafferent or by electrical stimulation of the cerebral nerves or connectives...
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[Three types of monkey striatal neuron impulse activity connected with behavioral changes].
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Animals↗
Prediction of trauma mortality using a neural network.
A neural network is a computerized construct consisting of input neurons (which process input data) connected to hidden neurons (to mathematically manipulate values they receive from all the input neurons) connected to output neurons (to output a prediction). Neural networks are created and trained via multiple iterations over data with known results. In 1993, 897 trauma patients were either declared dead in the emergency room (ER; 76 cases), admitted to the intensive care unit (427 cases, 36 deaths), or taken directly to the operating room (394 cases, 29 deaths). Using only data available from the ER, a neural network was created, and 628 cases were randomly selected for training. After 268 iterations, the network was trained to correctly predict death or survival in all 628 cases. This trained network was then tested on the other 269 cases without our providing the death or survival result. Its overall accuracy was 91 per cent (244 of 269 cases). It was able to predict correctly 60 per cent (12 of 20 cases) of the postoperative or post-intensive care unit admission deaths and 90 per cent (26 of 29 cases) of the deaths in the ER. Computerized neural networks can accurately predict a trauma patient's fate based on inital ER presentation. The theory and use of neural networks in predicting clinical outcome will be presented.
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Cytoglobin is a respiratory protein in connective tissue and neurons, which is up-regulated by hypoxia.
Cytoglobin is a recently discovered vertebrate globin distantly related to myoglobin, and its function is unknown. Here we present the first detailed analysis of the distribution and expression of cytoglobin. Northern and Western blotting experiments show the presence of cytoglobin mRNA and protein in a broad range of tissues. Quantitative PCR demonstrates an up-regulation of cytoglobin mRNA levels in rat heart and liver under hypoxic conditions (22 and 44 h of 9% oxygen). Immunofluorescence studies with three antibodies directed against different epitopes of the protein consistently show cytoglobin in connective tissue fibroblasts as well as in hepatic stellate cells. Cytoglobin is also present in chondroblasts and osteoblasts and shows a decreased level of expression upon differentiation to chondrocytes and osteocytes. Cytoglobin is located in the cytoplasm of these cell types. Evidence against an exclusively nuclear localization of cytoglobin, as recently proposed, is also provided by transfection assays with green fluorescent protein fusion constructs, which demonstrates the absence of an active nuclear import. The differential expression of cytoglobin argues against a general respiratory function of this molecule, but rather indicates a connective tissue-specific function. We hypothesize that cytoglobin may be involved in collagen synthesis. Cytoglobin expression was also observed in some neuronal subpopulations of the central and the peripheral nervous systems. Surprisingly, cytoglobin is localized in both the cytoplasm and nucleus of neurons, indicating a possible additional role of this protein in neuronal tissues.
Amino Acid Sequence↗
[Long-term changes in the efficiency of the excitatory and inhibitory connections in the neuronal micronetworks of the motor cortex induced by tetanization of the thalamic nuclei and the sensory cortex].
Assumption was verified that tetanization which produces long-term potentiation (LTP) and long-term depression (LTD) in the efficacy of thalamo-cortical inputs and the inputs from the sensory cortex to motor cortex results also in long-term modifications of the efficacy of interneuronal interactions in the motor-cortical networks. It was shown that monosynaptic excitation of one of the neighbouring neurons in the motor cortex by rhythmical stimulation of some thalamic nuclei or the sensory cortex resulted in LTP or LTD in the efficacy of excitatory and inhibitory transmission between the neurons of the network. The efficacy of the synapses formed by the terminals of one axon on the neighbouring cells could be variously modified. If the network contained the neurons with multiple convergent inputs the tetanization resulted in the LTP of most of them. In the neuronal networks which contained local feed-forward inhibitory circuits, LTP (LTD) of the inhibitory input to the target cell was accompanied by LTD (LTP) of the excitatory input to the same target cell. Different patterns of modified interneuronal connections were caused by tetanization of different brain structures (thalamic nuclei or sensory cortex).
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Morphological study of fetal neocortical transplant grafted to the cerebral corresponding area in young rats. Differentiation of immature neurons and reciprocal connections of fibers between graft-host brain.
The fetal neocortical transplant (E15-17 days gestation) of Wistar rat was grafted to the corresponding neocortical region (frontal-parietal lobe) of the same strain in young rats (4-5 weeks old). On the 7th, 15th, 30th, 60th, 150th day after transplantation, the sections cut through the middle area of graft-host brain were examined by HE, Nissl, Glees stain, immunohistochemical technique for GFAP and NF, Nissl, Glees stain, immunohistochemical technique for GFAP and NF, acetylcholinesterase (AChE) histochemistry as well as horseradish peroxidase (HRP) retrograde tracing with light microscope. Some of the sections were also examined with TEM. The result showed that most immature neurons within the graft can survive, grow, differentiate and mature, and are similar to the structure of the neocortical neurons of host brain. This study also provides patterns of integration of the interface between graft-host brain varying with the proliferation of reactive astrocyte as well as graft-host reciprocal connection of fibers.
Acetylcholinesterase↗