Realistic simulations of neuronal activity: a contribution to the debate on direct detection of neuronal currents by MRI

Many efforts have been done in order to preview the properties of the magnetic resonance (MR) signals produced by the neuronal currents using simulations. In this paper, starting with a detailed calculation of the magnetic field produced by the neuronal currents propagating over single hippocampal CA1 pyramidal neurons placed inside a cubic MR voxel of length 1.2 mm, we proceeded on the estimation of the phase and magnitude MR signals. We then extended the results to layers of parallel and synchronous similar neurons and to ensembles of layers, considering different echo times, voxel volumes and neuronal densities. The descriptions of the neurons and of their electrical activity took into account the real neuronal morphologies and the physiology of the neuronal events. Our results concern: (a) the expected time course of the MR signals produced by the neuronal currents in the brain, based on physiological and anatomical properties; (b) the different contributions of post-synaptic potentials and of action potentials to the MR signals; (c) the estimation of the equivalent current dipole and the influence of its orientation with respect to the external magnetic field on the observable MR signal variations; (d) the size of the estimated neuronal current induced phase and magnitude MR signal changes with respect to the echo time, voxel-size and neuronal density. The inclusion of realistic neuronal properties into the simulation introduces new information that can be helpful for the design of MR sequences for the direct detection of neuronal current effects and the testing of bio-electromagnetic models.

Dendritic action potentials connect distributed dendrodendritic microcircuits

Lateral inhibition of cells surrounding an excited area is a key property of sensory systems, sharpening the preferential tuning of individual cells in the presence of closely related input signals. In the olfactory pathway, a dendrodendritic synaptic microcircuit between mitral and granule cells in the olfactory bulb has been proposed to mediate this type of interaction through granule cell inhibition of surrounding mitral cells. However, it is becoming evident that odor inputs result in broad activation of the olfactory bulb with interactions that go beyond neighboring cells. Using a realistic modeling approach we show how backpropagating action potentials in the long lateral dendrites of mitral cells, together with granule cell actions on mitral cells within narrow columns forming glomerular units, can provide a mechanism to activate strong local inhibition between arbitrarily distant mitral cells. The simulations predict a new role for the dendrodendritic synapses in the multicolumnar organization of the granule cells. This new paradigm gives insight into the functional significance of the patterns of connectivity revealed by recent viral tracing studies. Together they suggest a functional wiring of the olfactory bulb that could greatly expand the computational roles of the mitral-granule cell network.

Parallel network simulations with NEURON

The NEURON simulation environment has been extended to support parallel network simulations. Each processor integrates the equations for its subnet over an interval equal to the minimum (interprocessor) presynaptic spike generation to postsynaptic spike delivery connection delay. The performance of three published network models with very different spike patterns exhibits superlinear speedup on Beowulf clusters and demonstrates that spike communication overhead is often less than the benefit of an increased fraction of the entire problem fitting into high speed cache. On the EPFL IBM Blue Gene, almost linear speedup was obtained up to 100 processors. Increasing one model from 500 to 40,000 realistic cells exhibited almost linear speedup on 2,000 processors, with an integration time of 9.8 seconds and communication time of 1.3 seconds. The potential for speed-ups of several orders of magnitude makes practical the running of large network simulations that could otherwise not be explored.

Input-output relations in the entorhinal cortex-dentate-hippocampal system: evidence for a non-linear transfer of signals

In the current study we analyzed the input-output relations in the entorhinal-dentate-hippocampal system, a major network involved in long-term memory. In anesthetized guinea pigs, the system was driven by activation of perforant path neurons in the entorhinal cortex (ENT), via presubicular fibers directly stimulated in the dorsal psalterium. Perforant path neuron discharge activated in parallel the dentate gyrus (DG) and hippocampal field CA2. Whereas the output from the DG activated hippocampal field CA3, the output from the sole field CA2 was sufficient for activation of field CA1. Signals from field CA3 operated in concert with CA2, likely contributing to discharge field CA1. These findings indicate the existence of two in parallel disynaptic systems: an ENT-CA2-CA1 and an ENT-DG-CA3 system. The convergence of the latter with the former gives origin the classical trisynaptic circuit, the ENT-DG-CA3-CA1 system. The input-output relations between the population excitatory postsynaptic potentials (pEPSP) evoked in the DG, CA3, CA2 and CA1 and the population spike (PS) evoked in the structure upstream (the input) were described by smooth sigmoid curves. In contrast, the input-output relations of the PS versus the pEPSP within each structure were described by steep sigmoid curves. The net input-output functions of the DG (ENT-DG system), field CA2 (ENT-CA2 system), field CA3 (ENT-DG-CA3 system) and field CA1 (ENT-CA2-CA1&ENT-DG-CA3-CA1 system) were described by sigmoid curves. While the DG and field CA2 exhibited steep sigmoids, fields CA3 and CA1 had less steep sigmoid functions. The present study demonstrates that all structures downstream to the ENT operate according to sigmoid input-output functions, characterized by specific parameters. These different behaviors may contribute to different memory processes. We additionally demonstrate that field CA1 can be activated by field CA2, independently from field CA3. This functional dissociation between CA3 and CA1 may subserve specific roles of each field in memory encoding/retrieval.

The role of distal dendritic gap junctions in synchronization of mitral cell axonal output

One of the first and most important stages of odor processing occurs in the glomerular units of the olfactory bulb and most likely involves mitral cell synchronization. Using a detailed model constrained by a number of experimental findings, we show how the intercellular coupling mediated by intraglomerular gap junctions (GJs) in the tuft dendrites could play a major role in sychronization of mitral cell action potential output in spite of their distal dendritic location. The model suggests that the high input resistance and active properties of the fine tuft dendrites are instrumental in generating local spike synchronization and an efficient forward and backpropagation of action potentials between the tuft and the soma. The model also gives insight into the physiological significance of long primary dendrites in mitral cells, and provides evidence against the use of reduced single compartmental models to investigate network properties of cortical pyramidal neurons.

Dendritic Ih selectively blocks temporal summation of unsynchronized distal inputs in CA1 pyramidal neurons

The active dendritic conductances shape the input-output properties of many principal neurons in different brain regions, and the various ways in which they regulate neuronal excitability need to be investigated to better understand their functional consequences. Using a realistic model of a hippocampal CA1 pyramidal neuron, we show a major role for the hyperpolarization-activated current, Ih, in regulating the spike probability of a neuron when independent synaptic inputs are activated with different degrees of synchronization and at different distances from the soma. The results allowed us to make the experimentally testable prediction that the Ih in these neurons is needed to reduce neuronal excitability selectively for distal unsynchronized, but not for synchronized, inputs.

The place of Belsey Mark IV fundoplication in the era of laparoscopic surgery☆

OBJECTIVES

Laparoscopic fundoplication to correct or avoid gastroesophageal reflux decreased Belsey Mark IV fundoplication (BMIV) dramatically worldwide. The purpose of this paper was to determine the role of BMIV and its current indications.

METHODS

We reviewed all patients who underwent fundoplication between April 1997 and December 2001. All patients underwent a complete work-up included barium meal, endoscopy, 24-h pH-metry and manometry preoperatively.

RESULTS

Sixty-two consecutive fundoplications were performed. There were 23 males and 39 females. Forty-six patients were treated by laparoscopic approach (37 patients with total and nine patients with partial fundoplication). BMIV was preferred in 16 patients with the following indications: reoperations for failed oesophageal surgery (5), hiatal hernia fixed in the chest (4), epiphrenic oesophageal diverticula (3), diffuse oesophageal spam (2), hiatal hernia associated with bullous emphysema (1), leiomyoma of the oesophago-gastric junction (1). Excellent to good results were reported in 14 patients and poor in two. Follow-up was completed in all patients.

CONCLUSIONS

BMIV remains a valid fundoplication although the current indications are now limited. The technique is to be considered an additional, but necessary, weapon for thoracic surgeons with interest in oesophageal disease.

Quantitative modeling of perception and production of time intervals

The accurate perception/production of durations in the seconds and minutes range is important in a number of everyday activities, but the lack of direct experimental evidence on the neural circuits that could be involved has precluded the detailed elucidation of the underlying physiological mechanisms. We show, using a basic biophysical model of a timekeeping system and experimental data on time intervals produced or estimated under different conditions, that experimental values, variability, and distributions can be quantitatively explained in terms of a background synaptic activity such as that generated by attention. The model provides a plausible neural substrate for encoding time intervals, and the findings suggest how it may interplay at the single neuron level with the attentional system, to elaborate a subjective representation of the elapsing time.

[Recent concepts regarding the treatment of traumatic hemopneumothorax]

BACKGROUND

The aim of the study was to examine the different therapeutic options for traumatic hemopneumothorax and in particular the latest concepts (videothoracoscopy).

METHODS

A retrospective study was performed on a group of 20 patients with post-traumatic hemothorax and/or pneumothorax attending the emergency department at Ospedale Vittorio Emanuele in Catania over the past 10 years where 2500 patients with abdominal and/or thoracic trauma were treated during the same period. The patients included 18 males and 2 females with a mean age of 35 years (range 13-70). The cause of injury was a gunshot wound in 8 patients, a blunt trauma following a road accident in 9 and a stab wound in 3 patients. A drainage tube was inserted in 15 patients (a videothoracoscopic approach was used in 2 cases); 3 patients required a diaphragmatic suture and 2 thoracotomy (inferior lobectomy and repair of the main left bronchus).

RESULTS

Three out of 20 patients died with an overall mortality rate of 15%. Two patients died respectively from severe brain and abdominal injuries associated with thoracic trauma, while the third patient died from septic complications.

CONCLUSIONS

In the past few years videothoracoscopy has allowed a different approach to be used in patients affected by hemo-pneumothorax since it allows a more accurate diagnosis (extent and origin of bleeding, identification of associated diaphragmatic lesion) and the possibility of treating pleuro-pulmonary injuries without performing extensive thoracotomies.

A single-trocar technique for minimally-invasive surgery of the chest

BACKGROUND

In thoracic surgery, the classic thoracoscope is used to perform simple maneuvers in the chest. We have devised a minimally invasive technique that requires only a single trocar. This technique is used in our department to diagnose and treat a wide range of thoracic pathologies.

METHODS

Between October 1998 and August 1999, 37 patients underwent surgery prospectively for a thoracic disease through a single trocar. There were 19 men and 18 women with a mean age of 60 years (range, 40-85). The trocar was flexible or soft and had a diameter of 15-20 mm. A 2-cm skin incision was made in the planned intercostal space. The chest drain was always inserted under video control.

RESULTS

The mean operative time was 53 +/- 5 min. One patient developed intraoperative bleeding that required intubation and a 5-cm mini-thoracotomy. In one patient with stage II empyema, it was necessary to insert another trocar. Chest tubes were removed after 77.7 +/- 7 h. Hospital stay was 4 +/- 1 days (range, 2-14). Histologic examination revealed malignant disease in 26 cases and benign disease in 11. Two patients (5.4%) developed wound infections. None of the patients had port site metastasis. There were no hospital deaths.

CONCLUSION

Because of its simplicity, we recommend the use of this mini-invasive technique in place of the classic thoracoscope or video-mediastinoscope.

A model of the effects of cognitive load on the subjective estimation and production of time intervals

How and where the brain calculates elapsing time is not known, and one or more internal pacemakers or others timekeeping systems have been suggested. Experiments have shown that the accuracy in estimating or producing time intervals depends on many factors and, in particular, both on the length of the intervals to be estimated and on the additional, and unrelated, cognitive load required during the task. The psychological 'attentional approach' is able to explain the experimental data in terms of perturbations of a cognitive timer. However, the basic biophysical mechanisms that could be involved at the single neuron level are still not clear. Here we propose a computational model suggesting how the process to focus the attention on a non-temporal task could alter the perception of time intervals as observed in the experiments. The model suggests that an attention-based excitatory and/or inhibitory background synaptic noise, impinging on the pacemaker circuit, could represent both qualitative and quantitative features of the cognitive load. These effects are predicted to be independent of the number, location or specific implementations of the internal timing systems.

[Systemic neoadjuvant chemotherapy in locally advanced gastric carcinoma: phase II study with 5-fluorouracil, epirubicin and etoposide]

Locally advanced gastric adenocarcinomas have a poor prognosis, particularly when the tumours are bulky, located in the cardia or when they present local/regional lymph node involvement. Neoadjuvant chemotherapy for locally advanced gastric cancer is an experimental treatment strategy that may increase resectability and improve survival in patients suffering from an almost uniformly fatal neoplasm. At our institution 11 patients younger than 70 years of age in good physical and mental condition with non-resectable adenocarcinomas of the stomach as determined by endoscopy, computed tomography scans and pathology examinations, were treated with combination chemotherapy [5-fluorouracil (375 mg/m2 i.v. for 5 days, epirubicin (60 mg/m2 i.v. on day 1), etoposide 80 mg/m2 on days 1, 2 and 3, leucovorin 100 mg/m2 for 5 days] every 4 weeks as neoadjuvant chemotherapy. The response to chemotherapy was evaluated after three courses. After three courses, we had one complete response, 8 partial responses or stable disease, and no response in two cases. One patient was still alive 36 months postoperatively. These preliminary results suggest that this protocol is an effective form of neoadjuvant chemotherapy for locally advanced gastric carcinoma.

Dendritic potassium channels in hippocampal pyramidal neurons

Potassium channels located in the dendrites of hippocampal CA1 pyramidal neurons control the shape and amplitude of back-propagating action potentials, the amplitude of excitatory postsynaptic potentials and dendritic excitability. Non-uniform gradients in the distribution of potassium channels in the dendrites make the dendritic electrical properties markedly different from those found in the soma. For example, the influence of a fast, calcium-dependent potassium current on action potential repolarization is progressively reduced in the first 150 micrometer of the apical dendrites, so that action potentials recorded farther than 200 micrometer from the soma have no fast after-hyperpolarization and are wider than those in the soma. The peak amplitude of back-propagating action potentials is also progressively reduced in the dendrites because of the increasing density of a transient potassium channel with distance from the soma. The activation of this channel can be reduced by the activity of a number of protein kinases as well as by prior depolarization. The depolarization from excitatory postsynaptic potentials (EPSPs) can inactivate these A-type K+ channels and thus lead to an increase in the amplitude of dendritic action potentials, provided the EPSP and the action potentials occur within the appropriate time window. This time window could be in the order of 15 ms and may play a role in long-term potentiation induced by pairing EPSPs and back-propagating action potentials.

Computational model of the effects of stochastic conditioning on the induction of long-term potentiation and depression

The long-term potentiation (LTP) or long-term depression (LTD) of synaptic strength are currently considered to be the first microscopic steps leading to learning and memory. The great majority of experiments (both in vitro and in vivo) studying the basic mechanisms of LTP and LTD induction use conditioning protocols in which the presynaptic stimuli are delivered at constant frequencies. This is not, however, what is commonly found in vivo, where a highly irregular spiking activity seems to drive most of the neuronal functions. Thus, some important aspects of the induction characteristics of LTP and LTD expressed in vivo might have been overlooked by the experiments. Using a simple schematic model for a synapse we show here that, in fact, the statistical properties of a presynaptic conditioning signal could change the probability to induce LTP and/or LTD, suggesting a new and faster operating mode for a synapse.

Clinical features and oesophageal motility in patients with tight fundoplication

OBJECTIVE

Tight fundoplication is a well known complication of surgery for gastroesophageal reflux. We have noted, in clinical experience, that some patients operated for gastro-oesophageal reflux develop pharyngo-oesophageal dysphagia. This study was undertaken to elucidate, by comparing motility data in patients with and without tight fundoplication, the pathophysiologic characteristics of the swallowing mechanism and to clarify the cause of dysphagia in these patients.

METHODS

Sixteen patients with postoperative persistent dysphagia following a fundoplication have been studied, of those 10 presented a lower oesophageal sphincter resting pressure above or equal to 30 mmHg (group A). Clinical work-up included a questionnaire, oendoscopy, manometry and barium meal or video-roentgenography. Oesophageal manometry was performed using a slow pull through technique while the stationary pull through technique was used for the pharyngo-oesophageal segment. The following parameters were evaluated: (a) amplitude of pharyngeal contraction; (b) upper oesophageal sphincter resting pressure; (c) amplitude of upper oesophageal sphincter contraction; (d) amplitude of oesophageal contraction; (e) lower oesophageal sphincter resting pressure. The results were compared to those of 21 patients who had a fundoplication with normal lower oesophageal sphincter pressure (group B).

RESULTS

in group A there were three males and seven females, with a mean age of 51 years (ranging from 28 to 60 years). Previous operations were Nissen in two and Nissen Rossetti in eight patients. Three out of 10 patients of group A presented pharyngo-oesophageal dysphagia. Mean lower oesophageal sphincter 36 versus 21 mmHg and upper oesophageal sphincter 86 versus 42 mmHg resting pressure, pharyngeal 147 versus 76 mmHg and oesophageal amplitude, upper oesophageal contraction 251 versus 103 mmHg were significantly higher in patients of group A versus group B. An increased number of repetitive contractions was also found in group A. The presence of a strong correlation was demonstrated between the pharyngeal amplitude and the closing tone of the upper oesophageal sphincter (R2 0.742 and R2 0.739) in both groups.

CONCLUSION

Tight fundoplication is, in our experience, always associated with total fundoplication. The appearance of pharyngo-oesophageal dysphagia in the postoperative period in patients operated on to correct gastroesophageal reflux using a total fundoplication, should not be under-estimated because it suggests an obstruction of the distal oesophagus.

Long-term potentiation and depression induced by a stochastic conditioning of a model synapse

Protracted presynaptic activity can induce long-term potentiation (LTP) or long-term depression (LTD) of the synaptic strength. However, virtually all the experiments testing how LTP and LTD depend on the conditioning input are carried out with trains of stimuli at constant frequencies, whereas neurons in vivo most likely experience a stochastic variation of interstimulus intervals. We used a computational model of synaptic transmission to test if and to what extent the stochastic fluctuations of an input signal could alter the probability to change the state of a synapse. We found that, even if the mean stimulation frequency was maintained constant, the probability to induce LTD and LTP could be a function of the temporal variation of the input activity. This mechanism, which depends only on the statistical properties of the input and not on the onset of additional biochemical mechanisms, is not usually considered in the experiments, but it could have an important role to determine the amount of LTP/LTD induction in vivo. In response to a change in the distribution of the interstimulus intervals, as measured by the coefficient of variation, a synapse could be easily adapted to inputs that might require immediate attention, with a shift of the input thresholds required to elicit LTD or LTP, which are restored to their initial conditions as soon as the input pattern returns to the original temporal distribution.

Role of an A-type K+ conductance in the back-propagation of action potentials in the dendrites of hippocampal pyramidal neurons

Action potentials elicited in the axon actively back-propagate into the dendritic tree. During this process their amplitudes can be modulated by internal and external factors. We used a compartmental model of a hippocampal CA1 pyramidal neuron to illustrate how this modulation could depend on (1) the properties of an A-type K+ conductance that is expressed at high density in hippocampal dendrites and (2) the relative timing of synaptic activation. The simulations suggest that the time relationship between pre- and postsynaptic activity could help regulate the amplitude of back-propagating action potentials, especially in the distal portion of the dendritic tree.

Energy efficient modulation of dendritic processing functions

The voltage dependent ionic conductances and the passive properties of the neural membrane determine how external inputs are processed by the dendritic tree, and define the computational characteristics of neurons. However, what controls these characteristics and how they are implemented at the single neuron level, in such a way that an external input results in the coding of the appropriate output, is essentially unknown. We show here that a slow inactivation of the Na+ channel, involved in the attenuation and/or failure of APs in the dendrites, acts as an active and energy efficient filter of synaptic input, and results in an activity-dependent control of the properties of individual neurons. Thus, the activation or expression of this mechanisms could be an efficient way to selectively modulate the input/output processing properties of dendrites, and could be needed to limit or suppress the onset of a number of pathological brain disorders.

Pedunculated intraluminal oesophageal fibrolipoma. A case report

Benign tumours of the oesophagus are rare. A patient with a pedunculated intraluminal tumour presented with dysphagia of recent onset and the tumour resected at open operation and histology was confirmed as a benign vascular fibrolipoma. We discuss the management and review the relevant literature.

Neuropsychological outcome of operated cerebral aneurysms: prognostic factors on 148 patients

OBJECTIVES

To analyse prognostic factors in patients operated upon for cerebral aneurysms. A previous investigation by our group showed that patients operated later than 10 days after bleeding have a worse neuropsychological prognosis, but the number of patients operated upon within 3 days was not sufficient. Here, a new sample of patients with early surgery is included in the analyses.

MATERIAL AND METHODS

Patients numbered 148 (65 with ACoA, 39 with MCA, and 44 with PCoA aneurysms): 56 were operated within 3 days, 44 within 4-10 days, and 48 after at least 10 days from bleeding. A standardized battery of 13 neuropsychological tests was adopted: we considered both the number of defective scores and the average performance.

RESULTS

Patients operated later than 10 days after bleeding had a worse prognosis than the 2 groups with early and intermediate surgery, which were not different. Aneurysm site was not relevant. Old age and low education were associated with a worse prognosis. Hunt and Hess scores at operation and post operation were also predictive of the outcome.

CONCLUSION

On the whole, patients operated upon later than 10 days after bleeding have a less favourable prognosis than those with earlier operation timing.

Possible roles of retrograde messengers on LTP, LTD, and associative memory

There are still no clear biophysical models for Associative Long-Term Potentiation (LTP) and Depression (LTD) in the hippocampus, where two populations of synapses targeted to the same receptive field are involved. Here we propose a model that allows an interpretation of the experiments in terms of the molecular processes that may be involved in associative memory. The model suggests that retrograde messengers could have a critical role in the induction and maintenance of associative LTP and LTD, by controlling the coupling between the two populations of synapses.

Modeling the attenuation and failure of action potentials in the dendrites of hippocampal neurons

We modeled two different mechanisms, a shunting conductance and a slow sodium inactivation, to test whether they could modulate the active propagation of a train of action potentials in a dendritic tree. Computer simulations, using a compartmental model of a pyramidal neuron, suggest that each of these two mechanisms could account for the activity-dependent attenuation and failure of the action potentials in the dendrites during the train. Each mechanism is shown to be in good qualitative agreement with experimental findings on somatic or dendritic stimulation and on the effects of hyperpolarization. The conditions under which branch point failures can be observed, and a few experimentally testable predictions, are presented and discussed.