A stochastic model for interacting neurons in the olfactory bulb

We focus on interacting neurons organized in a block-layered network devoted to the information processing from the sensory system to the brain. Specifically, we consider the firing activity of olfactory sensory neurons, periglomerular, granule and mitral cells in the context of the neuronal activity of the olfactory bulb. We propose and investigate a stochastic model of a layered and modular network to describe the dynamic behavior of each prototypical neuron, taking into account both its role (excitatory/inhibitory) and its location within the network. We adopt specific Gauss-Markov processes suitable to provide reliable estimates of the firing activity of the different neurons, given their linkages. Furthermore, we study the impact of selective excitation/inhibition on the information transmission by means of simulations and numerical estimates obtained through a Volterra integral approach.

Heart rate during exercise: mechanisms, behavior, and therapeutic and prognostic implications in heart failure patients with reduced ejection fraction

Exercise intolerance is a typical manifestation of patients affected by heart failure with reduced ejection fraction (HFrEF); however, the relationship among functional capacity, mortality, and exercise-induced heart rate response during exercise remains unclear in either sinus rhythm or atrial fibrillation subjects. Heart rate increase during incremental load exercise has a typical pattern in normal subjects, whereas it is commonly compromised in HFrEF patients, mainly due to the imbalance of the autonomic nervous system. In the present review, we aim to describe the behavior of heart rate during exercise in normal subjects and in HFrEF patients in sinus rhythm and atrial fibrillation, understanding and explaining the mechanism leading to a different exercise performance and functional limitation. Moreover, the role of chronotropic incompetence and the need of standardizing the cutoff criteria are also discussed in order to clarify the clinical importance, the prognostic relevance, and the potential therapeutic implications of this condition. Looking into the relative contribution and interaction of heart rate response during exercise might represent an important issue to guide individualized therapeutic interventions and prognostic assessment in HFrEF patients.

On two diffusion neuronal models with multiplicative noise: The mean first-passage time properties

Two diffusion processes with multiplicative noise, able to model the changes in the neuronal membrane depolarization between two consecutive spikes of a single neuron, are considered and compared. The processes have the same deterministic part but different stochastic components. The differences in the state-dependent variabilities, their asymptotic distributions, and the properties of the first-passage time across a constant threshold are investigated. Closed form expressions for the mean of the first-passage time of both processes are derived and applied to determine the role played by the parameters involved in the model. It is shown that for some values of the input parameters, the higher variability, given by the second moment, does not imply shorter mean first-passage time. The reason for that can be found in the complete shape of the stationary distribution of the two processes. Applications outside neuroscience are also mentioned.

From myocardium to the atherosclerotic plaque: new perspectives in cardiologic imaging

Molecular imaging is an innovative and promising approach in cardiology for functional characterization of atherosclerosis. Nuclear, ultrasound and magnetic resonance imaging have been used for assessment of atherosclerosis of large and small arteries in several clinical and experimental studies. Positron Emission Tomography with fluorodeoxyglucose can measure metabolic activity and vulnerability of atherosclerotic plaques, identifying individuals at risk of future cardiovascular events. Magnetic resonance imaging can quantify carotid artery inflammation using iron oxide nanoparticles as contrast agent. In addition, macrophage accumulation of iron particles in atherosclerotic plaques may allow monitoring of inflammation during drug therapy, whereas contrast-enhanced ultrasound imaging may detect plaque neovascularization. Currently, technical factors, including cardiac and diaphragmatic motion and small size of coronary vessels, limit routine application of these techniques for coronary imaging. Purpose of this review is to describe state of the art and potential areas of clinical applications of molecular imaging of atherosclerosis.

[Cardiovascular risk in systemic inflammatory diseases]

Systemic inflammatory diseases are associated with increased cardiovascular morbidity and mortality. The link between inflammatory and cardiovascular diseases can be attributed to the coexistence of classical risk factors and inflammatory mechanisms activated during systemic inflammatory diseases involving the immune system. Unfavorable metabolic effects of anti-inflammatory drugs can also contribute to increase cardiovascular risk. Yet, clinical implications of these findings are not entirely clear, and deeper knowledge and awareness of cardiac involvement in inflammatory diseases are necessary. The aim of this review is to summarize cardiac involvement in systemic inflammatory diseases and to identify aspects where evidence is currently lacking that would deserve further investigation in the future.

[From bench to bedside: new insights into the treatment of heart failure]

Despite significant advances in pharmacological and clinical treatment, heart failure remains a leading cause of morbidity and mortality worldwide. G-protein coupled receptors are a wide superfamily of plasma membrane receptors which represent an important target of heart failure drug therapy. Since heart failure is characterized by the overactivity of different neurohormones, such as catecholamines and angiotensin II, responsible for several detrimental effects on the cardiovascular system, over the last decade therapeutic strategies targeting beta-adrenergic and angiotensin receptors have been developed. Despite the introduction of successful drug classes, such as beta-adrenergic receptor blockers, angiotensin-converting enzyme inhibitors and sartans, heart failure still poses an enormous challenge, thus indicating the urgent need to develop innovative treatments that might counteract mechanisms involved in heart failure onset and progression. It is now established that a single receptor, activated by the same agonist, can elicit several different signaling pathways often resulting in opposite cellular responses, some beneficial and some detrimental. However, drugs currently used in heart failure target receptors on their extracellular domain by competing with the endogenous agonists. Thus, they can inhibit non-specifically all the receptor-related signaling pathways including those with beneficial activity whose blockade would not be desirable in heart failure. These observations stress the need for the generation of new therapeutic molecules able to target specific signaling pathways which might result in innovative therapies for cardiovascular disease.

[Endothelial dysfunction in type 2 diabetic patients with normal coronary arteries. A peripheral arterial tonometry study]

AIM

to evaluate endothelial function (EF) in diabetic and non-diabetic patients without CAD by peripheral artery tonometry (PAT) technique.

METHODS

a cohort of 94 patients (55 men and 39 postmenopausal women; mean age 63 +/- 9 years) undergoing coronary angiography was divided into 2 groups: 58 patients with DM and (group 1) and 36 patients without DM. Endothelial dysfunction (ED) was assessed by digital pulse amplitude, using a fingertip peripheral arterial tonometry (PAT). As a measure of ED, reactive hyperemia index (RHI) was calculated as the ratio of the digital pulse volume during reactive hyperemia following 5 min ischemia and its basal value.

RESULTS

prevalence of cardiovascular risk factors was similar between the two groups. RHI values were significantly lower in diabetic patients compared to non-diabetics (1.72 +/- 0.34 vs 2.00 +/- 0.44; p < 0.005) and they correlated with levels of glycosylated hemoglobin (p = 0.05; r = -0.266).

CONCLUSION

despite similar level of other risk factors, EF was much more impaired in diabetic patients than in non-diabetics. These evidences further support the impact of DM on cardiovascular risk.

Implantable cardioverter defibrillator to prevent sudden cardiac death in a patient with systemic sclerosis: A clinical case

Ventricular arrhythmias are frequent in patients with systemic sclerosis and may result in sudden cardiac death. We report the case of a patient with systemic sclerosis and recent syncopes in whom induction of unstable sustained ventricular tachycardia of 2 different morphologies accompanied by syncopal event was demonstrated at the electrophysiological study. He was then implanted a 3rd generation implantable cardioverter defibrillator and remained thereafter asymptomatic. We suggest that aggressive testing is warranted in systemic sclerosis patients with suspected malignant arrhythmias to identify candidates for defibrillator implantation and prevent sudden deaths.

[Clinical applications of MIBG SPECT in chronic heart failure]

Heart failure is characterized by several abnormalities of sympathetic cardiac activity that can be assessed by 123I metaiodobenzylguanidine single photon emission computed tomography (MIBG SPECT). This technique may be useful in the clinical management of heart failure patients. Abnormal MIBG uptake has been demonstrated to be a predictor of death and arrhythmic events in heart failure patients with a prognostic power incremental to that of conventional risk markers; it may also be useful to identify patients at low risk of arrhythmias despite current guideline indications for an implantable cardioverter-defibrillator (ICD) or patients at high risk for arrhythmias not fulfilling ICD indications. This review will focus on the clinical applications of MIBG SPECT in chronic heart failure, on the basis of the most recent evidence.

Assessment of cardiac sympathetic activity by MIBG imaging in patients with heart failure: a clinical appraisal

Cardiac sympathetic activity can be assessed by (123)I-labelled meta-iodobenzylguanidine (MIBG) scintigraphy. Abnormalities of sympathetic cardiac activity have been shown in patients with heart failure, resulting in reduced MIBG uptake. Abnormal MIBG uptake predicts cardiac death, arrhythmias and all-cause mortality in patients with heart failure with a prognostic power incremental to that of conventional risk markers, and may identify patients at low risk of arrhythmias despite current guideline indications for implantable cardioverter defibrillator or patients at high risk for arrhythmias not fulfilling implantable cardioverter defibrillator indications. Prospective outcome studies are needed to assess whether MIBG imaging will have an impact on the mortality and morbidity of patients with heart failure.

On a Stochastic Leaky Integrate-and-Fire Neuronal Model

The leaky integrate-and-fire neuronal model proposed in Stevens and Zador ( 1998 ), in which time constant and resting potential are postulated to be time dependent, is revisited within a stochastic framework in which the membrane potential is mathematically described as a gauss-diffusion process. The first-passage-time probability density, miming in such a context the firing probability density, is evaluated by either the Volterra integral equation of Buonocore, Nobile, and Ricciardi ( 1987 ) or, when possible, by the asymptotics of Giorno, Nobile, and Ricciardi ( 1990 ). The model examined here represents an extension of the classic leaky integrate-and-fire one based on the Ornstein-Uhlenbeck process in that it is in principle compatible with the inclusion of some other physiological characteristics such as relative refractoriness. It also allows finer tuning possibilities in view of its accounting for certain qualitative as well as quantitative features, such as the behavior of the time course of the membrane potential prior to firings and the computation of experimentally measurable statistical descriptors of the firing time: mean, median, coefficient of variation, and skewness. Finally, implementations of this model are provided in connection with certain experimental evidence discussed in the literature.

On a pulsating Brownian motor and its characterization

As a model of Brownian motor we consider the jump diffusion motion of a particle in the presence of an asymmetric periodic potential with a unique minimum and subject to half-period space shifts at the instants of occurrence of two Poisson processes. The relevant quantities, i.e., probability current, effective driving force, stall force, power and efficiency of the motor are explicitly calculated as averages of certain functions of the random variable representing the particle position.

On Myosin II dynamics in the presence of external loads

We address the controversial hot question concerning the validity of the loose coupling versus the lever-arm theories in the actomyosin dynamics by re-interpreting and extending the phenomenological washboard potential model proposed by some of us in a previous paper. In this new model a Brownian motion harnessing thermal energy is assumed to co-exist with the deterministic swing of the lever-arm, to yield an excellent fit of the set of data obtained by some of us on the sliding of Myosin II heads on immobilized actin filaments under various load conditions. Our theoretical arguments are complemented by accurate numerical simulations, and the robustness of the model is tested via different choices of parameters and potential profiles.

On the dynamics of a pair of coupled neurons subject to alternating input rates

We present a statistical analysis of the firing activity of two coupled neuronal units that interact according to a 'sending-receiving' model. The membrane potential's behavior of both units is described by the Stein equations under the additional assumption that the spikes released by the sending neuron constitute an extra excitation for the receiving one. We also assume the presence of an alternating behavior for the rates of inputs to the sending neuron. By means of ad hoc simulations, we obtain, and then discuss, some statistical results concerning the spike production times of the units within the subintervals of the alternating inputs, as well as the reaction times of the receiving neuron.

On some computational results for single neurons' activity modeling

The classical Ornstein-Uhlenbeck diffusion neuronal model is generalized by inclusion of a time-dependent input whose strength exponentially decreases in time. The behavior of the membrane potential is consequently seen to be modeled by a process whose mean and covariance classify, it as Gaussian-Markov. The effect of the input on the neuron's firing characteristics is investigated by comparing the firing probability densities and distributions for such a process with the corresponding ones of the Ornstein-Uhlenbeck model. All numerical results are obtained by implementation of a recently developed computational method.

On a non-Markov neuronal model and its approximations

Single neuron's activity modeling is considered with reference to some earlier contributions in which a non-Markov Gaussian process is assumed to describe the time course of the neuron's membrane potential. After re-formulating the problem in a rigorous framework and pinpointing the limits of validity of such a model, the available results on the firing probability density are compared with those obtained by us by means of an ad hoc numerical algorithm implemented for the leaky integrator diffusion firing model and with some data constructed by a simulation procedure of non-Markov Gaussian processes with pre-assigned covariances. Throughout this paper, the notion of 'correlation time' plays a fundamental role for the neuronal coding process modeling.