Abstract. The transition between Sinoatrial cells and Atrial cells in the Heart is not fully understood. Here we focus in cell-to-cell mathematical models involving typical Sinoatrial cells and Atrial cells connected with experimentally observed conductance values exclusively. We are interested mainly in the geometry of the microstructure of the conduction paths in the Sinoatrial Node. We show with some models that appropriate source-sink relationships between Atrial and Sinoatrial cells may occur according to certain geometric arrangements. Inter-phase between Atrial and Sinoatrial cellsIt has been observed in different species that going from the center of the sinoatrial node (SAN) in the heart toward the atrium, there is a transitional zone of cells having morphological and electrophysiological properties in-between to that of typical sinoatrial (SA) and atrial (A) cells [16]. The transitional cells have an aspect intermediate between that of typical nodal cells and that of the common atrial cells. Typical nodal cells have poor development of the contractile system and is assumed in general that they do not contract, moreover the existence of connexin43 is undetectable in the SA node center (see [3] and references therein), but posses automaticity in firing its action potential. On the contrary, atrial cells do contract themselves, but they require of an stimulus in order to contract, and they contain mainly connexin-43. This characteristics are included in the cells models used in this paper. A whole range of intermediate cells have been reported, but more important to the models in this paper, cells with one end connected to SA cells and the other end with A cells have been found [16]. The basic structure conforming the cytoarchitecture of this groups of cells consists of interdigitations of nodal and atrial bundles forming histological connections between nodal and atrial myocytes at regular distances [18].In [29] the authors, introduce a model of strands of atrial cells penetrating the SA node observed in the Pig. The model was constructed with 101 × 101 atrial and SA cells modeled with Oxsoft HEART V4.5. The lattice so constructed has a center of SA cells forming a circle of 30 cells of radius with twelve atrial interdigitations positioned at 30 deg intervals, where interdigitations are defined as sets of atrial cells at least ten cells distant from the node centre, and which are subtended by an angle of 15 deg. In that paper SA to SA conductance is g SA = 10 nS and the A to A conductance g A , varies from 10 nS to 250 nS.

This paper shows the simultaneous recording of electrical activity and the underlying ionic currents by using a gold substrate to culture NG108-15 cells. Cells grown on two different substrates (plastic Petri dishes and gold substrates) were characterized quantitatively through scanning electron microscopy (SEM) as well as qualitatively by optical and atomic force microscopy (AFM). No significant differences were observed between the surface area of cells cultured on gold substrates and Petri dishes, as indicated by measurements performed on SEM images. We also evaluated the electrophysiological compatibility of the cells through standard patch-clamp experiments by analyzing features such as the resting potential, membrane resistance, ionic currents, etc. Cells grown on both substrates showed no significant differences in their dependency on voltage, as well as in the magnitude of the Na+ and K+ current density; however, cells cultured on the gold substrate showed a lower membrane capacitance when compared to those grown on Petri dishes. By using two separate patch-clamp amplifiers, we were able to record the membrane current with the conventional patch-clamp technique and through the gold substrate simultaneously. Furthermore, the proposed technique allowed us to obtain simultaneous recordings of the electrical activity (such as action potentials firing) and the underlying membrane ionic currents. The excellent conductivity of gold makes it possible to overcome important difficulties found in conventional electrophysiological experiments such as those presented by the resistance of the electrolytic bath solution. We conclude that the technique here presented constitutes a solution to the problem of the simultaneous recording of electrical activity and the underlying ionic currents, which for decades, had been solved only partially.

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Bone mineral density (BMD) is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk. From a technical point of view Quantitative Computed Tomography (QCT) should be the gold standard in bone densitometry. On the other hand, it is known that a greater percentage increase in skin dose is needed as the patient size is increased: positive results and side effect of long-term steroid treatment as obesity have been found for Duchenne muscular dystrophy (DMD), characterized by a progressive muscle degeneration and substitution with fat. The present work is an effort to improve osteoporosis diagnostic efficacy in children by analyzing the trabecular bone texture in CT L3 vertebra by two methods which are independent of image intensity: fractal dimension with power spectrum and wavelet packets. As results, comparing healthy children (44 children both sexes) with osteoporotic subjects (13 adult women, aged 52-87 years) great differences were noticed in all image texture indicators (p<0.0146). For DMD children (7 boys, not overweight) classified by z-score as osteoporotic because of their low BMD, texture image analysis did not exhibited high spatial frequencies as in the osteoporotic group; the probability that these two groups were similar was weak (p<0.0059), suggesting a more similar bone condition to normal or osteopenia. None of the pediatric groups exhibited as high spatial frequencies as did the osteoporotic women group. These analyses could help to determine osteoporosis in children, where it is often a diagnostic challenge.

The aim of this work is to develop a computational model to study the extrinsic regulation of the heart rate variability (HRV) during sympathetic and/or vagal stimulation. The model here proposed is based on two recent models of the sinoatrial node cell (SANC) action potential and the influence of the autonomic nervous system (ANS) on the activity of ionic channels of SANCs. The HRV was simulated by applying a random frequency stimulation using both a normal and a beta probability density function (PDF) for different ranges of stimulation frequencies. The HRV was then analyzed by computing the scale exponent using detrended fluctuation analysis. We found that our model reproduces the value of the scale exponent observed in healthy humans (α=1.07± 0.05) when simultaneous vagal and sympathetic stimulus (with beta and normal PDFs, respectively) over the frequency range from 0 to 10 Hz are applied. Our model also predicts a Brownian motion behavior when the muscarinic receptors are blocked (α=1.8) and the white noise behavior when the b-adrenergic receptors are blocked (α=0.5). Our results shed light on how the ANS regulates the HRV in healthy conditions, where it is not enough to consider only one stimulation pathway with a simple normal PDF.