Fiber Longitudinal Direction Research Articles

Differential effects of quinidine, flecainide, and cibenzoline on anisotropic conduction in the isolated porcine heart.

Anisotropic conduction characteristics, expressed as the ratio of conduction velocities in the longitudinal (Vl) and transverse (Vt) fiber directions, may play a role in the mechanism of some ventricular tachycardias and is influenced by pharmacologic interventions. Discrepancies exist among the reported orientation-dependent effects of available Class I antiarrhythmic drugs. The aim of this study was to assess the respective effects of quinidine (Class IA), flecainide (Class IC), and cibenzoline (not subclassified) on the anisotropic conduction of porcine hearts, in corroboration of their effects on ventricular action potentials. We studied the actions of 3 and 10 microM quinidine, 1 and 3 microM flecainide, and 0.3 and 1 microM cibenzoline on Vl and Vt determined from 128 electrograms recorded with a plaque electrode on the anterior left ventricle of isolated perfused hearts (spacing 2.5 mm). Vl and Vt were computed from isochronal maps displaying ellipsoid activation during stimulation from the center of the plaque. The effects on the maximal rate of depolarization (Vmax) of action potentials were obtained from ventricular muscle exposed to the same drugs. Flecainide [1 microM] and cibenzoline [0.3 microM] did not alter Vl and Vt significantly. Quinidine [3 microM] predominantly depressed Vl at rapid pacing rates, but the Vl/Vt ratio was not significantly altered. Quinidine [10 microM] and flecainide [3 microM] reduced Vl and Vt in a frequency-dependent fashion. Conversely, cibenzoline [1 microM] mostly decreased Vl and thus decreased the Vl/Vt ratio and increased the Vl/Vt at all pacing rates. This different effect was not related to a greater depressant effect on Vmax Quinidine and flecainide act similarly on the anisotropic pattern of conduction (both drugs increase the Vl/Vt ratio), whereas cibenzoline exerts opposite effects. Orientation-dependent effects are different among Class I antiarrhythmic drugs and may be of importance in their therapeutic efficacy or proarrhythmic potential.

Circulating excitations and re-entry in the pregnant uterus.

The propagation of individual action potentials during spontaneous bursts of activity in isolated pregnant rat myometrium in the final stage of pregnancy was analyzed. Simultaneous recordings from 240 extracellular recording sites (inter-electrode distance 1 mm) made it possible to reconstruct, in spatial and temporal details, the conduction of the electrical impulse. On several occasions, the impulse was seen to be conducted in a circular fashion whereby the impulse repeatedly re-excited the myometrium. No evidence was found of circuits rotating around an area of depressed excitability or anatomical obstacle, suggesting that these circuits are similar to those proposed to occur in cardiac muscle by the "leading circuit" model. Because of the anisotropic conduction properties of the myometrium, several circuits revolved in an ellipse with the long axis parallel to the longitudinal fiber direction, similar to functional re-entry in the anisotropic ventricle. Of a total of 46 bursts analyzed, myometrial re-entry occurred in 10 of the bursts. Furthermore, re-entries were found at day 17, day 19 and day 21 ( = term) stages of pregnancy suggesting that re-entry may occur throughout the final stage of gestation. In conclusion, functional re-entry, previously shown in the myocardium, may also occur in the pregnant myometrium. The presence of re-entry in the uterus could underlie the mechanism for uterine tachysystole, leading to dysfunctional labor.

Spatial distribution of cardiac transmembrane potentials around an extracellular electrode: dependence on fiber orientation

Recent theoretical models of cardiac electrical stimulation or defibrillation predict a complex spatial pattern of transmembrane potential (Vm) around a stimulating electrode, resulting from the formation of virtual electrodes of reversed polarity. The pattern of membrane polarization has been attributed to the anisotropic structure of the tissue. To verify such model predictions experimentally, an optical technique using a fluorescent voltage-sensitive dye was used to map the spatial distribution of Vm around a 150-microns-radius extracellular unipolar electrode. An S1-S2 stimulation protocol was used, and vm was measured during an S2 pulse having an intensity equal to 10x the cathodal diastolic threshold of excitation. The recordings were obtained on the endocardial surface of bullfrog atrium in directions parallel and perpendicular to the cardiac fibers. In the longitudinal fiber direction, the membrane depolarized for cathodal pulses (and hyperpolarized for anodal pulses) but only in a region within 445 +/- 112 microns (and 616 +/- 78 microns for anodal pulses) from the center of the electrode (n = 9). Outside this region, vm reversed polarity and reached a local maximum at 922 +/- 136 microns (and 988 +/- 117 microns for anodal pulses) (n = 9). Beyond this point vm decayed to zero over a distance of 1.5-2 mm. In the transverse fiber direction, the membrane depolarized for cathodal pulses (and hyperpolarized for anodal pulses) at all distances from the electrode. The amplitude of the response decreased with distance from the electrode with an exponential decay constant of 343 +/- 110 microns for cathodal pulses and 253 +/- 91 microns for anodal pulses (n = 7). The results were qualitatively similar in both fiber directions when the atrium was bathed in a solution containing ionic channel blockers. A two-dimensional computer model was formulated for the case of highly anisotropic cardiac tissue and qualitatively accounts for nearly all the observed spatial and temporal behavior of vm in the two fiber directions. The relationships between vm and both the "activating function" and extracellular potential gradient are discussed.

Temperature dependence of the tensile behaviour of aramid/aluminium laminates

Aramid/aluminium laminates (ARALL® laminates) are a family of new hybrid composites made of alternating layers of thin aluminium alloy sheets with plies of epoxy adhesive prepreg containing unidirectional aramid fibres. The effect of elevated and cryogenic temperatures on these materials is critical to aerospace applications. ARALL 1, 2, 3, and 4 laminates have been tested in tension at temperatures ranging from −300F–400 °F (−184–204 °C) and at room temperature after exposure. This paper summarizes how tensile properties depend on temperature for these four ARALL laminates under the conditions described. At cryogenic temperatures, no degradation of ultimate tensile strengths, tensile yield strengths and moduli were found for either the longitudinal or transverse directions for ARALL 1–4 laminates. Furthermore, the mechanical properties remained the same or increased slightly as the temperature decreased. Longitudinal and transverse ultimate tensile strengths, tensile yield strengths, and moduli of ARALL 1–3 laminates at room temperature remain nearly constant after the laminates were exposed for 1, 10 and 100 h to temperatures up to 250 °F (121 °C), and up to 350 °F (177 °C) for ARALL 4 laminates. However, these properties determined at the elevated temperatures after 1, 10 and 100 h exposure showed a tendency to decrease with increasing temperature. The properties of ARALL laminates are much better in the longitudinal fibre direction than those of conventional monolithic aluminium alloys. Typical failure modes of the test specimens in the high-temperature range were examined using a scanning electron microscope. The discussions are also described in the paper.

Effects of cutouts on the dynamic response of curved rectangular composite panels

Numerical results obtained by the finite-element method were compared with experimental results obtained by holographic interferometry to quantify the effects of cutout size and orientation on the first five natural frequencies and mode shapes of a curved Gr-Ep panel. The clamped-clamped panels had a quasi-isotropic layup [0, - 45, 45, 90]s and measured 12 in. (30.48 cm) high with a 12-in. (30.48-cm) chord. Cutouts having dimensions of 2 x 2 in. (5.08 x 5.08 cm), 2x4 in. (5.08 x 10.16 cm) and 4 x 4 in. (10.16 X 10.16 cm) were centrally located in the panels. The effects of the orientation of the 2x4 in. (5.08 x 10.16 cm) cutout were studied. When the finite-element results were compared with the time-averaged holograms, the two techniques showed close correlation of both the natural frequencies and mode shapes. Increasing the cutout size resulted in the expected reduction in natural frequency. An unexpected mode switch was observed to occur in the first two modes as more material was removed from the panel. It was found that the 0-deg cutout orientation had a significant effect on the panel stiffness, whereas the other cutout orientations did not significantly effect the stiffness. Nomenclature El == Young's modulus in longitudinal fiber direction E2 = Young's modulus in the transverse fiber direction Gn = shear modulus Keq - equivalent spring stiffness m - mass t = panel thickness *>12, *>2i = Poisson's ratios p = mass density QJ = frequency

Patterns of parallel signal transmission between multiple alpha efferents and multiple Ia afferents in the cat semitendinosus muscle.

1. Certain features of signal transmission from multiple alpha-axons to multiple Ia fibres via extrafusal muscle tissue were studied by stimulating three ventral root filaments separately and simultaneously and by recording spike trains from three dorsal root filaments. 2. Post-stimulus time histograms (PSTH) revealed widely graded influences of single motor unit contractions on different muscle spindles, thus confirming results of Binder et al. (1976, 1980a). 3. In most instances, the signal transmission from a single motor unit to a single Ia fibre was disturbed to varying degrees by concomitant activity from other motor units. 4. This deterioration of signal transmission can probably be compensated for, at least in part, by correlations between discharge patterns of two or more Ia fibres induced by the "common input" of motor unit activity. 5. These correlations showed a topographical pattern such that a peak in the cross-correlogram (CCH) occurred at zero time for pairs of spindles located virtually in parallel to each other, and was shifted away from zero time as a function of the serial distance between spindles in the longitudinal muscle fibre direction. 6. Fusimotor innervation had complex effects on correlations between Ia fibre discharge patterns. It might de-correlate the latter as well as favour new correlations. 7. The possible role of topographical correlation patterns in the afferent reflex limb for tremor suppression is discussed.

2316. Acrylamide on the nerves again: Hopkins, A. (1970). The effect of acrylamide on the peripheral nervous system of the baboon. J. Neurol. Neurosurg. Psychiat.33, 805

The effect of pH on the kinetics of tremolite and anthophyllite dissolution was investigated at 25 °C in batch reactors over the pH range of 1–13.5, in inorganic buffered solutions. Dissolution rates were obtained based on the release of Si and Mg. Results obtained in this study show different behaviors for both minerals. For tremolite, dissolution rates show a noticeable dependence on pH between 1 and 8, decreasing as pH increases and reaching a minimum around neutral conditions. At basic pH this dependence becomes even stronger, but dissolution takes place together with collateral effects of saturation and carbonation. A preferential release of Ca and Mg is observed in acid media, lowering the Mg/Si ratio to the extent that Mg solubility decreases with pH. For anthophyllite, dissolution rates also show a strong dependence on pH, between 1 and 9.5. At the same pH, anthophyllite dissolves up to 8 times faster than tremolite. For pH > 9.5 this dependence is smooth, and it is probably associated with effects of saturation and carbonation. Dissolution is also non-stoichiometric with a faster release of Mg with respect to Si in acid media. SEM observations show differences in the breakage mechanism of the fibers. The anthophyllite particle breakage during dissolution consists of the splitting of bundle fibers parallel to the fiber longitudinal direction. However, for tremolite, other than fiber splitting, particles shorten induced by coalescence of etch pits developed perpendicular to c axe.