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1
Pospischil, Martin, Zuzanna Piwkowska, Michelle Rudolph, Thierry Bal et Alain Destexhe. « Calculating Event-Triggered Average Synaptic Conductances From the Membrane Potential ». Journal of Neurophysiology 97, no 3 (mars 2007) : 2544–52. http://dx.doi.org/10.1152/jn.01000.2006.
Texte intégralRésumé :
The optimal patterns of synaptic conductances for spike generation in central neurons is a subject of considerable interest. Ideally such conductance time courses should be extracted from membrane potential ( Vm) activity, but this is difficult because the nonlinear contribution of conductances to the Vm renders their estimation from the membrane equation extremely sensitive. We outline here a solution to this problem based on a discretization of the time axis. This procedure can extract the time course of excitatory and inhibitory conductances solely from the analysis of Vm activity. We test this method by calculating spike-triggered averages of synaptic conductances using numerical simulations of the integrate-and-fire model subject to colored conductance noise. The procedure was also tested successfully in biological cortical neurons using conductance noise injected with dynamic clamp. This method should allow the extraction of synaptic conductances from Vm recordings in vivo.
2
Frolov, Roman, Esa-Ville Immonen et Matti Weckström. « Visual ecology and potassium conductances of insect photoreceptors ». Journal of Neurophysiology 115, no 4 (1 avril 2016) : 2147–57. http://dx.doi.org/10.1152/jn.00795.2015.
Texte intégralRésumé :
Voltage-activated potassium channels (Kv channels) in the microvillar photoreceptors of arthropods are responsible for repolarization and regulation of photoreceptor signaling bandwidth. On the basis of analyzing Kv channels in dipteran flies, it was suggested that diurnal, rapidly flying insects predominantly express sustained K+ conductances, whereas crepuscular and nocturnally active animals exhibit strongly inactivating Kv conductances. The latter was suggested to function for minimizing cellular energy consumption. In this study we further explore the evolutionary adaptations of the photoreceptor channelome to visual ecology and behavior by comparing K+ conductances in 15 phylogenetically diverse insects, using patch-clamp recordings from dissociated ommatidia. We show that rapid diurnal flyers such as the blowfly ( Calliphora vicina) and the honeybee ( Apis mellifera) express relatively large noninactivating Kv conductances, conforming to the earlier hypothesis in Diptera. Nocturnal and/or slow-moving species do not in general exhibit stronger Kv conductance inactivation in the physiological membrane voltage range, but the photoreceptors in species that are known to rely more on vision behaviorally had higher densities of sustained Kv conductances than photoreceptors of less visually guided species. No statistically significant trends related to visual performance could be identified for the rapidly inactivating Kv conductances. Counterintuitively, strong negative correlations were observed between photoreceptor capacitance and specific membrane conductance for both sustained and inactivating fractions of Kv conductance, suggesting insignificant evolutionary pressure to offset negative effects of high capacitance on membrane filtering with increased conductance.
3
Apelblat, Alexander, et Josef Barthel. « Conductance Studies of Aqueous Succinic Acid ». Zeitschrift für Naturforschung A 47, no 3 (1 mars 1992) : 493–98. http://dx.doi.org/10.1515/zna-1992-0309.
Texte intégralRésumé :
Abstract Conductance measurements of aqueous solutions of succinic acid and of di-sodium succinate were performed from 278.15 to 308.15 K and the limiting conductances λ0 (1/2 Succ2- ) are reported. The Waiden product is independent of temperature: λ0(1/2 Succ2-)*η(T) = 0.503 ± 0.001. The salt conductances closely obey the Onsager limiting law. The evaluation of the equilibrium constants for the primary and secondary steps of dissociation, K1 and K2, and the limiting conductances of the hydrosuccinate ion, λ0(HSucc-), are discussed using the Quint and Viallard conductance equation
4
Melikyan, G. B., W. D. Niles, V. A. Ratinov, M. Karhanek, J. Zimmerberg et F. S. Cohen. « Comparison of transient and successful fusion pores connecting influenza hemagglutinin expressing cells to planar membranes. » Journal of General Physiology 106, no 5 (1 novembre 1995) : 803–19. http://dx.doi.org/10.1085/jgp.106.5.803.
Texte intégralRésumé :
Time-resolved admittance measurements were used to investigate the evolution of fusion pores formed between cells expressing influenza virus hemagglutinin (HA) and planar bilayer membranes. The majority of fusion pores opened in a stepwise fashion to semistable conductance levels of several nS. About 20% of the pores had measurable rise times to nS conductances; some of these opened to conductances of approximately 500 pS where they briefly lingered before opening further to semistable conductances. The fall times of closing were statistically similar to the rise times of opening. All fusion pores exhibited semistable values of conductance, varying from approximately 2-20 nS; they would then either close or fully open to conductances on the order of 1 microS. The majority of pores closed; approximately 10% fully opened. Once within the semistable stage, all fusion pores, even those that eventually closed, tended to grow. Statistically, however, before closing, transient fusion pores ceased to grow and reversed their conductance pattern: conductances decreased with a measurable time course until a final drop to closure. In contrast, pore enlargement to the fully open state tended to occur from the largest conductance values attained during a pore's semistable stage. This final enlargement was characterized by a stepwise increase in conductance. The density of HA on the cell surface did not strongly affect pore dynamics. But increased proteolytic treatment of cell surfaces did lead to faster growth within the semistable range. Transient pores and pores that fully opened had indistinguishable initial conductances and statistically identical time courses of early growth, suggesting they were the same upon formation. We suggest that transient and fully open pores evolved from common structures with stochastic factors determining their fate.
5
Carter, Gregory A., et Alan H. Teramura. « Nonsummer stomatal conductance for the invasive vines kudzu and Japanese honeysuckle ». Canadian Journal of Botany 66, no 12 (1 décembre 1988) : 2392–95. http://dx.doi.org/10.1139/b88-325.
Texte intégralRésumé :
A field study was conducted in Maryland to estimate nonsummer stomatal conductances on clear days for two invasive woody vine species common to the southeastern United States. Before the first frost in late October, stomatal conductances were similar for kudzu (Pueraria lobata) and Japanese honeysuckle (Lonicera japonica). When minimum predawn air temperature fell to −0.6 °C, kudzu leaves were irreversibly damaged, whereas maximum daily conductance in honeysuckle was unaffected. Maximum conductances in honeysuckle increased to 14 mm s−1 in late November and mid-December, similar to late-spring and summer values. When minimum air temperatures decreased to −2.8 °C in January, conductance still remained above 5 mm s−1. Maximum daily conductance was lowest in early March, corresponding with low leaf temperatures. By mid-April, maximum conductance increased to 10 mm s−1. Conductances suggested that rates of leaf gas exchange in honeysuckle during fall, winter, and spring were relatively high. Carbon gain during this period might thus contribute substantially to the invasive growth characteristic of the species.
6
Calkin, Howard W., Arthur C. Gibson et Park S. Nobel. « Xylem water potentials and hydraulic conductances in eight species of ferns ». Canadian Journal of Botany 63, no 3 (1 mars 1985) : 632–37. http://dx.doi.org/10.1139/b85-079.
Texte intégralRésumé :
Water potentials, flow rates, and anatomy of xylary elements were studied in eight species of ferns to assess the physical constraints that xylem structure presents to water flow. Comparisons were made among ferns of different leaf morphology as well as between a fern with vessels and ferns with tracheids only. Hydraulic conductance was measured by forcing a solution through excised plant segments. These hydraulic conductances were in close agreement with conductances calculated from water potential gradients and flows measured in intact plants. In three species, backflushing excised segments by forcing a basipetal flow increased subsequently measured conductances two- to six-fold, indicating that the xylem of these three species was partially blocked in intact plants. Hagen–Poiseuille estimates of conductance based on xylary element diameters were 1.8–2.7 times the conductances measured for excised segments. Hydraulic conductances of tracheids and vessels of ferns thus deviate from those of ideal capillaries of similar diameter to about the same extent as has been reported for tracheids in conifers and for vessels in dicotyledons.
7
Nakahari, T., et Y. Marunaka. « ADH-evoked [Cl-]i-dependent transient in whole cell current of distal nephron cell line A6 ». American Journal of Physiology-Renal Physiology 268, no 1 (1 janvier 1995) : F64—F72. http://dx.doi.org/10.1152/ajprenal.1995.268.1.f64.
Texte intégralRésumé :
The effect of antidiuretic hormone (ADH) on a distal nephron cell line (A6) was studied using the whole cell patch-clamp technique. A6 cells were cultured on a permeable support filter for 10-14 days in media containing 10% fetal bovine serum without supplemental aldosterone. In the unstimulated condition A6 cells had very small conductances of Na+,K+, and Cl-. Arginine vasotocin (AVT, 140 mU/ml, 280 nM) evoked a "transient" increase in whole cell currents as did dibutyryl-adenosine 3',5'-cyclic monophosphate (5 mM). These transients consisted of two components; one was the nonselective cation conductance, and the other was the Cl- conductance. Activation of these conductances was dependent on intracellular Cl- concentration ([Cl-]i). At low [Cl-]i (< or = 50 mM) both conductances were activated, whereas when [Cl-]i was 80 mM, only the Cl- conductance was activated. At high [Cl-]i (125 mM), both conductances were inhibited. It seems likely that the [Cl-]i maintained at a low level (< or = 50 mM) is an important requirement for A6 cells to respond to AVT.
8
Moore, L. K., E. C. Beyer et J. M. Burt. « Characterization of gap junction channels in A7r5 vascular smooth muscle cells ». American Journal of Physiology-Cell Physiology 260, no 5 (1 mai 1991) : C975—C981. http://dx.doi.org/10.1152/ajpcell.1991.260.5.c975.
Texte intégralRésumé :
Recent evidence suggest that coordination of blood flow in the microcirculation involves cell-to-cell coupling via gap junctions. In this study, using A7r5 cells as a model of vascular smooth muscle, we have characterized the gap junctions in terms of the unitary conductances of the observed channels, the responses to second messengers, and subunit protein composition. The cells were typically well coupled several hours after plating, with junctional conductances on the order 20-40 nS. Channels with mean conductances of 36 and 89 pS were observed in low-conductance cell pairs and in cell pairs whose macroscopic conductance was reduced by exposure to halothane. Connexin43 was the only known gap junction sequence detected by Northern blots (low and high stringency), immunoblots, or immunohistochemical studies. Junctional conductance was reduced 15% by 8-bromoadenosine 3',5'-cyclic monophosphate; 8-bromoguanosine 3',5'-cyclic monophosphate had no effect. The results suggest that connexin43 can form stable channels of at least two distinct conductances and gap junctions with differing responses to second messengers.
9
Senior, A., M. J. Kosch et F. Honary. « Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data ». Annales Geophysicae 26, no 12 (2 décembre 2008) : 3831–40. http://dx.doi.org/10.5194/angeo-26-3831-2008.
Texte intégralRésumé :
Abstract. Ground-based images of auroral optical emissions and cosmic radio noise absorption provide information on particle precipitation which enhances ionospheric conductances. Knowledge of this conductance field is important to understand the current systems associated with auroral features. Three methods of using ground-based optical and riometer data to estimate ionospheric conductances in the aurora are compared to conductances derived from incoherent scatter radar measurements. It is shown that a method using the 557.7 nm emission intensity alone gives the best results for the Pedersen conductance whilst a method using both this intensity and cosmic noise absorption is best for the Hall conductance. A method using cosmic noise absorption alone gives reasonable performance for the Hall conductance and the Hall/Pedersen conductance ratio, but performs poorly for the Pedersen conductance. It also appears to underestimate the Hall conductance significantly during times when softer precipitation is present, for example in discrete auroral arcs. There is some indication that the methods do not degrade noticeably for angles up to ~20° off magnetic zenith.
10
Abbott, L. F., et Gwendal LeMasson. « Analysis of Neuron Models with Dynamically Regulated Conductances ». Neural Computation 5, no 6 (novembre 1993) : 823–42. http://dx.doi.org/10.1162/neco.1993.5.6.823.
Texte intégralRésumé :
We analyze neuron models in which the maximal conductances of membrane currents are slowly varying dynamic variables regulated by the intracellular calcium concentration. These models allow us to study possible activity-dependent effects arising from processes that maintain and modify membrane channels in real neurons. Regulated model neurons maintain a constant average level of activity over a wide range of conditions by appropriately adjusting their conductances. The intracellular calcium concentration acts as a feedback element linking maximal conductances to electrical activity. The resulting plasticity of intrinsic characteristics has important implications for network behavior. We first study a simple two-conductance model, then introduce techniques that allow us to analyze dynamic regulation with an arbitrary number of conductances, and finally illustrate this method by studying a seven-conductance model. We conclude with an analysis of spontaneous differentiation of identical model neurons in a two-cell network.
11
Palmer, Lawrence G., et Gustavo Frindt. « Cl− channels of the distal nephron ». American Journal of Physiology-Renal Physiology 291, no 6 (décembre 2006) : F1157—F1168. http://dx.doi.org/10.1152/ajprenal.00496.2005.
Texte intégralRésumé :
Cl− currents were observed under whole cell clamp conditions in cells of the rat cortical collecting duct (CCD), connecting tubule (CNT), and thick ascending limb of Henle's loop (TALH). These currents were much larger in intercalated cells compared with principal cells of the CCD and were also larger in the TALH and in the CNT compared with the CCD. The conductance had no strong voltage dependence, and steady-state currents were similar in inward and outward directions with similar Cl− concentrations on both sides of the membrane. Current transients were observed, particularly at low Cl− concentrations, which could be explained by solute depletion and concentration in fluid layers next to the membrane. The currents had a remarkable selectivity among anions. Among halides, Br− and F− conductances were only 15% of that of Cl−, and I− conductance was immeasurably small. SCN− and OCN− conductances were ∼50%, and aspartate, glutamate, and methanesulfonate conductance was ∼5% that of Cl−. No conductance could be measured for any other anion tested, including NO3−, HCO3−, formate, acetate, or isethionate; NO3− and I− appeared to block the channels weakly. Conductances were diminished by lowering the extracellular pH to 6.4. The properties of the conductance fit best with those of the cloned renal anion channel ClC-K2 and likely reflect the basolateral Cl− conductances of the cells of these nephron segments.
12
Yoshii, K., L. E. Moore et B. N. Christensen. « Effect of subthreshold voltage-dependent conductances on the transfer function of branched excitable cells and the conduction of synaptic potentials ». Journal of Neurophysiology 59, no 3 (1 mars 1988) : 706–16. http://dx.doi.org/10.1152/jn.1988.59.3.706.
Texte intégralRésumé :
1. Impulse response functions were determined from complex point impedance and transfer functions from cultured NG-108 cells to simulate the propagation of a synaptic potential in response to the release of transmitter. In general, the flow of synaptic current has a much shorter duration than the normal membrane time constant, thereby making the use of impulse response functions useful approximations to synaptic events. 2. The resonance observed during the activation of the potassium conductance was reflected in the impulse response function as a pronounced damped oscillation. A comparison of the impulse response functions calculated from point impedance and transfer functions showed similar results for current injections in the growth cone. 3. In addition to the resonance effects of the voltage-dependent conductances on transfer and impulse response functions due principally to the activation of conductances for outward currents, transfer functions were measured during the activation of a steady-state negative conductance. Under these conditions the phase function approaches 180 degrees, indicating that the voltage response is out of phase with the current. 4. In the steady state, the effect of a negative conductance is to algebraically add to the positive conductances and generally decrease the absolute conductance unless there is a net negative current. The decreased conductance enhances the impulse response and the DC space constant, thus leading to a better propagation of slow potentials. This effect can be seen as a decrease in the electrotonic length, L, with intermediate depolarizations. At large depolarizations the steady-state activation of the K conductance generally dominates and leads to a greatly increased electrotonic length. 5. Both the net conductances and the associated kinetics play a role in shaping the potential changes during a synaptic current. This is especially critical if there is a net negative steady-state conductance. Under these conditions there is a surprising reduction in the impulse response function. 6. Thus, during a subthreshold activation of the voltage-dependent negative conductances, the observable synaptic potentials would be either large potential responses due to an apparent increase in the impedance (algebraic summation of positive and negative conductances with a net positive conductance) or a minimal response because of the phasic cancellation due to a net negative conductance. The latter condition could exist near the synaptic reversal potential due to a large synaptic drive and would appear experimentally as a form of inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)
13
Golowasch, Jorge, Mark S. Goldman, L. F. Abbott et Eve Marder. « Failure of Averaging in the Construction of a Conductance-Based Neuron Model ». Journal of Neurophysiology 87, no 2 (1 février 2002) : 1129–31. http://dx.doi.org/10.1152/jn.00412.2001.
Texte intégralRésumé :
Parameters for models of biological systems are often obtained by averaging over experimental results from a number of different preparations. To explore the validity of this procedure, we studied the behavior of a conductance-based model neuron with five voltage-dependent conductances. We randomly varied the maximal conductance of each of the active currents in the model and identified sets of maximal conductances that generate bursting neurons that fire a single action potential at the peak of a slow membrane potential depolarization. A model constructed using the means of the maximal conductances of this population is not itself a one-spike burster, but rather fires three action potentials per burst. Averaging fails because the maximal conductances of the population of one-spike bursters lie in a highly concave region of parameter space that does not contain its mean. This demonstrates that averages over multiple samples can fail to characterize a system whose behavior depends on interactions involving a number of highly variable components.
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Krieger, C., et T. A. Sears. « The development of voltege-dependent ionic conductances In murine spinal cord neurones in culture ». Canadian Journal of Physiology and Pharmacology 66, no 10 (1 octobre 1988) : 1328–36. http://dx.doi.org/10.1139/y88-217.
Texte intégralRésumé :
The development of voltage-dependent ionic conductances of foetal mouse spinal cord neurones was examined using the whole-cell patch-clamp technique on neurones cultured from embryos aged 10–12 days (E10–E12) which were studied between the first day in vitro (V1) to V10. A delayed rectifier potassium conductance (IK) and a leak conductance were observed in neurones of E10.V1, E11, V1, and E12, V1 as well as in neurones cultured for longer periods. A rapidly activating and inactivating potassium conductance (IA) was seen in neurones from E11, V2 and E12, V1 and at longer times in vitro. A tetrodotoxin (TTX) sensitive sodium-dependent inward current was observed in neurones of E11 and E12 from V1 onwards. Calcium-dependent conductances were not detectable in these neurones unless the external calcium concentration was raised 10- to 20-foid and potassium conductances were blocked. Under these conditions calcium currents could be observed as early as E11, V3 and E12, V2 and at subsequent times in vitro. The pattern of development of voltage-dependent ionic conductances in murine spinal neurones is such that initially leak and potassium currents are present followed by sodium current and subsequently calcium current.
15
Cao, Xiao-Jie, et Donata Oertel. « Temperature Affects Voltage-Sensitive Conductances Differentially in Octopus Cells of the Mammalian Cochlear Nucleus ». Journal of Neurophysiology 94, no 1 (juillet 2005) : 821–32. http://dx.doi.org/10.1152/jn.01049.2004.
Texte intégralRésumé :
Temperature is an important physiological variable the influence of which on macroscopic electrophysiological measurements in slices is not well documented. We show that each of three voltage-sensitive conductances of octopus cells of the mammalian ventral cochlear nucleus (VCN) is affected differently by changes in temperature. As expected, the kinetics of the currents were faster at higher than at lower temperature. Where they could be measured, time constants of activation, deactivation, and inactivation had Q10 values between 1.8 and 4.6. The magnitude of the peak conductances was differentially affected by temperature. While the peak magnitude of the high-voltage-activated K+ conductance, gKH, was unaffected by changes in temperature, the peak of the low-voltage-activated K+ conductance, gKL, was reduced by half when the temperature was lowered from 33 to 23°C ( Q10 = 2). Changing the temperature changed the kinetics and the magnitude of the hyperpolarization-activated mixed cation conductance, gh, but the changes in magnitude were transient. The voltage sensitivity of the three conductances was unaffected by temperature. The action of temperature on these conductances is reflected in the resting potentials and in the shapes of action potentials.
16
Taylor, Adam L., Timothy J. Hickey, Astrid A. Prinz et Eve Marder. « Structure and Visualization of High-Dimensional Conductance Spaces ». Journal of Neurophysiology 96, no 2 (août 2006) : 891–905. http://dx.doi.org/10.1152/jn.00367.2006.
Texte intégralRésumé :
Neurons, and realistic models of neurons, typically express several different types of voltage-gated conductances. These conductances are subject to continual regulation. Therefore it is essential to understand how changes in the conductances of a neuron affect its intrinsic properties, such as burst period or delay to firing after inhibition of a particular duration and magnitude. Even in model neurons, it can be difficult to visualize how the intrinsic properties vary as a function of their underlying maximal conductances. We used a technique, called clutter-based dimension reordering (CBDR), which enabled us to visualize intrinsic properties in high-dimensional conductance spaces. We applied CBDR to a family of models with eight different types of voltage- and calcium-dependent channels. CBDR yields images that reveal structure in the underlying conductance space. CBDR can also be used to visualize the results of other types of analysis. As examples, we use CBDR to visualize the results of a connected-components analysis, and to visually evaluate the results of a separating-hyperplane (i.e., linear classifier) analysis. We believe that CBDR will be a useful tool for visualizing the conductance spaces of neuronal models in many systems.
17
Butt, A. G., W. L. Clapp et R. A. Frizzell. « Potassium conductances in tracheal epithelium activated by secretion and cell swelling ». American Journal of Physiology-Cell Physiology 258, no 4 (1 avril 1990) : C630—C638. http://dx.doi.org/10.1152/ajpcell.1990.258.4.c630.
Texte intégralRésumé :
Increased basolateral membrane K conductance accompanies stimulation of Cl secretion across canine trachea. To assess the K conductance properties, we permeabilized the apical membranes with amphotericin B and monitored the current and conductance caused by K flow across the basolateral membranes. Under basal unstimulated conditions, two K conductances could be distinguished by blockers. One was inhibited only by barium; the other was sensitive also to quinidine and lidocaine. The permeabilities of the basal conductance pathways to K and Rb were similar (PK/PRb approximately equal to 1.5). The secretory agonist, epinephrine, selectively increased the quinidine-insensitive conductance, implicating it in the Cl secretory response. Cell swelling induced a third conductance with a low permeability to Rb (PK/PRb approximately equal to 10) that was quinidine sensitive. In tissues not treated with amphotericin, neither quinidine nor Rb-for-K replacement inhibited transepithelial Cl secretion. Thus neither of the quinidine-sensitive K conductances (basal or swelling induced) contribute to the increase in basolateral K conductance during Cl secretion. Cell shrinkage inhibited all three conductances and secretion, suggesting that the initial priority of the cell is volume regulation.
18
Golowasch, Jorge, Amitabha Bose, Yinzheng Guan, Dalia Salloum, Andrea Roeser et Farzan Nadim. « A balance of outward and linear inward ionic currents is required for generation of slow-wave oscillations ». Journal of Neurophysiology 118, no 2 (1 août 2017) : 1092–104. http://dx.doi.org/10.1152/jn.00240.2017.
Texte intégralRésumé :
Pacemaker neuron-generated rhythmic activity requires the activation of at least one inward and one outward current. We have previously shown that the inward current can be a linear current (with negative conductance). Using this simple mechanism, here we demonstrate that the inward current conductance must be in relative balance with the outward current conductances to generate oscillatory activity. Surprisingly, an excess of outward conductances completely precludes the possibility of achieving such a balance.
19
Endo, Toshiaki, et Ole Kiehn. « Asymmetric Operation of the Locomotor Central Pattern Generator in the Neonatal Mouse Spinal Cord ». Journal of Neurophysiology 100, no 6 (décembre 2008) : 3043–54. http://dx.doi.org/10.1152/jn.90729.2008.
Texte intégralRésumé :
The rhythmic voltage oscillations in motor neurons (MNs) during locomotor movements reflect the operation of the pre-MN central pattern generator (CPG) network. Recordings from MNs can thus be used as a method to deduct the organization of CPGs. Here, we use continuous conductance measurements and decomposition methods to quantitatively assess the weighting and phase tuning of synaptic inputs to different flexor and extensor MNs during locomotor-like activity in the isolated neonatal mice lumbar spinal cord preparation. Whole cell recordings were obtained from 22 flexor and 18 extensor MNs in rostral and caudal lumbar segments. In all flexor and the large majority of extensor MNs the extracted excitatory and inhibitory synaptic conductances alternate but with a predominance of inhibitory conductances, most pronounced in extensors. These conductance changes are consistent with a “push–pull” operation of locomotor CPG. The extracted excitatory and inhibitory synaptic conductances varied between 2 and 56% of the mean total conductance. Analysis of the phase tuning of the extracted synaptic conductances in flexor and extensor MNs in the rostral lumbar cord showed that the flexor-phase–related synaptic conductance changes have sharper locomotor-phase tuning than the extensor-phase–related conductances, suggesting a modular organization of premotor CPG networks consisting of reciprocally coupled, but differently composed, flexor and extensor CPG networks. There was a clear difference between phase tuning in rostral and caudal MNs, suggesting a distinct operation of CPG networks in different lumbar segments. The highly asymmetric features were preserved throughout all ranges of locomotor frequencies investigated and with different combinations of locomotor-inducing drugs. The asymmetric nature of CPG operation and phase tuning of the conductance profiles provide important clues to the organization of the rodent locomotor CPG and are compatible with a multilayered and distributed structure of the network.
20
Sceniak, Michael P., et Shasta L. Sabo. « Modulation of Firing Rate by Background Synaptic Noise Statistics in Rat Visual Cortical Neurons ». Journal of Neurophysiology 104, no 5 (novembre 2010) : 2792–805. http://dx.doi.org/10.1152/jn.00023.2010.
Texte intégralRésumé :
It has been shown previously that background synaptic noise modulates the response gain of neocortical neurons. However, the role of the statistical properties of the noise in modulating firing rate is not known. Here, the dependence of firing rate on the statistical properties of the excitatory to inhibitory balance (EI) in cortical pyramidal neurons was studied. Excitatory glutamatergic and inhibitory GABAergic synaptic conductances were simulated as two stochastic processes and injected into individual neurons in vitro through use of the dynamic-clamp system. Response gain was significantly modulated as a function of the statistical interactions between excitatory and inhibitory synaptic conductances. Firing rates were compared for noisy synaptic conductance steps by varying either the EI correlation or the relative delay between correlated E and I. When inhibitory synaptic conductances exhibited a short temporal delay (5 ms) relative to correlated excitatory synaptic conductances, the response gain was increased compared with noise with no temporal delay but with an equivalent degree of correlation. The dependence of neuronal firing rate on the EI delay of the noisy background synaptic conductance suggests that individual excitatory pyramidal neurons are sensitive to the EI balance of the synaptic conductance. Therefore the statistical EI interactions encoded within the synaptic subthreshold membrane fluctuations are able to modulate neuronal firing properties.
21
Tarran, R., M. A. Gray, M. J. Evans, W. H. Colledge, R. Ratcliff et B. E. Argent. « Basal chloride currents in murine airway epithelial cells : modulation by CFTR ». American Journal of Physiology-Cell Physiology 274, no 4 (1 avril 1998) : C904—C913. http://dx.doi.org/10.1152/ajpcell.1998.274.4.c904.
Texte intégralRésumé :
We have isolated ciliated respiratory cells from the nasal epithelium of wild-type and cystic fibrosis (CF) null mice and used the patch-clamp technique to investigate their basal conductances. Current-clamp experiments on unstimulated cells indicated the presence of K+ and Cl− conductances and, under certain conditions, a small Na+conductance. Voltage-clamp experiments revealed three distinct Cl− conductances. I tv-indep was time and voltage independent with a linear current-voltage ( I- V) plot; I v-actexhibited activation at potentials greater than ±50 mV, giving an S-shaped I- Vplot; and I hyp-act was activated by hyperpolarizing potentials and had an inwardly rectified I- Vplot. The current density sequence was I hyp-act = I v-act ≫ I tv-indep. These conductances had Cl−-to- N-methyl-d-glucamine cation permeability ratios of between 2.8 and 10.3 and were unaffected by tamoxifen, flufenamate, glibenclamide, DIDS, and 5-nitro-2-(3-phenylpropylamino) benzoic acid but were inhibited by Zn2+ and Gd3+. I tv-indep and I v-act were present in wild-type and CF cells at equal density and frequency. However, I hyp-actwas detected in only 3% of CF cells compared with 26% of wild-type cells, suggesting that this conductance may be modulated by cystic fibrosis transmembrane conductance regulator (CFTR).
22
Horisberger, J. D., et G. Giebisch. « Intracellular Na+ and K+ activities and membrane conductances in the collecting tubule of Amphiuma. » Journal of General Physiology 92, no 5 (1 novembre 1988) : 643–65. http://dx.doi.org/10.1085/jgp.92.5.643.
Texte intégralRésumé :
Membrane potentials and conductances, and intracellular ionic activities were studied in isolated perfused collecting tubules of K+-adapted Amphiuma. Intracellular Na+ (aNai) and K+ (aKi) activities were measured, using liquid ion-exchanger double-barreled microelectrodes. Apical and basolateral membrane conductances were estimated by cable analysis. The effects of inhibition of the apical conductance by amiloride (10(-5) M) and of inhibition of the basolateral Na-K pump by either a low K+ (0.1 mM) bath or by ouabain (10(-4) M) were studied. Under control conditions, aNai was 8.4 +/- 1.9 mM and aKi 56 +/- 3 mM. With luminal amiloride, aNai decreased to 2.2 +/- 0.4 mM and aKi increased to 66 +/- 3 mM. Ouabain produced an increase of aNai to 44 +/- 4 mM, and a decrease of aKi to 22 +/- 6, and similar changes were observed when the tubule was exposed to a low K+ bath solution. During pump inhibition, there was a progressive decrease of the K+-selective basolateral membrane conductance and of the Na+ permeability of the apical membrane. A similar inhibition of both membrane conductances was observed after pump inhibition by low K+ solution. Upon reintroduction of K+, a basolateral membrane hyperpolarization of -23 +/- 4 mV was observed, indicating an immediate reactivation of the electrogenic Na-K pump. However, the recovery of the membrane conductances occurred over a slower time course. These data imply that both membrane conductances are regulated according to the intracellular ionic composition, but that the basolateral K+ conductance is not directly linked to the pump activity.
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Britton, Oliver J., et Blanca Rodriguez. « A population of in silico models identifies the interplay between Nav 1.8 conductance and potassium currents as key in regulating human dorsal root ganglion neuron excitability ». F1000Research 11 (27 janvier 2022) : 104. http://dx.doi.org/10.12688/f1000research.74551.1.
Texte intégralRésumé :
Background: The Nav 1.8 sodium channel has a key role in generating repetitive action potentials in nociceptive human dorsal root ganglion neurons. Nav 1.8 is differentiated from other voltage-gated sodium channels by its unusually slow inactivation kinetics and depolarised voltage-dependence of activation. These features are particularly pronounced in the human Nav 1.8 channel and allow the channel to remain active during repolarisation. Gain-of-function mutations in Nav 1.8 have been linked to neuropathic pain and selective blockers of Nav 1.8 have been developed as potential new analgesics. However, it is not well understood how modulating the Nav 1.8 conductance alters neuronal excitability and how this depends on the balance of other ion channels expressed by nociceptive neurons. Methods: To investigate this, we developed a novel computational model of the human dorsal root ganglion neuron and used it to construct a population of models that mimicked inter-neuronal heterogeneity in ionic conductances and action potential morphology Results: By simulating changes to the Nav 1.8 conductance in the population of models, we found that moderately increasing the Nav 1.8 conductance led to increased firing rate, as expected, but increasing Nav 1.8 conductance beyond an inflection point caused firing rate to decrease. We found that the delayed rectifier and M-type potassium conductances were also critical for determining neuronal excitability. In particular, altering the delayed rectifier potassium conductance shifted the position of the Nav 1.8 inflection point and therefore the relationship between Nav 1.8 conductance and firing rate. Conclusions: Our results suggest that the effects of modulating Nav 1.8 in a nociceptive neuron can depend significantly on other conductances, particularly potassium conductances.
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Idoux, Erwin, Daniel Eugène, Antoine Chambaz, Christophe Magnani, John A. White et Lee E. Moore. « Control of Neuronal Persistent Activity by Voltage-Dependent Dendritic Properties ». Journal of Neurophysiology 100, no 3 (septembre 2008) : 1278–86. http://dx.doi.org/10.1152/jn.90559.2008.
Texte intégralRésumé :
Neural integrators and working memory rely on persistent activity, a widespread neural phenomenon potentially involving persistent sodium conductances. Using a unique combination of voltage-clamp, dynamic-clamp, and frequency-domain techniques, we have investigated the role of voltage-dependent conductances on the dendritic electrotonic structure of neurons of the prepositus hypoglossi nucleus (PHN), which is known to be involved in oculomotor integration. The PHN contains two main neuronal populations: type B neurons with a double afterhyperpolarization and type D neurons, which not only are oscillatory but also have a greater electrotonic length than that of type B neurons. The persistent sodium conductance is present in all PHN neurons, although its effect on the dynamic electrotonic structure is shown to significantly differ in the two major cell types present in the nucleus. The electrotonic differences are such that the persistent sodium conductance can be almost perfectly manipulated in a type B neuron using an on-line dynamic clamp to add or subtract virtual sodium ion channels. The dynamic-clamp results are confirmed by data-fitted models, which suggest that the persistent sodium conductance has two different roles depending on its somatic versus dendritic location: perisomatic conductances could play a major role in maintaining action potential discharge and dendritic conductances would be more involved in other computational properties, such as those involving remote synaptic processing or bistable events.
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Munger, Philip H., James M. Chandler et J. Tom Cothren. « Effect of Water Stress on Photosynthetic Parameters of Soybean (Glycine max) and Velvetleaf (Abutilon theophrasti) ». Weed Science 35, no 1 (janvier 1987) : 15–21. http://dx.doi.org/10.1017/s0043174500026722.
Texte intégralRésumé :
Greenhouse experiments were conducted to elucidate the effects of water stress on photosynthetic parameters of soybean [Glycine max(L.) Merr. ‘Hutton′] and velvetleaf (Abutilon theophrastiMedik. # ABUTH). Stomatal conductance of both species responded curvilinearly to reductions in leaf water potential. At leaf water potentials less negative than −2.5 MPa, stomatal conductance, net photosynthetic rate, and transpiration rate were greater in velvetleaf than in soybean. Soybean photosynthetic rate was linearly related to stomatal conductance. Velvetleaf photosynthetic rate increased linearly with stomatal conductances up to 1.5 cm s–1; however, no increase in photosynthetic rate was observed at stomatal conductances greater than 1.5 cm s–1, indicating nonstomatal limitations to photosynthesis. As water stress intensified, stomatal conductance, photosynthetic rate, and transpiration of velvetleaf declined more rapidly than in soybean.
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Moore, L. K., et J. M. Burt. « Gap junction function in vascular smooth muscle : influence of serotonin ». American Journal of Physiology-Heart and Circulatory Physiology 269, no 4 (1 octobre 1995) : H1481—H1489. http://dx.doi.org/10.1152/ajpheart.1995.269.4.h1481.
Texte intégralRésumé :
In this study we examined the effects of serotonin (5-hydroxytryptamine, 5-HT) on the function of gap junctions between smooth muscle cells isolated from human and pig coronary and rat mesentery arteries and between A7r5 cells (cell line derived from embryonic rat aorta). Mesentery and pig coronary cells expressed connexin (Cx) 43, and human coronary cells expressed Cx40. Mesentery and pig coronary cells each exhibited a single gap junction channel population with unitary conductances of 75 and 59 pS, respectively. Human coronary cells exhibited two channel populations with unitary conductances of 51 and 107 pS. The A7r5 cells express Cx40 and Cx43 and exhibit three channel populations with unitary conductances of 70, 108, and 141 pS. Under control conditions, junctional conductance between the four cell types ranged from 11 to 20 nS. During maximal stimulation with 5-HT (1-10 microM), junctional conductance increased (29-75%) in all four cell types. The unitary conductance profiles in the rat mesentery and pig coronary cells were unaffected by 5-HT, suggesting that the observed increase in macroscopic conductance reflects an increase in open probability. Unitary conductances were also unaffected in the human coronary and A7r5 cells. However, there was a reduced frequency of the 105-pS channel in the human coronary cells and of the 70- and 141-pS channels in the A7r5 cells. These changes in the relative frequency histograms suggest that the open probabilities of the various channel types are differentially affected by the 5-HT treatment.(ABSTRACT TRUNCATED AT 250 WORDS)
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Adams, P. R., S. W. Jones, P. Pennefather, D. A. Brown, C. Koch et B. Lancaster. « Slow synaptic transmission in frog sympathetic ganglia ». Journal of Experimental Biology 124, no 1 (1 septembre 1986) : 259–85. http://dx.doi.org/10.1242/jeb.124.1.259.
Texte intégralRésumé :
Bullfrog ganglia contain two classes of neurone, B and C cells, which receive different inputs and exhibit different slow synaptic potentials. B cells, to which most effort has been directed, possess slow and late slow EPSPs. The sEPSP reflects a muscarinic action of acetylcholine released from boutons on B cells, whereas the late sEPSP is caused by a peptide (similar to teleost LHRH) released from boutons on C cells. During either sEPSP there is a selective reduction in two slow potassium conductances, designated ‘M’ and ‘AHP’. The M conductance is voltage dependent and the AHP conductance is calcium dependent. Normally they act synergistically to prevent repetitive firing of action potentials during maintained stimuli. Computer stimulation of the interactions of these conductances with the other five voltage-dependent conductances present in the membrane allows a complete reconstruction of the effects of slow synaptic transmission on electrical behaviour.
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Skryma, R., N. Prevarskaya, P. Vacher et B. Dufy. « Voltage-dependent ionic conductances in Chinese hamster ovary cells ». American Journal of Physiology-Cell Physiology 267, no 2 (1 août 1994) : C544—C553. http://dx.doi.org/10.1152/ajpcell.1994.267.2.c544.
Texte intégralRésumé :
Chinese hamster ovary (CHO) cells are becoming a widely used biological material. A number of studies report membrane ion conductance changes after transfection of channels and receptors, but there are few data available on the properties of membrane ion conductances of CHO cells before transfection. In this work we studied voltage-dependent ionic conductances in cultures of CHO native (CHO-K1) cells. Three types of voltage-dependent ionic conductances were identified: 1) a K+ conductance showing sensitivity to Ca2+ and a unit conductance of approximately 210 pS in symmetrical 150 mM K+ outside-out patches (this conductance, which did not inactivate during a 160-ms pulse, was inhibited by 30 nM charybdotoxin but not by 30 mM extracellular tetraethylammonium); 2) a rapidly activating and inactivating tetrodotoxin (TTX)-sensitive inward current, peaking at about -10 to 0 mV (this current showed characteristics similar in many respects to Na+ current recorded in neurons); and 3) another voltage-dependent inward current, which had slow inactivation, was TTX insensitive but was blocked by Co2+ (current was also carried by Ba2+, peaked at approximately 0 to +10 mV, was identified as a Ca2+ conductance, and was inhibited by dihydropyridines but not by 10 microM omega-conotoxin). Cell-attached patch recordings of single Ca2+ channel currents demonstrated a unitary conductance of approximately 20 pS.
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Helman, S. I., et X. Liu. « Substrate-dependent expression of Na+ transport and shunt conductance in A6 epithelia ». American Journal of Physiology-Cell Physiology 273, no 2 (1 août 1997) : C434—C441. http://dx.doi.org/10.1152/ajpcell.1997.273.2.c434.
Texte intégralRésumé :
A6 epithelia grown in tissue culture vary enormously in their baseline rates of Na+ transport due to differences in growth media, serum, and other unknown factors. To evaluate the effect(s) of substrates on expression of Na+ transport, we determined short-circuit currents, open-circuit voltages, and electrical resistances of mature confluent A6 epithelia grown on a variety of commercially available permeable supports. Because the cells, growth conditions, and all other factors were the same, differences in transport could be attributed alone to the substrate on which the cells were grown. Tissues were grown on both large- and small-diameter inserts of the same type with differing ratios of edge length to area so that the contribution of the edge and tight junction conductances to the combined shunt conductance of the inserts could be evaluated. Shunt and cellular conductances and the cellular Thevenin electromotive force were determined after aldosterone stimulation and amiloride inhibition of Na+ transport. Marked and extreme differences were observed not only for expression of Na+ transport (controls, 0.09-3.94 microA/cm2; aldosterone, 1.53-28.2 microA/cm2) due to changes of apical membrane conductance but also for the development of junctional conductances (3,250 to < infinity omega.cm2) and edge conductances (13,175 to < infinity omega.cm) among substrates.
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Yang, Yang, Timothy Adowski, Bina Ramamurthy, Andreas Neef et Matthew A. Xu-Friedman. « High-speed dynamic-clamp interface ». Journal of Neurophysiology 113, no 7 (avril 2015) : 2713–20. http://dx.doi.org/10.1152/jn.00543.2014.
Texte intégralRésumé :
The dynamic-clamp technique is highly useful for mimicking synaptic or voltage-gated conductances. However, its use remains rare in part because there are few systems, and they can be expensive and difficult for less-experienced programmers to implement. Furthermore, some conductances (such as sodium channels) can be quite rapid or may have complex voltage sensitivity, so high speeds are necessary. To address these issues, we have developed a new interface that uses a common personal computer platform with National Instruments data acquisition and WaveMetrics IGOR to provide a simple user interface. This dynamic clamp implements leak and linear synaptic conductances as well as a voltage-dependent synaptic conductance and kinetic channel conductances based on Hodgkin-Huxley or Markov models. The speed of the system can be assayed using a testing mode, and currently speeds of >100 kHz (10 μs per cycle) are achievable with short latency and little jitter.
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Kullmann, Paul H. M., Diek W. Wheeler, Joshua Beacom et John P. Horn. « Implementation of a Fast 16-Bit Dynamic Clamp Using LabVIEW-RT ». Journal of Neurophysiology 91, no 1 (janvier 2004) : 542–54. http://dx.doi.org/10.1152/jn.00559.2003.
Texte intégralRésumé :
The dynamic-clamp method provides a powerful electrophysiological tool for creating virtual ionic conductances in living cells and studying their influence on membrane potential. Here we describe G-clamp, a new way to implement a dynamic clamp using the real-time version of the Lab-VIEW programming environment together with a Windows host, an embedded microprocessor that runs a real-time operating system and a multifunction data-acquisition board. The software includes descriptions of a fast voltage-dependent sodium conductance, delayed rectifier, M-type and A-type potassium conductances, and a leak conductance. The system can also read synaptic conductance waveforms from preassembled data files. These virtual conductances can be reliably implemented at speeds ≤43 kHz while simultaneously saving two channels of data with 16-bit precision. G-clamp also includes utilities for measuring current-voltage relations, synaptic strength, and synaptic gain. Taking an approach built on a commercially available software/hardware platform has resulted in a system that is easy to assemble and upgrade. In addition, the graphical programming structure of LabVIEW should make it relatively easy for others to adapt G-clamp for new experimental applications.
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Davies, J. A., et M. Lester. « The relationship between electric fields, conductances and currents in the high-latitude ionosphere : a statistical study using EISCAT data ». Annales Geophysicae 17, no 1 (31 janvier 1999) : 43–52. http://dx.doi.org/10.1007/s00585-999-0043-3.
Texte intégralRésumé :
Abstract. The relationship between electric fields, height-integrated conductivities and electric currents in the high-latitude nightside electrojet region is known to be complex. The tristatic nature of the EISCAT UHF radar facility provides an excellent means of exploring this interrelationship as it enables simultaneous estimates to be made of the full electric field vector and the ionospheric Hall and Pedersen conductances, further allowing the determination of both field-perpendicular electric current components. Over 1300 h of common programme observations by the UHF radar system provide the basis of a statistical study of electric fields, conductances and currents in the high-latitude ionosphere, from which preliminary results are presented. Times at which there is significant solar contribution to the ionospheric conductances have been excluded by limiting the observations according to solar zenith angle. Initial results indicate that, in general, the times of peak conductance, identified from the entire set of EISCAT observations, do not correspond to the times of the largest electric field values; the relative contribution of ionospheric conductance and electric field to the electrojet currents therefore depends critically on local time, a conclusion which corroborates work by previous authors. Simultaneous measurements confirm a tendency for a decrease in both Hall and Pedersen conductances to be accompanied by an increase in the electric field, at least for moderate and large electric field value, a tendency which is also identified to some extent in the ratio of the conductances, which acts as an indicator of the energy of precipitating particles.Key words. Ionosphere (auroral ionosphere; electric fields and currents)
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Maltenfort, Mitchell G., Carrie A. Phillips, Martha L. McCurdy et Thomas M. Hamm. « Determination of the Location and Magnitude of Synaptic Conductance Changes in Spinal Motoneurons by Impedance Measurements ». Journal of Neurophysiology 92, no 3 (septembre 2004) : 1400–1416. http://dx.doi.org/10.1152/jn.00873.2003.
Texte intégralRésumé :
The relation between impedance change and the location and magnitude of a tonic synaptic conductance was examined in compartmental motoneuron models based on previously published data. The dependency of motoneuron impedance on system time constant (τ), electrotonic length (L), and dendritic-to-somatic conductance ratio (ρ) was examined, showing that the relation between impedance phase and ρ differed markedly between models with uniform and nonuniform membrane resistivity. Dendritic synaptic conductances decreased impedance magnitude at low frequencies; at higher frequencies, impedance magnitude increased. The frequency at which the change in impedance magnitude reversed from a decrease to an increase—the reversal frequency, Fr—was a good estimator of electrotonic synaptic location. A measure of the average normalized impedance change at frequencies less than Fr, cuΔZ, estimated relative synaptic conductance. Fr and cuΔZ provided useful estimates of synaptic location and conductance in models with nonuniform (step, sigmoidal) and uniform membrane resistivity. Fr also provided good estimates of spatial synaptic location on the equivalent cable in both step and sigmoidal models. Variability in relations between Fr, cuΔZ, and conductance location and magnitude between neurons was reduced by normalization with ρ and τ. The effects on Fr and cuΔZ of noise in experimental recordings, different synaptic distributions, and voltage-dependent conductances were also assessed. This study indicates that location and conductance of tonic dendritic conductances can be estimated from Fr, cuΔZ, and basic electrotonic motoneuron parameters with the exercise of suitable precautions.
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Machin, J., J. Smith et G. Lampert. « EVIDENCE FOR HYDRATION-DEPENDENT CLOSING OF PORE STRUCTURES IN THE CUTICLE OF PERIPLANETA AMERICANA ». Journal of Experimental Biology 192, no 1 (1 juillet 1994) : 83–94. http://dx.doi.org/10.1242/jeb.192.1.83.
Texte intégralRésumé :
Integumental electrical resistances were measured on the antenna, pronotum, forewing, second abdominal tergite, fifth abdominal sternite and the femur of the third leg in restrained male and female Periplaneta americana. The same measurements, excepting those on antenna and wing, were made on last-instar nymphs. Electrical contact was made through two externally applied glass tubes filled with cockroach Ringer with a combined contact area of about 2 mm2. Resistances corresponding to current flows through two thicknesses of integument were measured using a current-clamping amplifier. Calculated electrical conductances tended to be higher in the antennae, pronotum and abdominal tergites than in the legs and wings. Conductances of the pronotum and abdominal tergites were significantly higher (P<0.001) in males than in females. The literature suggests that male abdominal tergites are the site of sex pheromone production. In nymphs as well as adults, the conductances of all areas, except the antennae of males, decreased following dehydration and a decline in animal water content. In most cases the magnitude of the decrease was tightly correlated with initial hydrated conductance. The data suggest that variations in regional conductances in hydrated animals are principally due to differences in dermal gland density. We argue that the decrease in conductance following dehydration is evidence of a mechanism closing dermal gland openings in times of water stress.
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Cardozo, David L. « A model for understanding membrane potential using springs ». Advances in Physiology Education 29, no 4 (décembre 2005) : 204–7. http://dx.doi.org/10.1152/advan.00067.2004.
Texte intégralRésumé :
In this report, I present a simple model using springs to conceptualize the relationship between ionic conductances across a cellular membrane and their effect on membrane potential. The equation describing the relationships linking membrane potential, ionic equilibrium potential, and ionic conductance is of similar form to that describing the force generated by a spring as a function of its displacement. The spring analogy is especially useful in helping students to conceptualize the effects of multiple conductances on membrane potential.
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Haldar, Purushottam, et Bijan Das. « Electrical Conductances of Tetrabutylammonium Bromide, Sodium Tetraphenylborate and Sodium Bromide in 2-Ethoxyethanol in the Temperature Range 35–50°C ». Zeitschrift für Physikalische Chemie 218, no 5 (1 mai 2004) : 599–610. http://dx.doi.org/10.1524/zpch.218.5.599.30503.
Texte intégralRésumé :
AbstractThe electrical conductances of the solutions of tetrabutylammonium bromide (Bu4NBr), sodium tetraphenylborate (NaPh4B) and sodium bromide (NaBr) in 2-ethoxyethanol have been reported at 35, 40, 45 and 50°C. The conductance data have been analyzed by the 1978 Fuoss conductance–concentration equation in terms of the limiting molar conductance (Λ0), the association constant (KΛ) and the association diameter (R). The ionic contributions to the limiting molar conductances (Λ0) have been estimated using the “reference electrolyte” tetrabutylammonium tetraphenylborate (Bu4NPh4B). Appreciable ionic association was observed for all of these electrolytes which were found to exist in the form of solvent-separated ion-pairs in 2-ethoxyethanol. The tetrabutylammonium and tetraphenylborate ions are found to remain scarcely solvated whereas the sodium and bromide ions undergo substantial solvation in 2-ethoxyethanol medium in the temperature range investigated here.
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Gill, Dip Singh, et Dilbag Rana. « Preparation of Some Novel Copper(I) Complexes and their Molar Conductances in Organic Solvents ». Zeitschrift für Naturforschung A 64, no 3-4 (1 avril 2009) : 269–72. http://dx.doi.org/10.1515/zna-2009-3-416.
Texte intégralRésumé :
Abstract Attempts have been made to prepare some novel copper(I) nitrate, sulfate, and perchlorate complexes. Molar conductances of these complexes have been measured in organic solvents like acetonitrile (AN), acetone (AC), methanol (MeOH), N,N-dimethylformamide (DMF), N,Ndimethylacetamide (DMA), and dimethylsulfoxide (DMSO) at 298 K. The molar conductance data have been analyzed to obtain limiting molar conductances (λ0) and ion association constants (KA) of the electrolytes. The results showed that all these complexes are strong electrolytes in all organic solvents. The limiting ionic molar conductances (λo± ) for various ions have been calculated using Bu4NBPh4 as reference electrolyte. The actual radii for copper(I) complex ions are very large and different in different solvents and indicate some solvation effects in each solvent system
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Soybel, D. I., S. W. Ashley et L. Y. Cheung. « Basolateral K+ conductances in surface epithelium of Necturus antrum : effects of Ca2+ and divalent cations ». American Journal of Physiology-Gastrointestinal and Liver Physiology 262, no 4 (1 avril 1992) : G651—G659. http://dx.doi.org/10.1152/ajpgi.1992.262.4.g651.
Texte intégralRésumé :
Intracellular microelectrode techniques were used to characterize basolateral membrane K+ conductances in isolated Necturus antral mucosa. Exposure of tissues to progressively higher levels of serosal K+ (4, 20, 40, or 60 mM) resulted in progressively greater depolarizations of basolateral membrane potentials and decreases in membrane resistance, consistent with the presence of a significant K+ conductance. Ba2+ (2 mM) partially blocks these conductances. Exposure of tissues to increased levels of serosal Ca2+ (from 1.8 to 6.8 mM) elicited significant hyperpolarization of basolateral potentials and decreases in basolateral resistance. These effects are also elicited by Sr2+ (5 mM), but not by Mg2+ (5 mM). Ba2+ (5 mM) elicits complex and time-dependent effects, but transiently elicits an effect similar to high Ca2+. Ion substitutions in the serosal perfusate suggest that the Ca(2+)-induced effects are due to enhancement of basolateral K+ conductances. Further work is necessary to identify the processes that mediate this increase in basolateral K+ conductance and to evaluate the physiological significance of this change in membrane permeability to K+.
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Takahashi, Masaaki, et Kiyohisa Natsume. « Automatic Estimation of the Dynamics of Channel Conductance Using a Recurrent Neural Network ». Advances in Artificial Neural Systems 2009 (20 novembre 2009) : 1–10. http://dx.doi.org/10.1155/2009/724092.
Texte intégralRésumé :
In order to simulate neuronal electrical activities, we must estimate the dynamics of channel conductances from physiological experimental data. However, this approach requires the formulation of differential equations that express the time course of channel conductance. On the other hand, if the dynamics are automatically estimated, neuronal activities can be easily simulated. By using a recurrent neural network (RNN), it is possible to estimate the dynamics of channel conductances without formulating the differential equations. In the present study, we estimated the dynamics of the Na+ and K+ conductances of a squid giant axon using two different fully connected RNNs and were able to reproduce various neuronal activities of the axon. The reproduced activities were an action potential, a threshold, a refractory phenomenon, a rebound action potential, and periodic action potentials with a constant stimulation. RNNs can be trained using channels other than the Na+ and K+ channels. Therefore, using our RNN estimation method, the dynamics of channel conductance can be automatically estimated and the neuronal activities can be simulated using the channel RNNs. An RNN can be a useful tool to estimate the dynamics of the channel conductance of a neuron, and by using the method presented here, it is possible to simulate neuronal activities more easily than by using the previous methods.
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Syal, Vijay K., Suvarcha Chauhan, Alok Katoch et Mohinder S. Chauhan. « Conductance studies of some 1:1 electrolytes in acetone + dimethylsulphoxide mixtures at 25 °C ». Collection of Czechoslovak Chemical Communications 56, no 9 (1991) : 1803–12. http://dx.doi.org/10.1135/cccc19911803.
Texte intégralRésumé :
Precise molar conductances of Bu4NBPh4, Bu4NI, Bu4NNO3, AgNO3, LiNO3, NaBPh4 and KI have been measured in acetone (Ac), dimethylsulphoxide (DMSO) and Ac + DMSO mixtures over the whole solvent composition range at 25°C. The conductance data has been analysed by computer using Shedlovsky conductance equation. Limiting ionic conductances have been calculated by using Gill’s model. Evaluation of solvated radii of Li+, Na+, K+ and Ag+ ions in Ac + DMSO mixtures shows strong preferential solvation for Li+ ion by DMSO. Na+ and Ag+ ions are found to be preferentially solvated by DMSO in Ac-rich region and by Ac in DMSO rich regions. K+, NO3- and I- ions show no preferential solvation in Ac + DMSO mixtures.
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Ismael, Ali K., et Colin J. Lambert. « Single-molecule conductance oscillations in alkane rings ». Journal of Materials Chemistry C 7, no 22 (2019) : 6578–81. http://dx.doi.org/10.1039/c8tc05565c.
Texte intégral
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Cheng, C. Y., et C. K. Chen. « Efficiency Optimizations of an Irreversible Brayton Heat Engine ». Journal of Energy Resources Technology 120, no 2 (1 juin 1998) : 143–48. http://dx.doi.org/10.1115/1.2795025.
Texte intégralRésumé :
A steady-flow approach for finite-time thermodynamics is used to calculate the maximum thermal efficiency, its corresponding power output, adiabatic temperature ratio, and thermal-conductance ratio of heat transfer equipment of a closed Brayton heat engine. The physical model considers three types of irreversibilities: finite thermal conductance between the working fluid and the reservoirs, heat leaks between the reservoirs, and internal irreversibility inside the closed Brayton heat engine. The effects of heat leaks, hot-cold reservoir temperature ratios, turbine and compressor isentropic efficiencies, and total conductances of heat exchangers on the maximum thermal efficiency and its corresponding parameters are studied. The optimum conductance ratio could be found to effectively use the heat transfer equipment, and this ratio is increased as the component efficiencies and total conductances of heat exchangers are increased, and always less than or equal to 0.5.
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Merlin, Didier, Lianwei Jiang, Gregg R. Strohmeier, Asma Nusrat, Seth L. Alper, Wayne I. Lencer et James L. Madara. « Distinct Ca2+- and cAMP-dependent anion conductances in the apical membrane of polarized T84 cells ». American Journal of Physiology-Cell Physiology 275, no 2 (1 août 1998) : C484—C495. http://dx.doi.org/10.1152/ajpcell.1998.275.2.c484.
Texte intégralRésumé :
Monolayers of the human colonic epithelial cell line T84 exhibit electrogenic Cl− secretion in response to the Ca2+ agonist thapsigargin and to the cAMP agonist forskolin. To evaluate directly the regulation of apical Cl−conductance by these two agonists, we have utilized amphotericin B to permeabilize selectively the basolateral membranes of T84 cell monolayers. We find that apical anion conductance is stimulated by both forskolin and thapsigargin but that these conductances are differentially sensitive to the anion channel blocker DIDS. DIDS inhibits thapsigargin-stimulated responses completely but forskolin responses only partially. Furthermore, the apical membrane anion conductances elicited by these two agonists differ in anion selectivity (for thapsigargin, I− > Cl−; for forskolin, Cl− > I−). However, the DIDS-sensitive component of the forskolin-induced conductance response exhibits anion selectivity similar to that induced by thapsigargin (I− > Cl−). Thus forskolin-induced apical anion conductance comprises at least two components, one of which has features in common with that elicited by thapsigargin.
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Sellin, J. H., A. Hall, E. J. Cragoe et W. P. Dubinsky. « Characterization of an apical sodium conductance in rabbit cecum ». American Journal of Physiology-Gastrointestinal and Liver Physiology 264, no 1 (1 janvier 1993) : G13—G21. http://dx.doi.org/10.1152/ajpgi.1993.264.1.g13.
Texte intégralRésumé :
Rabbit cecum in vitro exhibits electrogenic Na+ absorption not blocked by amiloride but inhibited by the amiloride analogue phenamil, suggesting transport mediated by modified Na+ channels in the apical membrane. To further characterize the mechanism(s) of Na+ absorption, microelectrode impalements of single epithelial cells were performed to measure intracellular potential difference (psi mc) and fractional resistance of the apical membrane, to characterize ionic conductances of the apical and basolateral membranes, and to determine the response to phenamil. The electrical potential profile of cecum (psi mc = -31 +/- 2 mV, fractional resistance = 0.71 +/- 0.03) was qualitatively similar to distal colon. The apical membrane exhibited responses suggesting both Na+ and K+ conductances, whereas the basolateral membrane appeared to have a predominant K+ conductance. Phenamil elicited a depolarization of psi mc and a decrease in fractional resistance; neither response is consistent with inhibition of an apical Na+ conductance. Studies were performed in apical membrane vesicles to characterize ionic conductances by a second independent methodology. These additional studies confirmed the presence of an apical Na+ conductance not inhibited by either amiloride or phenamil. Thus both microelectrode impalement and vesicle studies demonstrated an apical membrane Na+ conductance in rabbit cecum; this is the likely mechanism of electrogenic Na+ absorption in this epithelium. However, the anomalous response to phenamil suggests that the inhibitory effect of this agent is not directly on the conductance. The cecal transporter may be one of a family of cation channels related to, but significantly different from, the classic Na+ channel found in distal colon and other tight epithelia.
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Rathour, Rahul Kumar, et Rishikesh Narayanan. « Influence fields : a quantitative framework for representation and analysis of active dendrites ». Journal of Neurophysiology 107, no 9 (1 mai 2012) : 2313–34. http://dx.doi.org/10.1152/jn.00846.2011.
Texte intégralRésumé :
Neuronal dendrites express numerous voltage-gated ion channels (VGICs), typically with spatial gradients in their densities and properties. Dendritic VGICs, their gradients, and their plasticity endow neurons with information processing capabilities that are higher than those of neurons with passive dendrites. Despite this, frameworks that incorporate dendritic VGICs and their plasticity into neurophysiological and learning theory models have been far and few. Here, we develop a generalized quantitative framework to analyze the extent of influence of a spatially localized VGIC conductance on different physiological properties along the entire stretch of a neuron. Employing this framework, we show that the extent of influence of a VGIC conductance is largely independent of the conductance magnitude but is heavily dependent on the specific physiological property and background conductances. Morphologically, our analyses demonstrate that the influences of different VGIC conductances located on an oblique dendrite are confined within that oblique dendrite, thus providing further credence to the postulate that dendritic branches act as independent computational units. Furthermore, distinguishing between active and passive propagation of signals within a neuron, we demonstrate that the influence of a VGIC conductance is spatially confined only when propagation is active. Finally, we reconstruct functional gradients from VGIC conductance gradients using influence fields and demonstrate that the cumulative contribution of VGIC conductances in adjacent compartments plays a critical role in determining physiological properties at a given location. We suggest that our framework provides a quantitative basis for unraveling the roles of dendritic VGICs and their plasticity in neural coding, learning, and homeostasis.
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Verheijck, E. Etienne, Ronald Wilders, Ronald W. Joyner, David A. Golod, Rajiv Kumar, Habo J. Jongsma, Lennart N. Bouman et Antoni C. G. van Ginneken. « Pacemaker Synchronization of Electrically Coupled Rabbit Sinoatrial Node Cells ». Journal of General Physiology 111, no 1 (1 janvier 1998) : 95–112. http://dx.doi.org/10.1085/jgp.111.1.95.
Texte intégralRésumé :
The effects of intercellular coupling conductance on the activity of two electrically coupled isolated rabbit sinoatrial nodal cells were investigated. A computer-controlled version of the “coupling clamp” technique was used in which isolated sinoatrial nodal cells, not physically in contact with each other, were electrically coupled at various values of ohmic coupling conductance, mimicking the effects of mutual interaction by electrical coupling through gap junctional channels. We demonstrate the existence of four types of electrical behavior of coupled spontaneously active cells. As the coupling conductance is progressively increased, the cells exhibit: (a) independent pacemaking at low coupling conductances, (b) complex dynamics of activity with mutual interactions, (c) entrainment of action potential frequency at a 1:1 ratio with different action potential waveforms, and (d) entrainment of action potentials at the same frequency of activation and virtually identical action potential waveforms. The critical value of coupling conductance required for 1:1 frequency entrainment was <0.5 nS in each of the five cell pairs studied. The common interbeat interval at a relatively high coupling conductance (10 nS), which is sufficient to produce entrainment of frequency and also identical action potential waveforms, is determined most by the intrinsically faster pacemaker cell and it can be predicted from the diastolic depolarization times of both cells. Evidence is provided that, at low coupling conductances, mutual pacemaker synchronization results mainly from the phase-resetting effects of the action potential of one cell on the depolarization phase of the other. At high coupling conductances, the tonic, diastolic interactions become more important.
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Bal, Ramazan, et Donata Oertel. « Potassium Currents in Octopus Cells of the Mammalian Cochlear Nucleus ». Journal of Neurophysiology 86, no 5 (1 novembre 2001) : 2299–311. http://dx.doi.org/10.1152/jn.2001.86.5.2299.
Texte intégralRésumé :
Octopus cells in the posteroventral cochlear nucleus (PVCN) of mammals are biophysically specialized to detect coincident firing in the population of auditory nerve fibers that provide their synaptic input and to convey its occurrence with temporal precision. The precision in the timing of action potentials depends on the low input resistance (∼6 MΩ) of octopus cells at the resting potential that makes voltage changes rapid (τ ∼ 200 μs). It is the activation of voltage-dependent conductances that endows octopus cells with low input resistances and prevents repetitive firing in response to depolarization. These conductances have been examined under whole cell voltage clamp. The present study reveals the properties of two conductances that mediate currents whose reversal at or near the equilibrium potential for K+ over a wide range of extracellular K+ concentrations identifies them as K+ currents. One rapidly inactivating conductance, g KL, had a threshold of activation at −70 mV, rose steeply as a function of depolarization with half-maximal activation at −45 ± 6 mV (mean ± SD), and was fully activated at 0 mV. The low-threshold K+ current ( I KL) was largely blocked by α-dendrotoxin (α-DTX) and partially blocked by DTX-K and tityustoxin, indicating that this current was mediated through potassium channels of the Kv1 (also known as shakeror KCNA) family. The maximum low-threshold K+conductance ( g KL) was large, 514 ± 135 nS. Blocking I KL with α-DTX revealed a second K+ current with a higher threshold ( I KH) that was largely blocked by 20 mM tetraethylammonium (TEA). The more slowly inactivating conductance, g KH, had a threshold for activation at −40 mV, reached half-maximal activation at −16 ± 5 mV, and was fully activated at +30 mV. The maximum high-threshold conductance, g KH, was on average 116 ± 27 nS. The present experiments show that it is not the biophysical and pharmacological properties but the magnitude of the K+ conductances that make octopus cells unusual. At the resting potential, −62 mV, g KLcontributes ∼42 nS to the resting conductance and mediates a resting K+ current of 1 nA. The resting outward K+ current is balanced by an inward current through the hyperpolarization-activated conductance, g h, that has been described previously.
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Prinz, Astrid A., et Peter Fromherz. « Effect of Neuritic Cables on Conductance Estimates for Remote Electrical Synapses ». Journal of Neurophysiology 89, no 4 (1 avril 2003) : 2215–24. http://dx.doi.org/10.1152/jn.00956.2002.
Texte intégralRésumé :
The conductance of electrical synapses is usually estimated from voltage recordings at the neuronal somata under the assumption that each cell is isopotential. This approach neglects effects of intervening neurites. For a cell pair with unbranched neurites and an electrical synapse at their ends, we used cable theory to derive an analytical expression that relates the synaptic conductance to voltage recordings at the cell bodies and to the neurite properties. The equation implies that the conventional method significantly underestimates the actual synapse conductance if the neurite length is comparable to the electrotonic length constant and if the synaptic conductance is similar to the serial neurite conductance. For an experimental test, we cultured pairs of snail neurons on protein patterns, resulting in a geometry that matched the theoretical model. Using the isopotential theory, we estimated the synapse conductances and found them to be rather weak. To obtain the cable properties, we recorded spatiotemporal maps of signal propagation in the neurites using a voltage-sensitive dye. Fits of these maps to a passive cable model showed that the snail neurons are electrotonically rather compact. Given these features of our experimental system, the synaptic conductances derived with the nonisopotential model deviated from the estimates of the isopotential theory by about 13%. This discrepancy, although small, shows that even in electrotonically compact neurons coupled by weak synapses the impact of the neuritic cables on conductance estimates cannot be neglected. When applied to less compact and more strongly coupled cell pairs in vivo, our approach can supply the realistic estimates of synaptic conductances that are necessary for a better understanding of the role of electrical coupling in neural systems.
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Kaka, Kosrat N., Anis A. Al-Najar et Wali M. Hamad. « The Audio Frequency Conductance Study of Some Metal Succinate Salts in Aqueous Medium at Different Temperatures (Part I : Magnesium, Manganese (II), Barium and Copper Succinates) ». Journal of Chemistry 2013 (2013) : 1–10. http://dx.doi.org/10.1155/2013/858374.
Texte intégralRésumé :
The audio electrical conductances of aqueous solutions of magnesium, manganese II, barium, and copper succinates have been measured at various temperatures in the range of 298.15 K to 313.15 K, using an audio frequency conductance bridge. The evaluation of conductance data was carried out by minimisation technique using the theoretical equations of the complete and modified forms of Pitts (P) and Fuoss-Hsia (F-H), each a three-parameter equation, association constant (KA), molar conductance (Λm), and distance parameter (a). Quantitative results showed that these salts do not behave as “strong” electrolytes, and that their dissociations are far from complete. The abnormally low conductances of these electrolytes are not due to the presence of electrically neutral molecules but to the ion-pair formation. The Walden product values, as well as the standard thermodynamics functions (ΔH∘,ΔG∘,ΔS∘) for the association reaction at the four temperatures studied, have been evaluated.
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Antar, Mohamed A., et Syed M. Zubair. « Thermoeconomic Considerations in the Optimum Allocation of Heat Transfer Inventory for Refrigeration and Heat Pump Systems ». Journal of Energy Resources Technology 124, no 1 (1 mars 2002) : 28–33. http://dx.doi.org/10.1115/1.1446070.
Texte intégralRésumé :
Thermoeconomics is defined as attaching monetary values to heat exchanger conductances of a given plant. In this study, optimum allocation of heat transfer inventory for heat exchangers in a refrigeration system with specified power input or cooling capacity, and for a heat pump with specified heating capacity is investigated. The ratio of hot- to cold-end conductance unit cost ratio, G, was considered in the analysis as an additional parameter of considerable importance to the designer. A closed-form expression is given in terms of unit cost of conductances of both the heat exchangers. The results show a strong dependence of the total cost on the absolute temperature ratios as well as the hot- to the cold-end conductance cost ratio. It is demonstrated in the illustrative example that for G=0.1, the conductance of the hot-end heat exchanger is about three times the cold-end heat exchanger.