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The nature of electrical propagation in cardiac muscle   总被引:7,自引:0,他引:7  
It has long been appreciated that cardiac muscle is composed of individual cells connected by low-resistance connections, but most concepts of cardiac impulse conduction have been based on a simplified model of propagation assuming continuously uniform intracellular resistivity in the direction of propagation. In this article we describe the development of the application of the theory of continuous media to propagation in cardiac muscle and review some of the successes achieved with this theory. New evidence is cited that propagation in cardiac muscle often displays a discontinuous nature. We consider the hypothesis that this previously unrecognized aspect of propagation can be explained by discontinuities in axial resistance related to known structural complexities of cardiac muscle. A major implication is that the combination of discontinuities of effective axial resistivity at several size levels can produce a wide variety of complex abnormalities of propagation, including most currently known cardiac conduction disturbances that have been considered to require spatial nonuniformity of membrane properties.  相似文献   

3.
A computer model of a one-dimensional cardiac fibre of resistively coupled cells is used to investigate the influence of the junction resistance on the nature of conduction. The results of the simulations are presented and indicate that the effect of the junction on both intracellular and extracellular waveshape and on the velocity of propagation depends on the size and frequency of the coupling resistance and the kinetics of the active membrane. Significant changes in these factors are not observed without the generation of prepotentials in the action potential upstroke. The absence of this ‘signature’ in microelectrode recordings of activity in ventricular muscle suggests that under normal conditions cardiac tissue behaves as a functional syncytium.  相似文献   

4.
The effect of desensitized cholinoceptors on the time course of end-plate currents was evaluated in frog skeletal muscle at a high (physiological) level of acetylcholine secretion in the presence of active acetylcholinesterase, with two-electrode recording of the membrane potential. When the number of cholinoceptors was small so that they did not appreciably affect the amplitude of end-plate currents or the parameters of onequantum responses (miniature currents), the decay of multiquantum currents was significantly accelerated. Moreover, the presence of cholinoceptors drastically reduced the ability of acetylcholinesterase inhibitors to prolong the decay of end-plate currents. It is suggested that desensitized cholinoceptors in a synapse with a physiological level of acetylcholine secretion and active acetylcholinesterase may bind free acetylcholine with high affinity and thus supplement the well-known physiological role of acetylcholinesterase in limiting the reactivation of postsynaptic membrane cholinoceptors Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 119, No. 6, pp. 578–581, June, 1995 Presented by A. D. Ado, Member of the Russian Academy of Medical Sciences  相似文献   

5.
Bidomain theory for cardiac tissue assumes two interpenetrating anisotropic media--intracellular (i) and extracellular (e)--connected everywhere via a cell membrane; four local parameters sigma(i,e)(l,t) specify conductivities in the longitudinal (l) and transverse (t) directions with respect to cardiac muscle fibers. The full bidomain model for the propagation of electrical activation consists of coupled elliptic-parabolic partial differential equations for the transmembrane potential upsilon(m) and extracellular potential phi(e), together with quasistatic equations for the flow of current in the extracardiac regions. In this work we develop a preliminary assessment of the consequences of neglecting the effect of the passive extracardiac tissue and intracardiac blood masses on wave propagation in isolated whole heart models and describe a decoupling procedure, which requires no assumptions on the anisotropic conductivities and which yields a single reaction-diffusion equation for simulating the propagation of activation. This reduction to a decoupled model is justified in terms of the dimensionless parameter epsilon = (sigma(i)(l)sigma(e)(t) - sigma(i)(t)sigma(e)(l))/(sigma(i)(l) + sigma(e)(l))(sigma(i)(t) + sigma(e)(t)). Numerical simulations are generated which compare propagation in a sheet H of cardiac tissue using the full bidomain model, an isolated bidomain model, and the decoupled model. Preliminary results suggest that the decoupled model may be adequate for studying general properties of cardiac dynamics in isolated whole heart models.  相似文献   

6.
We have previously reported a depolarization-activated 4-aminopyridine-resistant transient outward K+ current with inward rectification (I to.ir) in canine and guinea pig cardiac myocytes. However, molecular identity of this current is not clear. The present study was designed to investigate whether Kir2.1 channel carries this current in stably transfected human embryonic kidney (HEK) 293 cells using whole-cell patch-clamp technique. It was found that HEK 293 cells stably expressing human Kir2.1 gene had a transient outward current elicited by voltage steps positive to the membrane potential (around −70 mV). The current exhibited a current–voltage relationship with intermediate inward rectification and showed time-dependent inactivation and rapid recovery from inactivation. The half potential (V 0.5) of availability of the current was −49.4 ± 2.1 mV at 5 mM K+ in bath solution. Action potential waveform clamp revealed two components of outward currents; one was immediately elicited and then rapidly inactivated during depolarization, and another was slowly activated during repolarization of action potential. These properties were similar to those of I to.ir observed previously in native cardiac myocytes. Interestingly, inactivation of the I to.ir was strongly slowed by increasing intracellular free Mg2+ (Mg2+ i , from 0.03 to 1.0, 4.0, and 8.0 mM). The component elicited by action potential depolarization increased with the elevation of Mg2+ i . Inclusion of spermine (100 μM) in the pipette solution remarkably inhibited both the I to.ir and steady-state current. These results demonstrate that the Mg2+ i -dependent current carried by Kir2.1 likely is the molecular identity of I to.ir observed previously in cardiac myocytes.  相似文献   

7.
The objective of the study was to explore the effect of electrode encapsulation by fibrous scar tissue on electrical potential distributions and auditory nerve fibre excitation patterns. A finite element model in combination with an auditory nerve fibre model was used to predict changes in threshold currents and auditory nerve fibre excitation patterns. The model showed that electrical potentials at the target nerve fibres and the electrode contacts changed in the presence of encapsulation tissue. This led to changes in threshold currents and spread of excitation. The effect of electrode encapsulation on threshold currents and spread of excitation depended on the thickness of the perilymph layer separating the fibrous tissue encapsulation and the electrode array, nerve fibre survival status, electrode geometry and configuration, and array location. Model results suggested that arrays located close to the modiolus were most sensitive to threshold changes caused by electrode encapsulation (changes were between −0.26 and 2.41 dB), whereas encapsulation of an electrode array had less effect on threshold currents when the array was located in a lateral position in the scala tympani (changes were between −0.64 and 1.5 dB). For medially located arrays, changes in the spread of excitation varied between an increase of 0.21 mm and a decrease of 0.33 mm along the length of the basilar membrane, and an increase of 0.18 mm and a decrease of 0.66 mm along the length of the basilar membrane were calculated for laterally located arrays.  相似文献   

8.
We examined the effects of niflumic acid (NFA), a chloride channel blocker, on the hyperpolarization-activated current (Ih) in newt rod photoreceptors. At 100 μM, NFA delayed the activation of Ih induced by hyperpolarizing voltage pulses to −83 mV from a holding potential of −43 mV, and reduced the steady-state current. However, reduction by NFA was weakened when Ih was activated by hyperpolarizing steps to −123 mV, suggesting that these effects were voltage-dependent. The suppressive effects of NFA on Ih were accompanied by a negative shift in activation voltage. NFA also delayed the relaxation of Ih tail currents, showing that this drug also inhibited deactivation of the current. The reversal potential and the fully activated conductance were not affected. These observations suggest that NFA reduces Ih by modifying the gating kinetics of the underlying channels. The suppressive actions of NFA remained when intracellular Ca2+ was strongly chelated, and the failure of suppression by NFA in inside-out patches suggests that the agent may act on the Ih channel from the extracellular side. These results, obtained in rod photoreceptors, are consistent with similar effects of NFA on If in cardiac myocytes, suggesting that both currents share similar pharmacological properties.  相似文献   

9.
A theoretical model of a cardiac muscle fiber (strand) based on core conductor principles and which includes a periodic intercalated disc structure has been developed. The model allows for examination of the mechanism of electrical propagation in cardiac muscle on a microscopic cell-to-cell level. The results of the model simulations demonstrate the discontinuous nature of electrical propagation in cardiac muscle and the inability of classical continuous cable theory to adequately describe propagation phenomena in cardiac muscle.  相似文献   

10.
Experiments on the preparation of frog sartorius muscle showed that electrical impedance myogram of evoked muscle contractions is the sum of electric processes accompanying contraction of some muscle fibers or muscle regions. Inverted muscle response probably results from heterotopic excitation of distant muscle regions under conditions of reduced excitability of muscle fibers adjacent to the electrode. During centrifugal propagation of contractions, especially in altered muscles, the negative wave on the myogram corresponding to impedance decrease is sometimes followed by a positive wave. The possibility for recording of heterotopic contractions indicates that the impedance of the electrode-muscle interface produces no considerable effects on the shape of electrical impedance myogram. These results hold much promise for the diagnostics of neuromuscular diseases. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 129, No. 5, pp. 518–520, May, 2000  相似文献   

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