Electrotonic potential

About: Electrotonic potential is a research topic. Over the lifetime, 118 publications have been published within this topic receiving 5915 citations.

Cable properties of smooth muscle.

Abstract: 1. The cable properties of smooth muscle of guinea-pig taenia coli were studied by intracellular recording of electrotonic potentials produced by square current pulses and alternating current applied with external electrodes.2. An electrical model of the smooth muscle was constructed to test how the junctional resistance between cells affected the cable properties. The model consisted of a series of short cables (representing cells) which were connected by junctional resistances.3. It was concluded, from the experiments on the living tissue and on the model, that the electrotonic potential in smooth muscle can be expressed by the ordinary cable equation used for nerve and skeletal muscle fibres, even though the junctional resistance is of the same order of magnitude as that of the myoplasmic resistance.4. The cable equation was used to analyse the membrane parameters from the electrotonic potential, from the time course of the foot of the spike and from the conduction velocity. The analysis indicates that the smooth muscle has a membrane capacity of 2-3 muF/cm(2) and a membrane resistance of 30-50 kOmega cm(2).

Mechanism of block by ZD 7288 of the hyperpolarization-activated inward rectifying current in guinea pig substantia nigra neurons in vitro.

Abstract: 1. The effects of the novel bradycardic agent 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD 7288) (Zeneca) were investigated on the hyperpolarization-activated cationic current (Ih) in guinea pig substantia nigra pars compacta neurons in vitro, using a single-microelectrode current-clamp/voltage-clamp technique. 2. Under current-clamp conditions, injection of large negative current pulses (0.1-0.5 nA, 400 ms) evoked a slow depolarizing "sag" in the electrotonic potential due to activation of the slow inward (anomalous) rectifier. In voltage-clamp recordings, hyperpolarizing voltage steps from a holding potential of -60 mV (close to resting potential) elicited slow inward current relaxations with kinetic properties similar to those seen for other neuronal Ihs. 3. ZD 7288 (10-100 microM) produced a consistent abolition of the electrotonic potential sag with no effect on membrane potential or spike properties. Under voltage clamp, Ih amplitude was clearly reduced in a time- and concentration-dependent manner (apparent half-maximum blocking concentration = 2 microM); full block of Ih was typically achieved after 10-15 min of exposure to 50 microM ZD 7288, with no significant recovery observed after 1 h of washing. 4. A similar (although more rapid) block of Ih was seen after application of 3-5 mM Cs+ (partially reversible after 30 min of washing). 5. Partial block of Ih by 10 microM ZD 7288 was accompanied by a reduction in the maximum amplitude of the Ih activation curve, a small negative shift in its position on the voltage axis, and a linearization of the steady-state current-voltage relationship. The estimated Ih reversal potential, however, remained unaffected. 6. In 10 microM ZD 7288, the time course of Ih activation and deactivation was significantly slowed (within the range of -70 to -120 mV for the activation time constant and -70 to -90 mV for the inactivation time constant). 7. Blockade of Ih by ZD 7288 or Cs+ was independent of prior Ih activation (i.e., non-use dependent). 8. Intracellular loading with ZD 7288 also abolished the sag in the electrotonic voltage response and Ih relaxations, suggesting an intracellular site of action. By contrast, intracellular Cs+ had no effect on Ih properties. 9. Block of Ih by ZD 7288 (but not Cs+) was relieved by prolonged cell hyperpolarization, manifested as a slowly developing (half-time approximately 20 s) inward current at a holding potential of -100 mV. 10. We propose that ZD 7288, when applied externally, may behave as a "lipophilic" quaternary cation, capable of passing into the cell interior to block Ih channels in their closed state; this compound may thus prove a useful research tool, in place of Cs+, for studying the properties and significance of Ih currents in controlling neuronal function.

The Electrical Properties of the Muscle Fibre Membrane

Abstract: An analysis is made of the electric properties of frog muscle using the 9rectangular pulse9 technique of Hodgkin & Rushton (1946). The experiments were made on isolated fibres and small bundles of the M. adductor magnus and on the M. extensor longus dig. IV. Sub-threshold currents of about 0.1 sec. duration were applied and wave form and attenuation of the extrapolar potential changes determined. In the vicinity of the cathode the relation between current and voltage across the fibre membrane is non-linear, and there is evidence of a local electric response with currents of more than 30% threshold strength. At the anode, however, the membrane behaves as a conductor of approximately constant resistance (section A). At an average temperature of 22 degrees C, the following mean values were obtained: (a) Fibre diameter: $75\mu $ in the bundles of M. adductor magnus, and $45\mu $ in M. extensor longus dig. IV. (b) Characteristic length of the muscle fibres: 0.65 and 1.1 mm. respectively. (c) Membrane time constant: 9 and 18.5 msec. (d) Specific resistance of the myoplasm: about 230 $\Omega \text{cm}$. (e) Transverse resistance of the fibre membrane: 1500 and 4300 $\Omega \text{cm.}^{2}$. (f) Membrane capacity: about 5 $\mu $F/cm.$^{2}$. The numerical differences between isolated fibres and whole muscle arose chiefly from a different value of the membrane resistance, the significance of which is discussed (section section B1 and D). The value of the membrane capacity of muscle is about five times higher than that reported for various other cell membranes and confirmed here for isolated crustacean nerve fibres. The large membrane capacity must be an important factor in determining the slow electrical time scale of muscle. The relations between the electric constants of the resting muscle fibre and some of its physiological properties (time factor of excitation, propagation velocity, rate of decline of the end-plate potential) are discussed (section E). The location of the membrane, or interface, at which electromotive changes occur is discussed, and a number of reasons are given which indicate that the site of the electrotonic potential changes must be at the surface of the muscle fibres.

Membrane currents in the rabbit sinoatrial node cell as studied by the double microelectrode method.

Abstract: When a strand of the rabbit sinoatrial node tissue was shortened by ligation, the spatial decay of electrotonic potential decreased and the input impedance increased. In a piece of the tissue 0.2-0.3 mm in diameter apparently uniform current spread was obtained. Action potentials recorded from three different sites in this small piece occurred simultaneously and were superimposable. In voltage clamp experiments using the double microelectrode method, the membrane potential was usually held at -30 to -40 mV, where no net current flowed. When membrane potential was suddenly changed from the holding potential, the sign and the time course of the ionic current varied with membrane potential. Hyperpolarization gave an inward current which increased with time. Depolarization gave a transient inward current followed by sustained outward current, and repolarization gave an outward current tail which exponentially subsided with a time constant of 0.37 s. The membrane time constant was 12.0 ms. When the specific membrane capacitance was assumed to be 1 muF/cm2, the specific membrane resistance at the resting potential was 12 Komega cm2. The peak of the transient inward current on depolarization was 1.3 X 10(-5) A/cm2.