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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 355 (1975), S. 361-364 
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Cs-Ions ; Potassium Channel
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Voltage clamp experiments done on nodes of Ranvier show that external application of Cesium blocks the inward but not the outward potassium currents. Internal application of Cs ions reduces the outward K-current and the inward K-current is not affected. These results support the hypotheses that K ions cross the K-channel after dehydration at superficial sites where competition may occur with Cs-ions.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 357 (1975), S. 145-148 
    ISSN: 1432-2013
    Keywords: Sodium Permeability ; Delayed Currents ; Node of Ranvier
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Voltage clamp experiments carried out on nodes of Ranvier of myelinated fibres ofRana esculenta showed that a small fraction of sodium channels fail to inactivate. Thus during long lasting depolarizing pulses there is a small Na-current superimposed on the leakage and potassium currents. This late Na-current appears more marked in sensory fibres than in motor ones.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 358 (1975), S. 111-124 
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Potassium Permeability ; Ionic Accumulation ; Series Resistance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary 1. Voltage clamp experiments were carried out on frog myelinated fibres to study the origin of the transient inward current occuring when the membrane is repolarized after long lasting depolarizing pulses (tail current denominated “I p” by Frankenhaeuser). 2. The “tail” of inward current measured during repolarization after break of the depolarizing pulse is insensitive to external application of TTX, is abolished by external treatment with TEA or Cs and decreases when the outward K-current during the pulse is diminished. 3. The time course of the “tail” current is exponential. Its direction depends on the duration of the depolarizing pulse and on the membrane potential level at repolarization. 4. It is concluded that the tail of inward current during repolarization is carried by K-ions accumulated in the perinodal space during a depolarizing pulse. The data suggest that the tail reflects the time course of the restoration of the K-concentration to its initial level. The tail current itself contributes to this restoration depending on the Em value at repolarization. 5. It is shown that one of the two phenomenological models proposed by Frankenhaeuser and Hodgkin to account for the external potassium accumulation observed in the squid giant axon may be also applied to the Ranvier node. Assuming that the thickness of the space is 2900 Å and that the K-permeability of the barrier is 0.019 cm/sec, it is possible to account for the observed changes in [K]0 during a long lasting depolarizing pulse. 6. The existence of such a barrier would introduce an electrical resistance in series with the nodal membrane of roughly 150000 Ω.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Pflügers Archiv 317 (1970), S. 287-302 
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Voltage-Clamp ; Sodium Pump ; Anomalous Rectification ; Membrane Channels ; Ranvier-Schnürring ; Voltage-Clamp ; Natriumpumpe ; Abnormale Gleichrichtung ; Membrankanäle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Voltage-clamp experiments have been performed in order to investigate the after effects of Na-accumulation in myelinated nerve fibres from the frogRana esculenta. 1. The ionic current associated with a depolarizing test-pulse adjusted to the sodium equilibrium potential has been measured following conditioning volleys of depolarizing pulses. 2. A marked shift ofE Na towards less positive potentials has been observed following volleys of more than 50 msec; the time course of recovery ofE Na showed a rapid phase followed by a slow one. Both phases proved to be insensitive to suabain, DNP and K-free solution. The slow recovery phase was, however, very much prolonged when the conditioning volleys were applied to a node superfused with Li-Ri. 3. The shift ofE Na was associated with a drastic reduction of the outward potassium current and with anomalous rectification; the reduction of the potassium outward current, however, was observed when both the sodium and potassium ions moved from inside to the external solution. 4. The results may be explained by an influx of sodium ions into the nerve fibre which are temporarily accumulated near the inner surface of the membrane. They compete with potassium ions at the inner opening of the potassium “channel” and thereby reduce the number of potassium ions which are available to move through the membrane under the influence of the electrochemical gradient.
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  • 5
    ISSN: 1432-2013
    Keywords: Node of Ranvier ; Potassium conductance ; Cation selectivity ; Gating mechanism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary 1. Voltage clamp experiments were carried out on single myelinated fibres of the frog to analyse the changes of the potassium conductance (g K ) resulting from variations of the external K-concentration ([K]0). The use ofg K is justified by the observation that instantaneous K-currents vary linearly withE despite asymmetries between [K]0 and [K]i. 2. At constant membrane potential and for inward going currents,g K increases as a Michaëlian function of [K]0. We propose, therefore, that external K-ions bind specific membrane sites controllingg K . The apparent equilibrium constant (K app ) of the binding reaction decreases with depolarization following a double exponential function. The rate constants of this function depend on the external Ca-concentration. 3. An empirical equation is derived which satisfactorily describes the variation ofg K as a function ofE and [K]0. The fact that either the increase in [K]0 or the membrane depolarization can lead to the opening of the channels is regarded as evidence that the binding of K to specific sites (“K-receptors”) is a necessary step for the formation of conducting channels. The affinity of the receptor-sites for K-ions depends on the electrical field in the membrane. 4. K-free Ringer solutions markedly reduce the steady-state K-current and its rate of activation. These effects are not observed if the lack of potassium is compensated by addition of caesium. It is concluded that Cs can replace K at the sites controlling the formation of the channel. 5. High external concentrations of Cs-ions reduce the inward K-currents when [K]0 is high and tend to increase outward K-currents, especially when [K]0 is low. These results are explained assuming that Cs-ions enter the channel with K-ions moving in single file. They stop the in-moving file when reaching a selectivity structure deeply located in the channel. 6. The fraction of the membrane thickness (δ) at which Cs-ions occupy a blocking position in the K-channel, when K and Cs are present at high concentrations in the external medium, is estimated to be: δ=0.6. From this result, it is deduced that the voltagesensitive K-receptors are located in the external half of the membrane.
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