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Cephalopod NeurobiologyNeuroscience Studies in Squid, Octopus and Cuttlefish$
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N. Joan Abbott, Roddy Williamson, and Linda Maddock

Print publication date: 1995

Print ISBN-13: 9780198547907

Published to Oxford Scholarship Online: March 2012

DOI: 10.1093/acprof:oso/9780198547907.001.0001

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Voltage Dependence of Sodium Channel Inactivation In The Squid Giant Axon

Voltage Dependence of Sodium Channel Inactivation In The Squid Giant Axon

(p.107) 8 Voltage Dependence of Sodium Channel Inactivation In The Squid Giant Axon
Cephalopod Neurobiology

Nikolaus G. Greeff

Ian C. Forster

Oxford University Press

This chapter shows that high resolution recording of gating current from the squid giant axon detects a slow component during the inactivation phase of sodium ionic current. From the combined data of ionic and gating current obtained with a new approach (isochronic analysis), an estimate of about 1.2 electron charges can be made for the voltage dependence of microscopic sodium inactivation, i.e., the quantal gating charge of the open-to-inactivated transition. This method also reveals that the conventional rate analysis of the inactivation of sodium ionic currents will estimate the voltage dependence correctly between −10 and +40 mV but not at lower voltages. The initiation of a nerve action potential depends on the activation of voltage-gated sodium channels, which leads to their opening and the inward flow of sodium ions. The termination of the action potential is then ensured by the subsequent inactivation process. The conformational state of the inactivated channel is a closed one that differs from the resting closed state to which the channels return upon repolarization. Both processes, activation and inactivation, occur faster at more depolarized voltages, indicating that they are driven by the electric field across the membrane. The knowledge of the quantal inactivation gating charge leads to the question about the molecular structural element in the channel protein that represents the voltage sensor of the inactivation gate.

Keywords:   sodium channel, inactivation phase, quantal gating, depolarized voltages, voltage gated, squid giant axon

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