Voltage-sensitive sodium permeability

Ciguatoxin binds tightly to voltage-sensitive sodium channels. This binding leads to increased opening of sodium channels and subsequent increased cell membrane permeability to sodium. As a result, the electrical potential of involved cells is altered. 

Sodium channels open more rapidly than K+ channels because they are more voltage sensitive and a small depolarization is sufficient to open them. Larger changes in membrane potential associated with further cell excitation are required to open the less voltage-sensitive K+ channels. Therefore, the increase in the permeability of K+ ions occurs later than that of Na+ ions. This is functionally significant because if both types of ion channels opened concurrently, the change in membrane potential that would occur due to Na+ ion influx would be cancelled out by K+ ion efflux and the action potential could not be generated. 

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