<prompt> Refractory period of a myelinated nerve is due to:Voltage-dependence of Na channelsTime-dependence of Na channelsVoltage-dependence of K channelsTime-dependence of K channels  <response>: Only when the membrane has repolarized up to about - 40 mV to - 50 mV that the Na channels recover from their inactivated state and go into resting state. All this while, the membrane is refractory; that is, the membrane can excite again only when many of the Na channels have recovered which itself depends on the membrane potential. Hence, refractory period is due to the voltage-dependance of the Na channels. 