Answer and Explanation:
Under squid physiological conditions, let's explore the effect on membrane potential (Em) when the membrane is depolarized by opening Na-channels (increasing gNa to 200) with different Cl-conductance levels:
(a) At a relatively low Cl-conductance (10% of gK):
When the Cl-conductance is relatively low, the influx of Na+ ions will cause depolarization of the membrane potential (Em) towards a less negative value. This is because the opening of Na+ channels will lead to an increase in positive charges inside the cell, shifting Em towards a more positive value. Since the Cl-conductance is low, the Cl- ions won't contribute significantly to the overall membrane potential changes, and their effect on Em will be minimal.
(b) At a relatively high Cl-conductance (500%):
With a higher Cl-conductance, the influx of Cl- ions will have a more significant impact on the membrane potential changes. When the Na+ channels open and Na+ ions enter the cell, the influx of positive charges will depolarize the membrane potential. However, the presence of the high Cl- conductance allows Cl- ions to exit the cell, effectively counteracting the depolarizing effect of the Na+ influx. The efflux of Cl- ions contributes to maintaining the overall membrane potential near its resting state. Therefore, with a high Cl-conductance, the effect on Em will be less pronounced compared to the scenario with low Cl-conductance.
In summary, under squid physiological conditions, when the membrane is depolarized by opening Na-channels with different Cl-conductance levels:
(a) At a relatively low Cl-conductance, the membrane potential (Em) will be depolarized towards a less negative value due to the influx of Na+ ions, with a minimal effect from Cl- ions.
(b) At a relatively high Cl-conductance, the depolarizing effect of the Na+ influx will be counteracted by the efflux of Cl- ions, resulting in a less pronounced change in Em compared to the scenario with low Cl-conductance.