AAMC FL1 B/B #14

[MC789]

Can someone explain why my answer choice is incorrect? I chose B: an increase in extracellular Na+ concentration because I recently did content review for action potentials and learnt that action potential/depolarizations are spread by Na+ rushing INTO the membrane. So my reasoning for stopping the spreading of the firing was to increase extracellular Na+ concentration so that way the Na+ inside the membrane will prefer to flow down their concentration gradient, exiting the membrane and stopping depolarization --> stopping the firing.

Though the AAMC's correct choice makes sense, I don't understand why my answer can not be right also.

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[TrustTheProcess76]

If you increase the threshold of firing, the membrane will need to depolarize to fire an Action Potential. Remember, Paps are all or nothing. If you increase the threshold for AP to fire, it makes it harder to get epileptic symptoms

Also: I didn't pick B, because if I increase sodium concentration outside of the cell, this will create an even bigger gradient for Sodium to flow into cell and cause an AP. (charges move from high density to low density, or higher + potentials to lower + potentials)

 

[MC789]

omg thats so true! Na+ would want to flow DOWN its gradient and increasing the outside Na concentration will just further cause inflow! my thinking got clouded under the pressure of time. Thanks!!

 

[sapientnarwhal]

Your reasoning is incomplete. Changes to the concentrations of relevant anions/cations leads to changes in the resting membrane potential (GHK equation). Increasing EC [Na+] will increase the resting membrane potential, thereby bringing it closer to the threshold potential and increasing excitability. Of course the Vm is dictated by the reversal potentials of the relevant cations/anions as well as their relative permeabilities, so while you are correct that the gradient is increased, it is also important to note that the effect is an elevated resting membrane potential.

The effect of the new gradient on the AP is more complicated. In short, the duration of the AP is shorter and the amplitude of the AP is higher, but I am not sure as to how the absolute refractory period is affected. If the duration of the absolute refractory period is unaffected, then more AP's could be generated per unit time. I suppose this might increase the frequency of the synchronous firing, but IMO the aforementioned increased excitability is the more pertinent aspect to why B is incorrect.

 

[letsgetstarted1234]

Adding to this,

what would decreasing depolarization stage do?

wouldn't it hyperpolarize the cell more? because the cell is less depolarized when the hyperpolarization begins?