CAIE A-Level Biology A2 15.1.8 Refractory Period Questions

any three from:

A - any one reason from:

1 Na / sodium, channels already open OR

$\mathrm{Na}^{+}$has already entered neurone OR

no more $\mathrm{Na}^{+}$channels to open OR

less $\mathrm{Na}^{+}$outside to diffuse in OR

less steep $\mathrm{Na}^{+}$concentration gradient ;

B - any one reason from:

2 sodium channels are, inactive / unresponsive OR potassium channels are open OR

membrane is, impermeable / less permeable, to $\mathrm{Na}^{+} \mathrm{OR}$

membrane is more permeable to $\mathrm{K}^{+}$;

C - any one reason from:

3 harder to reach threshold OR

potassium channels are (still) open OR sodium-potassium pumps need to restore the resting potential ;

4 hyperpolarisation at C ;