-1
$\begingroup$

it does for K+ and Na+ but not calcium? why?

$\endgroup$

closed as unclear what you're asking by kmm, WYSIWYG Feb 5 at 15:21

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

3
$\begingroup$

The equilibrium potential at any point in time is determined by the relative permeability of ions and the reversal potentials for those ions, according to the Goldman equation/GHK equation. The voltage of the cell moves towards that equilibrium potential at a rate that depends on the membrane time constant (determined by the capacitance and resistance/conductance of the membrane).

In a typical neuron, during the rising phase of an action potential, the conductance of sodium compared to other ions is very large, large enough that you can almost ignore the other ions, so the peak of the action potential gets fairly close to the sodium reversal potential. Often there is also permeability to calcium, which pushes it even closer to sodium reversal.

However, in cardiac pacemaker cells (and in cardiac myocytes), although there is a big increase in calcium permeability, the relative permeability to other ions isn't quite as big, so the "weighted average" of ion conductances and their reversal potentials gives an equilibrium potential somewhere in the middle.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.