ou know the neurons are excitable cells they have membrane which can be polarized, depolarized and repolarized.

During rest when there is no stimulation (due to lack of stimulus) the membrane have positive charge outside and negative charge inside due to different concentration of positive and negative ions (Na,K,Ca,Cl ions etc.).

The membranes have ion channels which are selectively permeable for different ions, the channels allow the passage of only one type of ion and resists others.

Channels may be: Leak channels which always remains open.

                        Voltage gated channel which remains closed but can open due to specific stimulus.

The excitability is due to the voltage gated channels.

At resting potential the axonal membrane is comparatively more permeable to potassium ions & nearly impermeable to Na ions, during resting stage inside the membrane there is high concentration of K ions and negatively charged proteins and low concentration of Na ions, in contrast out side the membrane there is high concentration of Na ions and low concentration K ions.

When a stimulus is applied to a site on the neuron the closed gated Na channels suddenly open & allowing Na ions to enter in to neuron due to this the polarity of membrane changes at the site of stimulus (out side become negative and inside become positive) & the site become depolarized means resting potential become action potential at that particuler point  but there is limit of membrane action potential its maximum value is +30 mV, at this maximum value the Na voltage gated channels closed (the time to reach resting potential -70 mV to maximum action potential +30 is very short lived 0.5 mili sec. ) and the K voltage gated channels open and K ions diffuse out side (after 0.5 mili sec. i.e. within a fraction of second) the membrane to restore the resting potential this is necessary to take new stimulus.

but why does the k+ diffuse outside and not inside to restore resting potential, since we have more k+ inside at rest

in Resting state- Na-Ka pump is open ( 3 Na ion outside and 2Ka ion inside ) and positive charge is maintained outside and negative charge inside = polarised state.       

When nerve recive the stimuli ,Na voltage getted channel opens and Na start entering inside to create overall positive charge inside and negative charge outside = depolarised state.        

But there is a problem!! More + inside and more negative outside...so now ka voltage gatted channel will open,, and ka ion will diffuse( move ) outside to create more +charge outside and more  negative charge inside.....

Now to atain resting state again ,3 Na ion move outside and 2 ka ion inside through Na-Ka pump( it is always open) - repolarisation...

Hope you get you point clear now!!!