Nerves

Resting potential and action potential.

Resting

When we are resting, we don't feel anything. This is because our cell membranes are polarised.

This essentially means the outside of the membrane is positively charged in comparison to the inside. This is very simply due to the higher concertration of positive ions outside the cell.

The resting potential of a polarised membrane is -70 mV. 

Don't be fooled, just because we are resting doesn't mean our cells are! A lot of action is involved in maintaining this resting potential and it involves sodium-potassium pumps and potassium ion channels. 

As you should know from AS, the sodium potasssium pump involves active transport and thus ATP. What does it do? Well it pumps three sodium ions out of the cell and two potassium ions in. 

The membrane isn't permiable to sodium ions, so as they're pumped out against their concerntration gradient, they can't simply diffuse back in. This creates a sodium ion electochemical gradient. As sodium ions are positvely charged, there is a postive charge on the outside of the cell in comparison to the inside.

The cell membrane is permiable to potassium ions, and so they simple diffuse (facilitated diffusion) back down their concertrated gradient out of the cell via the potassium ion channels. 

As you can see, a lot of work goes into not creating a nerve impulse, so don't be mislead by the term "resting potential"!. 

Stimulated cells

So what happens when our cells when they are stimulated? 

When our cells are stimulated they become excited, a process known as excitation, and it causes sodium ion channels to open. (Don't confuse these with the sodium-potassium pumps - they're something completely different!)

This makes the membrane more permiable to sodium, and so sodium ions diffuse down their sodium ion electrochemcial gradient (Ring any bells? Remember this…