Neurones

The resting membrane potential:

In a neurone's resting state, the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside. So the membrane is polarised. The voltage across the membrane when it's at rest is about 70mV.

Movement of sodium and potassium ions - 

• The sodium potassium pump moves sodium ions out of the neurone, but the membrane isn't permeable to sodium ions, so they can't diffuse back in. This creates a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside.
• The sodium potassium pumps also move potassium ions into the neurone. 
• When the cell's at rest, most potassium ion channels are open. This means that the membrane is permeable to potassium ions, so some diffuse back out through potassium ion channels. 

Action potentials:

 1. Stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.

2. Depolarisation - if the potential difference reaches the threshold(around -55mV), more sodium ion channels open. More sodium ions diffuse into the neurone.

3.Repolarisation - at a potential difference of around +30mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.

4.Hyperpolarisation - potassium ion channels are slow to close so there's a slight 'overshoot' where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential(i.e less than -70mV)

5. Resting potential - the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting…