Explain, in terms of the movement of ions, the shape of a typical action potential.

First, draw the graph of time against voltage for a typical action potential and label with points 1-8 (see whiteboard). 1. at rest, the membrane is polarised at -60mv (this is the potential inside the cell compared to the outside of the cell). There is a higher concentration of Na+ (sodium) ions outside of the cell and a high concentration of K+ (potassium) inside of the cell. 2. In a sensory neurone, a stimulus causes Na+ channels to open. These may be sensitive to stretch or a particular chemical. Some Na+ diffuses into the cell. This will cause the inside of the cell to become more positive and the potential difference will increase, or become less negative. 3. If the membrane potential reaches -50mV, this is the threshold for voltage gated sodium channels to open. Voltage gated channels open and this allows more Na+ into the cell, further depolarising it and creating a positive feedback cycle in which more Na+ channels open. 4. The potential difference now reaches +40 mV. The inside of the cell is positive compared to the outside. 5. The voltage gated Na+ channels are only transiently open. After a set period of time, they inactivate / close. Sodium stops entering the cell. 6. During the preceeding period of time, voltage gated K+ channels have slowly been activating. They are also activated by the depolarisation, but take longer to open. The concentration of K+ is higher inside of the cell, so once open these channels also K+ to exit the cell. This starts to make the inside of the cell more negative, and it begins to repolarise. 7. At this point, the potential difference 'overshoots' slightly leading to hyperpolarisation, where the potential difference is more negative than at resting potential. 8. By this point, the original potential has been restored. This is partly due to the action of the sodium potassium pump.