Explain how an action potential is generated in a sensory neurone.

DIAGRAM
At rest, there is a potential difference across the cell surface membrane of -60mV maintained by sodium-potassium ion pumps which pump 3 Na+ ions out of the cell for every 2 K+ ions pumped in. When a stimulus is detected by a receptor some of the sodium ion channels open, allowing Na+ ions to diffuse into the cell down the concentration gradient. This causes a depolarisation of the membrane known as a generator potential. If this generator potential exceeds the threshold potential (-50mV), voltage-gated sodium ion channels will open triggering diffusion of more Na+ ions into the neurone and the membrane will depolarise, generating an action potential of +40mV.
Once the action potential is reached, the sodium ion channels close and potassium ion channels open. K+ ions diffuse out of the cell causing repolarisation of the membrane. However this overshoots leading to hyperpolarisation. At this point the potassium ion channels also close and the sodium-potassium ion pumps return the potential difference to -60mV via active transport.