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When we talk about a neuron firing, we are actually referring to the action potential. The action potential is a nerve impulse which travels from the axon hillock to the end of the axon, signalling the axon terminals to release their neurotransmitters. Before we start there are a few things you need too know:










Now, lets take a look at a cross section of a neuron.

There is a few thing you will notice from the image above.

Firstly the positions of sodium and potassium. Sodium ions are outside, while the potassium ions are inside. An easy way to remember this is a salty banana, (potassium inside and sodium outside).

Secondly, the negatively charged proteins within the neuron. These proteins can not exit the cell.

Another thing you will see while talking about action potentials is the the graph to the right.

This graph records the voltage within the cell.

Notice how the voltage changes as an action potential goes through the neuron.

Resting potential

At this stage the neuron is not stimulated and said to be at rest.

The protein channels are closed and the voltage inside is -70mV.

Depolarisation

The neuron has just been stimulated. Sodium channels open allowing positively charged sodium ions to flood into the cell through diffusion.

The neurons voltage becomes +30mV.

Re-polarisation

Once the neuron reaches +30mV the sodium channels close and potassium channels open. This causes a flood of positively charged potassium ions out of the cell.

 


Hyper polarise

As the potassium is released out of the cell the negative charge of the cell increases. Since the potassium pumps close slowly the neuron becomes hyper polarised.

The neuron now has a voltage of  -90mV


 


Refractory period

This period occurs after the action potential. During this time the neuron can not conduct another action potential.
The sodium potassium pumps work to restore the original gradients. Pumping 2 potassium ions for every 3 sodium ions to their original sides.

After the refractory period this section is returned to -70mV and is ready for another action potential.


To the left is the complete graph of an action potential

Once the sodium ions have entered the cell, some will diffuse down the axon depolarising the adjacent region of the axon membrane. This triggers another action potential.

Bellow is a complete animation of an action potential. Click start to view

Click here for questions

Click here for information on neurotransmitters

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