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:
The nerve cell operates on an all or nothing principle. By this I mean the neuron has to have sufficient stimulation to fire. If the stimulate is too weak the neuron won’t fire.
The intensity of the stimulus is actually determine by the rate of firing. For example a pin prick may cause a neuron to fire twice, where as a knife stabbing may cause the neuron to fire multiple times.
Voltage is difference between how positive something is and how negative something is. Taking a positive ion and moving across the membrane creates a voltage.
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.
In myelinated neurons, the action potential occurs in the Nodes of Ranvier. The animation to the right shows how much faster the signal can be sent down in a myelinated cell compared to an un-myelinated 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
