You already could read what a neuron is and how it works. What still has to be explained indepth is the way the Action Potential functions. But before we can we have to explain a few more basics about the electric charge of a neuron and its Resting Potential.

Neurons send messages via the use of electrochemicals, this means that chemicals cause an electrical signal. These chemicals all have an electrical charge and are therefore called ions.

Ions either have a positive or negative charge, a few examples are:

• Sodium (Na+): 1 +.
• Potassium (K+): 1+.
• Calcium 2++.
• Chloride (Cl-): 1-.

There are also some negatively charged protein molecules.

Remember that opposites attract when it comes to ions, this is how electricity moves through cables. We saw how each Nerve Cell has a membrane and since it is semi- permeable it will allow some ions through and will block others affecting the balance between inside and outside of a neuron.

Obviously a neuron is not always firing signals and this is when it's at rest. At this point the charge on the inside of the neuron is negative relative to the outside.
Normally the concentrations of different ions would try to balance out on both sides of the membrane, diffusing from areas of higher concentration to areas of lower concentration. But this is where the semi- permeable membrane plays comes to play, seeing that it will only allow some ions to pass through the ion- channels and blocking others.

When the neuron is at rest, leaking channels let some of the Potassium Ions K+ move across the membrane while the Cloride Ions Cl- and Sodium Ions Na+ can do so less easily. The negative Protein Molecules A- inside the neuron can't cross the membrane at all and only very small amounts of Sodium diffuse in.
In addition to these selective Ion- Channels you have a Pump which uses energy to move 3 Sodium Ions out of the neuron for every 2 Potassium Ions it puts in.
When all of these different forces are balanced out, you can measure the difference in voltage between the inside and the outside of the neuron. This is what is refered to as the Resting Potential. The resting membrane potential of the neuron is about -70mV (mV= MilliVolt.) So inside of the neuron is 70mV less than the outside. At rest there are relatively more Sodium Ions outside each neuron and more Potassium Ions inside.

Now that we looked into the Neuron at rest we can more easily understand how the Action Potential works. Remember that the neuron sends information via its axon, away from the cell body, towards the next neuron? This is simplified what the Action Potential is, an impulse of electric activity. This is kick- started by a stimulus which opens selective ion-channels bringing the Resting Potential of -70mV to 0mV causing depolarization of the cell.

The depolarization triggered by the stimulus, brings the Resting Potential to 0mV. When the depolarization reached -55mV (Depolarization of about 15mV)an Action Potential will be fired by the Neuron. What is so important about this is that unless this treshold is reached, nothing will happen. But when it is reached, a fixed sized Action Potential will be fired off. This means there is no inbetween for an Action Potential, it is all or nothing. When treshold is reached a full Action Potential will be fired.

So what exactly is the process of depolarization? We already know it starts by a stimulus. What comes next are a few consecutive, but overlapping, steps in which ions are exchanged through the neuron's membrane.

• A stimulus causes depolarization and when treshold is reached, Sodium Channels start to open. Since there are many more Sodium Ions on the outside of the cell, which is negatively charged compared to the outside, Sodium Ions push into the cell. Because Sodium is a positively charged Ion the untill then negatively charged Cell is slowly becoming more positive and thus depolarizing. More Sodium Channels open, further depolarizing the cell and increasing the membranes permeabliliy to sodium by over 1000 times. The Membrane Potential has now gone from -70mV to +30mV.
• But as the cell starts to reach 0mV, the Sodium entry starts to slow down and the Ion Channels slowly start to close. This adds to the lessoning of the permeability of the cell's membrane and less Sodium Ions enter and then finally stop when the membrane reaches a depolarization of +30mV.
• The neuron is now depolarized and as the membrane potential reaches +30 it's the turn of the Potassium Channels to open, which take longer. Now the positively charged Potassium Ions rush out of the cell reversing the depolarization. Now the cell is nearly impermeable to Sodium and permeable to Potassium.
• At the same time that the Potassium Channels opened the Sodium Channels had started to close. Because of the slow closing of the Potassium Channels repolarization goes past the Resting Potential of -70mV, and reach -75mV which is refered to as hyperpolarization.
• Now that all Channels have been closed, the ion concentrations go back to their resting levels which in this case is -70mV.