Resting potential: approximately -70millivolts Action potential: between -55 and -65 millivolts

The fluid surrounding neurons is high in sodium, but the selective permeability of neurons keeps it out for the most part. This leads to a high concentration of sodium attracted to the negativity of the neuron, but unable to actually get in. Since the concentration of sodium is so high, even diffused, it mostly gathers around neurons. (Think little bits of iron around a cylindrical magnet).

When the sodium channels open, the sodium rushes inside the cell, and the influx of sodium raises the voltage positively. The net negative charge is due to the ratio of sodium outside of the cell (pre-signal) to the amount of potassium within the cell. Since there is so much sodium outside the cell, the relative negativity within the cell allows the positive ions to flow in.

Since the inside of cell is negative at resting potential, and the outside is highly positive with its concentration of sodium, when neurotransmitters signal that sodium channels should open, the sodium flows into the neuron until the channel closes, or the cell reaches action potential. Action potential is also negative, but more positive than resting potential. This is why the cell is considered 'depolarized' right before it reaches action potential.

If the inside of the cell was not negative, then the sodium wouldn't behave this way, which would basically make your brain unable to send signals. Additionally, the sodium potassium pump, if you're familiar with it, is what allows the cell to return to resting potential, and the external concentration of sodium to stay high.