To my understanding, the steps of an action potential are as follows:
The neuron is at rest--there is a negative charge (K ions) inside the cell, and a positive charge (Na ions) outside the cell. Pumps work hard, pumping in K and pumping out Na to maintain this polarization.
Excitatory NTs bind with the dendrites. As soon as it's over a certain threshold it triggers an action potential.
The inside of the cell depolarizes, and the depolarized "chunk" propagates through the axon.
Now here's my confusion:
A. How do neurotransmitters manage to depolarize the inside of the cell? Do they force the cell to give up pumping out Na ions? Do the neurotransmitters themselves contain positively charged ions that the ion pumps are not sensitive to?
B. When the cell depolarizes, the electrical impulse travels down the axon. When this happens, Na+ and K+ ions rapidly pass through the membrane as the signal fires. (Like this picture) http://en.wikipedia.org/wiki/Action_potential#mediaviewer/File:Action_Potential.gif
Why are ions being pumped in and out of the axon in such a way to propagate an action potential? Why isn't it just a positively charged signal running through the axon, without regard to the outside environment? (please let me know if this is unclear!)