The resting membrane potential...
Understanding the resting membrane potential is the first step in understanding exactly how the action potential is able to travel throughout the body...
Like most cells, neuronal cells (or commonly known as neurons) contain a cellular membrane. It's this membrane that allows an action potential to occur. This is only able to happen because of difference in voltage between the inside and outside of the cell. This difference in voltage at rest is what's termed the "resting membrane potential," and it typically sits around -70mV in most cells.*
*depending on the neuron RMPs can vary from -40 to -90mV, but the majority stay around -70mV at rest.
This difference in charges forms whats known as a concentration gradient and is created by the different concentrations of electrically charged particles, particularly potassium (K+) and sodium (Na+).
It is important here to know that the interior of the cell contains a greater concentration of potassium (K+) ions and a lower concentration of sodium (Na+) ions relative to the outside, and it is this relative difference that creates this -70mV charge.
Within the cell's membrane are various proteins that act as channels for ions to come and go. These channels vary in the way that they allow K+ and Na+ in and out of the cell. Some are "leaky" channels, that allow either K+ or Na+ (depending on the channel) to freely leave the cell down their concentration gradients. Others are voltage or chemically gated channels that are more involved with the action potential than the RMP.
So picture this, a cell membrane with a leaky K+ channel, and a leaky Na+ channel. At rest, the cell is only slightly permeable to Na+ ions, but very permeable to K+ ions (~75 times more). Therefore, the high efflux of K+ ions is the main reason the RMP exists. Since there are more K+ ions inside the cell, they will pass through the channel down their concentration gradient to the outside of the cell. The opposite will occur with Na+. These ions will go down their concentration gradient INTO the cell through their own set of "leaky" channels.
So logically, you may think that at some point, this efflux and influx of ions would in time balance out, causing this membrane to become electrically neutral, and if you're thinking that, you're thinking the right way, however,these ions are constantly being actively replaced to their origins by what's known as an ATPase pump.
This pump is a protein channel similar to the leaky channels, however, this is an active pump whereas the leaky channels are passive. "Active" simply mean energy is required to drive the movement. In this case, ATP powers the channel to take replace 3 Na+, and 2 K+, to their origins against the concentration gradients (hence energy is required). The odd number of Na+ keeps the concentration constantly changing and the membrane potential from becoming electrically neutral.
Remember, 2 K+'s in for every 3 Na's out. Since there's two K's in "kink," it might help you remember this. 2 K's go IN the cell, for 3 Na's out. KINK.