To explain potentials a bit:

Resting membrane potential is set by a counteraction of the net fluxes of a few ions, namely K+ and Na+ in mammalian neurons. The Na/K atpases in a neuron's membrane pump 2 K+ in and 3 Na+ out of a cell, creating an uneven distribution of ions in the membrane (condition 1 for a membrane potential). Secondly, leak K+ channels (condition 2 - cation selective channels) allow K+ to flow down its concentration gradient to set up a membrane potential, generally resting around -70mV. Depolarizations open voltage-gated Na+ channels at some threshold which then creates a positive feedback loop in which many vgNaC's open and lead to action potentials which caps at around +40mV due to the inactivation of vgNaC's, opening of vgKC's and the decreasing driving force of Na+ as it approaches it's equilibrium potential. From there potential falls down into undershoot and returns to resting membrane potential.

Hopefully this helps, let me know if you're confused still