Terms to know about cell firing

Na+ (sodium)	The major positive ion in the fluid outside the cell: it can leak into the cell.
K+ (potassium)	The major positive ion inside the cell. It can leak in and out.
Na+/K+ ATPase, or Na+/K+ pump	An enzyme that uses ATP for fuel, to pump the Na+ back out of the cell and pump the K+ back into the cell
Cell resting potential (also called Em)	This is the charge inside the cell when it is at rest. It is around -90 millivolts, because the cell contains negatively charged proteins.
Ion channels or ion gates	Proteins in the cell membrane that let particular ions through
Ligand-gated Na+ channels	These are gates for Na+, and they will open when a chemical from outside the cell binds to them (like a molecule you're smelling or tasting). Then Na+ can go into the cell.
Voltage-gated Na+ channels	These are gates for Na+ that will open when the cell charge becomes more positive (for instance, when you get an electric shock). Then Na+ can go into the cell.
Depolarization	This is when Na+ gates open and Na+ goes into the cell, making it become more positive.
Threshold potential (Et)	As the cell gets more positive, it reaches a charge at which the voltage-gated Na+ channels will open. This is the threshold potential. It is around -50 to -70 mV.
Action potential (Ea)	When Na+ has flooded into the cell, the cell becomes as positive as it can. This is its action potential.
Voltage-gated K+ channels	These K+ channels open when the cell reaches action potential, and let the K+ flow out of the cell.
Repolarization	This is when the K+ leaves the cell, making the cell become negative again.
Refractory period	This is a short period after the cell repolarizes, when it can't fire again until it has 'reset' itself.
Hyperpolarized	A cell whose resting potential is too negative - that is, instead of being -90mV, it might be -120 mV. This cell will not fire as easily as usual.
Hypopolarized	A cell whose resting potential is not negative enough - instead of being -90 mV, it might be -70 mV. This cell will fire easily.
Sensitivity	How easily a cell fires. This depends on how hard it is to raise the cell's charge from its resting potential to the threshold potential.
K+ effect on resting potential	Increasing the amount of K+ outside the cell will make the resting potential more positive and hypopolarize the cell. Decreasing the amount of K+ outside the cell will make the resting potential more negative and hyperpolarize the cell.
Extracellular Ca2+	When Ca2+ is outside the cell, it can block the Na+ channels. This makes it harder for the cell to fire.