Action Potential

Resting Membrane Potential

In a resting neuron, the charge inside its membrane is relatively negative compared to the exterior. This charge separation across the membrane is called the membrane potential.
The membrane potential across a membrane of a resting neuron is called the resting membrane potential, which is around -70 mV. The resting membrane potential provides energy for the generation of a nervous impulse due to any kind of stimuli.
The process of generation a resting membrane potential is called polarization. Neurons become polarized do to several process that happen at the same time.
Large protein molecules that are negatively charge are found in the intracellular fluid, but not outside. These proteins cannot pass through the cell membrane. The neural membrane is permeable to small negatively ions, such as chlorine.
The biggest contributor to the resting membrane potential is called sodium-potassium exchange pump. This system, along with the use of ATP, is able to exchange sodium ions (negative) with potassium ions (positive).
The sodium-potassium exchange pump exchanges three sodium ions for two potassium ions. As a result, the outside becomes positive. However, these ions can leak slowly by diffusion, messing up the whole system, that why there is a constant exchange, keeping the -70 mV.

Action Potential 

To a nerve impulse is composed by a series of action potentials.
Action potential is divided into three main phases:

1. Depolarization: when the membrane potential is reduced to less the -70 mV, this membrane is said to be depolarized. If the membrane potential is reduced to -55 mV or less, a dramatic change occur in the neuron, called action potential. Action potential is said to be "all-or-none" effect because if the membrane potential become between -55 mV and -70mV, nothing really happens. That is why -55 mV is such a important mark to the  Action potential. Thus, we call -55 mV, the Thershold potential. During this process, voltage-gated sodium channels open in a way that let a lot of sodium ions in the membrane, creating an extreme positive membrane potential.
2. Repolarization: As a result the voltage-gated potassium gates open and let potassium ions in, which will take out the cell the positive gradient, creating a less negative environment inside the neuron. Therefore, there is a return to its previous polarization.
3. Refractory Period: Period at witch the neuron cannot receive any order action potential.