How is a resting potential achieved?

At resting potential the voltage of the cell is ~ -65mV A neurone sends an impulse from sensory receptorsThis causes sodium ion channels to open on the dendritesThis allows the movement of Na+ ions into the cell which causes the cell membrane to be depolarisedIf this depolarisation reaches the threshold potential (which is ~ -55mV) the voltage-gated sodium channels will be activated which will lead to an action potential being formed (at ~ +40mV)Once the voltage-gated sodium channels close, the voltage-gated potassium channels open allowing K+ ions to leave the cell, which causes repolarisation of the cell membrane.The K+ ions continue to leave the cellOnce a certain amount of K+ ions have left the cell, the cell becomes so polarized that is known as hyperpolarisation (~ -90mV), at which point the voltage-gated potassium channels close.Finally, the Sodium-potassium pump returns the cell to the resting membrane potential.Action potentials are an all or nothing response because once the threshold is reached (~ -55mV) each action potential always depolarises the axon to the same voltage (+ ~40mV) by voltage-gated sodium channels. So no matter how far beyond the threshold the action potentials reach, they always depolarise the cell membrane of the axon to the same voltage.The refractory period is the period in an action potential where the axon can't be depolarised to initiate a new action potential. It limits the frequency of action potentials and ensures action potentials only travel in one direction.