Depolarization: graded and action potential.
Stimuli that increase membrane permeability to K+ produce graded hyperpolarizations. A stronger stimulus produces a larger hyperpolarization.		Stimuli that increase membrane permeability to Na+ produce graded depolarizations. A stronger stimulus produces a larger depolarization.		Some channels are voltage-gated: a depolarization causes more Na+ channels to open. A depolarization that reaches the threshold (-55 mV) triggers positive feedback and causes a massive voltage change called action potential (30 mV).