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).