After a stimulus results in initial depolarization of a region of cell membrane of a neuron, a threshold voltage is reached. once this happens, which of the following takes place? a. a large number of voltage-gated sodium channels quickly open, causing a rapid change in membrane potential. b. voltage-gated sodium channels quickly close, voltage-gated potassium channels open, and recovery begins. c. a large number of voltage-gated potassium channels quickly open, causing a rapid change in membrane potential. d. voltage-gated sodium and potassium channels quickly close, and recovery begins.