Which structure surrounds the cytoplasm of the cell and separates the interior of the cell from the external environment?

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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the correct answer is option b

behavioral response need communication and thus co-ordination at several levels such as interaction among cells, organ system and the entire organism.

behavioral response produces both internal and environmental stimulus.