A traveling nerve impulse does not depolarize the area immediately behind it, because:
It is hyperpolarised
It is refractory
It is not self-propagating
The conduction is always ohodromic
When an AP is initiated at a point, the point goes into refractoriness when the Na+ entered through that point has excited the adjacent (2nd) point. so it will travel in single direction Hyperpolarization event is after the refractory period.
Excitation and Action Potiential