<prompt> A travelling nerve impulse does not depolarize the area immediately behind it, because:It is hyperpolarizedIt is refractoryIt is not self-propagatingThe condition is always orthodromic  <response>: Ans. is b i.e. It is refractory Ganong writes -Once initiated, a moving impulse does not depolarize the area behind it to the firing level, because this area is refractory 