Final answer:
The statement is true; voltage-gated Na+ channels inactivate shortly after opening, leading to an absolute refractory period which prevents the action potential from moving backward along the axon.
Step-by-step explanation:
The statement that voltage-gated Na+ channels become automatically inactivated shortly after opening, which ensures that the action potential cannot move backward along the axon, is True. During an action potential, the depolarization of the neuron's membrane potential leads to the opening of these channels. Once the membrane potential reaches around +40 mV, the inactivation gate of the Na+ channel closes. This inactivation occurs nearly simultaneously with the peak of depolarization, resulting in a brief period known as the absolute refractory period. During this time, no new action potential can be initiated in the same region, preventing the backward movement of the action potential along the axon. It is only after the inactivation gate re-opens and the activation gate closes, resetting the channel to its original state, that another action potential can be possibly generated.