The action potential can move only in one direction because the recently depolarized area of the membrane is an absolute refractory period, and cannot generate an action potential. What is occurring durring the refractory period at the molecular level?

The refractory period describes a period between the initiation of an action potential (where Na+ channels are active/open), and immediately after the action potential’s peak. In the absolute refractory period, action potentials cannot be generated, as Na+ channels are in an inactive or closed state, they usually take around one or two minutes.

After the absolute refractory period the relative refractory period (RRP) occurs. During the RRP, it is hard to send another action potential. Na+ channels are once again open, but the cell remains hyperpolarized- its membrane potential, remains negative. Recovery from inactivation is voltage and time-dependent; Action potentials would require an influx of more positively charged ions. These must be more than a specific threshold in order to have the cells send along more action potentials which helps with figuring out stimulus intensity.