Define threshold, and relative and absolute refractory period. explain the state of ion channels and the stimulus amplitude required to generate an action potential during each.

Threshold is a specific voltage that is needed by a neuron to generate action potential  

the absolute refractory period is period in which neurons are unable to generate second action potential. And relative refractory period is interval following absolute refractory period.

Normally -70mV voltage is required to stimulate an action potential  

Explanation:

The relative refractory period is the period occur shortly after the depolarization of a nerve fiber when partial repolarization has occurred and need a greater stimulus which can stimulate a second action potential.

During an action potential, sodium channels (voltage gated)  open, and sodium rushes into the cell  because there is more sodium outside the cell than inside and after specific time period sodium channels closed and at the same  time potassium channels opens which let the positive ions (potassium ions) to goes out of the cell which makes axon negative and again ready for other action potential

The action potential generates at  −70 mV which raised it to threshold potential which is above −55mV then rapidly rises to a peak potential of +40 mV and then the potential  drops and overshoots to −90 mV, and the resting potential of −70 mV is reestablished.