5. Describe the one condition that has been varied among the four trials in Model 2.

Physiologically, after an action potential has occurred, the sodium ions  channels are closed up momentarily.After this moment, no matter the extent of stimulation of the neuron,action potential will not be generated,(absolute refractive period) because the Voltage gated sodium channels will remain closed up.This intervals of non responsive of he action potential is called refractive period.it is the period for the plasma membrane to relax and prepare for another action potential propagation, provided the stimulus is greater than the normal stimulation for the neuron(relative refractive period)

Its duration depends on age groups, thus in  teenagers(18yrs)  about minutes and up to 20 hours in adults.

Its major significance it to ensure  animals adaptability and adjustment to a stimulus, so that the number of action  potential propagated  per minute is controlled.  Thus the length of the refractive period is a measure of the maximum frequency  of  transmission of action potential along a neuron.

Explanation: