Why does the threshold increase when the interval between the stimuli decreases?

Some sodium channels have been inactivated and cannot be reopened immediately.

Explanation: A stimuli is a term used to describe any sensation on a living organism such as plants or animals which is likely to create a form of response or responses by that living organism.

Sodium channels are specifically connected with stimuli transfer, once the intervals between stimuli decrease it will affect the response provided as the sodium channels may not be able to reopen immediately in order to manage the next stimuli.