What phase of Action Potential (AP) exactly defines the Relative Refractory Period?

Question

I am posting below, word for word, two statements made on Relative Refractory period - the first from the text: Human Physiology for Medical Students by Magdi Sabry, and the second from a web page with the URL: https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module5/readings/refractory_periods.html.

In addition, I've attached a photo from Google which appears to be in support of the second statement. But which of these is correct?

1. "Relative refractory period (RRP)...corresponds to the remaining part of the descending limb of the AP till the start of after-depolarization (i.e. during the later 2/3 of RP)"

2."...there are two types of refractory periods; the absolute refractory period, which corresponds to depolarization and repolarization, and the relative refractory period, which corresponds to hyperpolarization. "

Answer 1

Neither is an accurate definition; both are correct and not conflicting if you interpret "corresponding" to mean "typically/approximately corresponding".

The absolute refractory period is any time in which you cannot evoke another action potential no matter the strength of stimulus, due to an action potential in progress/that has just occurred.

The relative refractory period is any time in which you can trigger an action potential but need a stronger stimulus than normal.

Neither of these definitions refers to specific phases of the action potential in terms of voltage or change in voltage. Yes, typically the absolute period includes the ascending and descending phases of the AP, but there's no reason it can't last longer depending on the gating properties of the sodium channels. Similarly, the relative refractory period starts with the end of the absolute refractory period and continues through the hyperpolarization phase, but there is no reason it cannot end sooner or later, again depending on gating properties.