The rules for Cortex (and BSG and Serenity) have characters make an Endurance check once their stun and wound=their life points.  While this works just fine, my experience in fights is that even the smallest damage can sometimes take the wind out of your sails for a moment.  So I propose a new rule for STUN…

The character takes an injury — stun or wound — that is more than either the die in their Vitality or Willpower, requires them to make an Endurance test equal to the number of damage (stun+wound) taken.  You don’t pass out , but you lose any more actions for that round.  If you fumble, you are stunned until you either spend a plot point, or make an Easy Endurance test.

If the character takes enough damage to require an Endurance test to not pass out, a success leaves them stunned for a turn, unless they blow a plot point or rolled an extraordinary success on the test.