The aforementioned dark ritual that kept him in control as the already cut off blood supply to his brain was still in effect either by the throat gouging or by its constriction.  The dark ritual was taken by every Knight of his order, A dark bargain with the old evil dying gods of his world and gave his order the ability to keep going until the last drop of blood had left their system or their head was cut off.  Nergal used this better than anyone in his order using his blood magics at dangerous levels as the dark magics that governed his body kept his spirit in control of his body.  In past encounters before coming to shinra it was not unheard of him to use all but a last pint of his own blood, loosing it profusely through various wounds on his body.  Nergal though on occasion did have to push beyond that, that's what the putricide was for, almost loosing every bit of his blood the putricide would be pushed into his system.  He would use his blood, still going strong and surprising his prey, and when his body was almost drained he would change over.  So, having almost no blood going to his noggin' and still being able to function was his style routinely with each fight and this exchange was not different. 

Of course now that his blood was going directly through the creatures tongue...and into its head and digestive track.  Well...the bombs effects would be even worse now.