What is mental effort?

I understand the biological necessity for Mental Effort as it stops you over stressing your body and burning more calories than needed but I don't understand how it works.

Example, when you do excersize you get to a mental barrier at which point it becomes mentally difficult to continue exercising.  You may just be using one arm with moderately elevated heart rate and breathing and yet it changes from a physical effort into a mental one.

I don't know how the system works but I guess that a chemical signal in the brain depleats the longer it is used and mental effort affects the production of the chemical?  Compounds like adrenaline adjust the takeup of the signals, not giving more strengtg but making it more easily available.

It seems that a drug which affects this chemical would be highly addictive but also highly useful.