In the Phillipine-American war, US marines ran into a style of warrior who could put themselves into a state where they would feel no pain, and only stop fighting when their wounds caused them to keel over dead, not yield to their pain. Many of these raids would consist of Filipinos with machetes charging at American squads, being shot up but taking several marines with them because of their ability to continue ignore pain (leading eventually to the general issue status of the Colt .45 in the American army).
Go back a millennium and you'll see Norse warriors who fought with animal like ferocity. Not heeding defense, not heeding wounds, not heeding burns or the freezing cold on their often naked or half naked bodies. What my question is, could these two styles of fighting be related and if so, are they caused by the same thing?