Reactive Circuits are components of the Mjolnir Powered Assault Armor system created by the UNSC for the Spartan-II and III soldiers. Reactive circuits are installed throughout the armor and are linked to the Spartan Neural Interface. They amplify the wearer's reaction time by connecting directly to the thoughts of the wearer, making it much easier to control and allowing for the soldier to be both more efficient and have a higher survival rate in combat.