In a normal airplane, hydraulics or even cable-pulled systems are used to power the control surfaces. In a missile, that seems infeasible. What device is used to move the control surfaces, and where does it get its power?
Battery powered servo actuators.
The control section consists of control electronics, actuator batteries, and four independently controlled servo-actuators.
Further on the actual AIM-120 control mechanism: (Courtesy of the Turkish AF http://web.deu.edu.tr/atiksu/ana44/air3.html)
The Weapons Control Unit (WCU)-11/B consists of four independently controlled electro-mechanical servo actuators, four lithium-aluminum batteries connected in parallel, and a steel fuselage section that is bolted to the propulsion section aft skirt. Each actuator consists of a brushless DC motor ballscrew, an infinite resolution potentiometer directly coupled to the output shaft, and pulse width modulated control electronics. The output shaft is engaged directly to a squib actuated lock so that it does not interfere with the fin (control surface) installation and removal.