In an actuator having a reversible motor before a step-down gear, this can be optimized kinetically, geometrically and in terms of material engineering when different step-down ratios become active according to the different torque requirement for application and for release, in a manner dependent on direction of rotation, for example of a parking brake operated by an electric motor. For this purpose, two differently dimensioned idler wheels of the step-down gears that mesh, for example, with a stepped motor pinion, act via freewheels acting in opposite directions on a common gear shaft. Depending on direction of rotation of the motor, the gear shaft thereby rotates at different rpms, delivering correspondingly different torques.