An actuation unit has a primary motor and a transmission mechanism for transmitting motion generated by the primary motor to a control member. The transmission mechanism includes a first transmission member connected to a drive shaft of the primary motor, a second transmission member drivingly connected to the control member, and a coupling member shiftable between an engagement position and a disengagement position. The transmission mechanism further includes a first elastic member interposed between the first transmission member and the second transmission member to cause rotation of the second transmission member relative to the first transmission member when the coupling member is in the disengagement position, a second elastic member for elastically urging the coupling member towards the engagement position, and an auxiliary actuation device for shifting the coupling member from the engagement position to the disengagement position.