A surgical instrument comprises a shaft, wherein a deflectable articulation section is formed at the shaft, a proximal handling portion at a proximal end of the shaft, a distal effector at a distal end of the shaft, and a deflection mechanism for controlling a bending state of the articulation section. The deflection mechanism comprises a first pull element for a deflection movement and a second pull element for a return movement. The first pull element and the second pull element are jointly pretensioned during the movement of the articulation section. An interface is provided where the instrument is demountable into a distal shaft assembly and a proximal handle piece, wherein the effector is controlled via an actuation mechanism that passes through the interface in a mounted state. The actuation mechanism comprises a push piece having a distal end that is coupled with the effector and a proximal end that is actuable by a pushing movement.