In this paper, corrections of the order of V/m to the nuclear structure function are investigated. V is the average potential energy of a nucleon in a nucleus, m is the nucleon mass. To this order, both the binding of the nucleon in the single-particle (impulse) approximation and the scattering of a virtual γ-quantum on the carriers of the NN interaction should be taken into account. In this case the momentum sum rule is automatically satisfied. In the infinite momentum frame this corresponds to the inclusion of non-nucleonic degrees of freedom which are responsible for the interaction between nucleons.