The improvement of the materials characterization techniques in the last years has given access to new important information about the microstructure of polycrystalline metals. From experimental studies of deformed polycrystals, plastic strain within grains is known to be strongly heterogeneous and intermittent. As a consequence of the collective motion of dislocations, sample surfaces are indeed characterized by the presence of slip lines and slip bands (as slip traces). In the present study, a new micromechanical approach is developed to derive the mechanical fields (stresses, distortion, lattice curvature, elastic energy) arising from the presence of an inelastic strain field representing a typical internal "microstructure" as the one observed during the plastification of metallic polycrystals. This "microstructure" is due to the formation of discrete (spatial-temporal) intra-granular plastic slip heterogeneities which are modelled using discrete distributions of circular glide dislocation loops for a grain embedded in an infinite elastic matrix. Then, field equations have been solved using the method of Fourier Transforms. In contrast with the mean field approach based on the Eshelby formalism, it is then found that stress and lattice curvature fields are not more uniform inside the grain. A grain boundary layer actually appears where strong gradients occur and whose thickness depends on the introduced internal lengths. These results are compared with experimental measurements of local lattice rotation fields obtained by orientation imaging mapping (OIM). The model is able to capture different behaviours between near grain boundary regions and grain interior. The model was also develop to allow the study of more complex microstructures like the dislocation cells