  The validity of the approximations done in the mean field description of the
quasi-elastic excitation of medium-heavy nuclei is discussed. A test of the
reliability of the plane wave Born approximation is presented. The uncertainty
related to the choice of the electromagnetic nucleon form factors is discussed.
The effects produced by the meson exchange currents generated by the exchange
of a single pion are studied, as well as the need of including relativistic
corrections to the one-body currents. The results of the continuum shell model
and those of the Fermi gas model are compared and the need of treating the
emitted nucleon within a relativistic framework is studied. We analyze the
results of Random Phase Approximation calculations not only in terms of their
effects on the mean field responses, but also in terms of the theoretical
consistency in the choice of the effective residual interaction. The role of
the final state interaction is investigated. A microscopic justification of the
use of complex optical potential to describe the final state interaction is
provided. The effects of the final state interaction on the mean field
responses are studied with a sum rule conserving model. A comparison with the
experimental data measured in the C12 and Ca40 nuclei is shown.
