  Using the quark-meson coupling (QMC) model, we study the longitudinal
response function for quasielastic electron scattering from nuclear matter. In
QMC the coupling constant between the scalar ($\sigma$) meson and the nucleon
is expected to decrease with increasing nuclear density, because of the
self-consistent modification of the structure of the nucleon. The reduction of
the coupling constant then leads to a smaller contribution from relativistic
RPA than in the Walecka model. However, since the electromagnetic form factors
of the in-medium nucleon are modified at the same time, the longitudinal
response function and the Coulomb sum are reduced by a total of about 20% in
comparison with the Hartree contribution. We find that the relativistic RPA and
the nucleon structure variation both contribute about fifty-fifty to the
reduction of the longitudinal response.
