  The splitting between the charge-longitudinal and spin-transverse responses
is explained in a model whose inputs are the effective interactions in the
particle-hole channels in the first order boson loop expansion. The interplay
between $\omega$-meson exchange and box diagrams mainly governs the
longitudinal response, while in the transverse one the direct $\Delta$
excitations almost cancel the one-loop correction and the response is ruled by
the $\rho$-meson rescattering.
