  A microscopic basis is provided for the spin-fermion model used to describe
the physics of the underdoped cuprates. In this way, a spin-fermion coupling is
shown to take care of the local no double occupancy constraint, which is
ignored in the weakly coupled regime. This ingredient is proved to be essential
to capture the physics of strong correlations, however. We elaborate further on
our recent proposal for the strong-coupling version of the spin-fermion model
that may prove to be the correct starting point to deal with the strong
correlation physics displayed by the underdoped cuprates.
