  Sturmian theory for nucleon-nucleus scattering is discussed in the presence
of all the phenomenological ingredients necessary for the description of
weakly-bound (or particle-unstable) light nuclear systems. Currently, we use a
macroscopic potential model of collective nature. The analysis shows that the
couplings to low-energy collective-core excitations are fundamental but they
are physically meaningful only if the constraints introduced by the Pauli
principle are taken into account. The formalism leads one to discuss a new
concept, Pauli hindrance, which appears to be important to understand the
structure of weakly-bound and unbound systems.
