  Here we present the spatially resolved study of six Galactic planetary
nebulae (PNe), namely IC 4593, Hen 2-186, Hen 2-429, NGC 3918, NGC 6543 and NGC
6905, from intermediate-resolution spectra of the 2.5 m Isaac Newton Telescope
and the 1.54 m Danish telescope. The physical conditions (electron densities,
N$_{e}$, and temperatures, T$_{e}$), chemical compositions and dominant
excitation mechanisms for the different regions of these objects are derived,
in an attempt to go deeper on the knowledge of the low-ionization structures
(LISs) hosted by these PNe. We reinforce the previous conclusions that LISs are
characterized by lower (or at most equal) N$_{e}$ than their associated rims
and shells. As for the T$_{e}$, we point out a \textit{possible} different
trend between the N and O diagnostics. T$_e$[NII] does not show significant
variations throughout the nebular components, whereas T$_e$[OIII] appears to be
slightly higher for LISs. The much larger uncertainties associated with the
T$_e$[OIII] of LISs do not allow robust conclusions. Moreover, the chemical
abundances show no variation from one to another PN components, not even
contrasting LISs with rims and shells, as also found in a number of other
works. By discussing the ionization photon flux due to shocks and stellar
radiation, we explore the possible mechanisms responsible for the excitation of
LISs. We argue that the presence of shocks in LISs is not negligible, although
there is a strong dependence on the orientation of the host PNe and LISs.
