  Nuclear spirals naturally form as a gas response to non-axisymmetry in the
galactic potential, even if the degree of this asymmetry is very small. Linear
wave theory well describes weak nuclear spirals, but spirals induced by
stronger asymmetries in the potential are clearly beyond the linear regime.
Hydrodynamical models indicate spiral shocks in this latter case that,
depending on how the spiral intersects the x2 orbits, either get damped,
leading to the formation of the nuclear ring, or get strengthened, and
propagate towards the galaxy centre. Central massive black hole of sufficient
mass can allow the spiral shocks to extend all the way to its immediate
vicinity, and to generate gas inflow up to 0.03 M_sun/yr, which coincides with
the accretion rates needed to power luminous local Active Galactic Nuclei.
