  We study the role of massive perturbers (MPs) in deflecting stars and
binaries to almost radial ("loss-cone") orbits, where they pass near the
central massive black hole (MBH), interact with it at periapse, and are
ultimately destroyed. MPs dominate dynamical relaxation when the ratio of the
2nd moments of the MP and star mass distributions,
mu_2=(N_p*<M_p^2>)/(N_s*<M_s^2>), satisfies mu_2>>1. We compile the MP mass
function from published observations, and show that MPs in the nucleus of the
Galaxy (mainly giant molecular clouds), and plausibly in late type galaxies
generally, have 10^2<mu_2<10^8. MPs thus shorten the relaxation timescale by
10^1-10^7 relative to 2-body relaxation by stars alone. We show this increases
by 10-1000 the rate of \emph{large}-periapse interactions with the MBH, where
loss-cone refilling by stellar 2-body relaxation is inefficient. We extend the
Fokker-Planck loss-cone formalism to approximately account for relaxation by
rare encounters with MPs. We show that binary stars--MBH exchanges driven by
MPs can explain the origin of the young main sequence B stars that are observed
very near the Galactic MBH, and increase by orders of magnitude the ejection
rate of hyper-velocity stars. In contrast, the rate of \emph{small}-periapse
interactions of single stars with the MBH, such as tidal disruption, is only
increased by a factor of a few. We suggest that MP-driven relaxation plays an
important role in the 3-body exchange capture of single stars on very tight
orbits around the MBH. These captured stars may later be disrupted by the MBH
via tidal orbital decay or direct scattering into the loss cone; captured
compact objects may inspiral into the MBH by the emission of gravitational
waves from zero-eccentricity orbits.
