  We have experimentally demonstrated both heating and cooling of electrons in
an ultracold neutral plasma (UNP) by embedding Rydberg atoms into the plasma
soon after its creation. We have determined the relationship between the
initial electron temperature, $T_{e,i}$, and the binding energy of the added
Rydberg atoms, $E_b$, at the crossover between heating and cooling behaviors
(that is, the binding energy of the atoms, which, when they are added to the
plasma, neither accelerate or slow down the plasma expansion). Specifically,
this condition is $|E_b| \approx 2.7 \times k_B T_{e,i}$ when the diagnostic
used is the effect of the Rydberg atoms on the plasma asymptotic expansion
velocity. Additionally, we have obtained experimental estimates for the amount
of heating or cooling which occurs when the Rydberg binding energy does not
satisfy the crossover condition. The experimental results for the crossover
condition, and the degree of heating or cooling away from the crossover, are in
agreement with predictions obtained from numerical modeling of the interactions
between Rydberg atoms and the plasma. We have also developed a simple intuitive
picture of how the Rydberg atoms affect the plasma which supports the concept
of a `bottleneck' in the Rydberg state distribution of atoms in equilibrium
with a co-existing plasma.
