  Brownian dynamics simulations are used to study highly entangled solutions of
semiflexible polymers. Bending fluctuations of semiflexible rods are
signficantly affected by entanglement only above a concentration $c^{**}$,
where $c^{**}\sim 10^{3}L^{-3}$ for chains of similar length $L$ and
persistence length. For $c > c^{**}$, the tube radius $R_{e}$ approaches a
dependence $R_{e} \propto c^{-3/5}$, and the linear viscoelastic response
develops an elastic contribution that is absent for $c < c^{**}$. Experiments
on isotropic solutions of $F$-actin span concentrations near $c^{**}$ for which
the predicted asymptotic scaling of the plateau modulus $G \propto c^{7/5}$ is
not yet valid.
