  We study the dynamics of an entangled, isotropic solution of polar filaments
coupled by molecular motors which generate relative motion of the filaments in
two and three dimensions. We investigate the stability of the homogeneous state
for constant motor concentration taking into account excluded volume and
entanglement. At low filament density the system develops a density
instability, while at high filament density entanglement effects drive the
instability of orientational fluctuations.
