  Motivated by the formation of ring-like filament structures in the cortex of
plant and animal cells, we study the dynamics of a two-dimensional layer of
cytoskeletal filaments and motor proteins near a surface by a general continuum
theory. As a result of active processes, dynamic patterns of filament
orientation and density emerge via instabilities. We show that
self-organization phenomena can lead to the formation of stationary and
oscillating rings. We present state diagrams which reveal a rich scenario of
asymptotic behaviors and discuss the role of boundary conditions.
